Field of Science

Showing posts with label Fabidae. Show all posts
Showing posts with label Fabidae. Show all posts

Malpighiales: A Glorious Mess of Flowering Plants

Ixonanthes reticulata, from here.


There is no denying that the advent of molecular analysis revolutionised the world of plant phylogeny. Previously an uncertain landscape of shifting sands, beset by the eroding forces of convergent evolution and morphological plasticity, the higher relationships of flowering plants have begun to resolve into a much clearer view than before. But some of the revealed vistas have been unexpected, and have led to quagmires of their own.

The Malpighiales are one clade that has become generally recognised as a result of molecular analyses, but remain almost impossible to characterise morphologically. Part of that difficulty is a consequence of sheer diversity: the clade includes about 16,000 species worldwide. The bulk of these species are tropical; it has been estimated that 40% of the world's tropical rain-forest understory is composed of Malpighiales (Xi et al. 2012). Only a relative minority of Malpighiales are found in more temperate parts of the world, though that minority still includes such familiar plants as violets, willows and spurges. The ranks of Malpighiales include some of the most bizarrely modified of all flowering plants: the endoparasitic Rafflesiaceae and the aquatic Podostemaceae.

Fruit of the jellyfish tree Medusagyne oppositifolia, photographed by Christopher Kaiser-Bunbury. The jellyfish tree is restricted to the Seychelles and critically endangered; the few surviving trees occupy marginal habitat where seedling germination seemingly cannot occur.


Though molecular analyses have been fairly consistent in supporting the Malpighiales as a whole, relationships within the Malpighiales long proved more recalcitrant. As a result, its species have been placed in up to 42 different families, these families varying wildly in diversity. At one end of the scale, the Euphorbiaceae has been home to over 5700 species, even in its modern restricted sense (earlier botanists recognised a Euphorbiaceae that was considerably larger). At the other end, the Malesian vine Lophopyxis maingayi and the jellyfish tree Medusagyne oppositifolia of the Seychelles have each been considered distinctive enough and of uncertain enough affinities to be placed in their own monotypic families. Many of these families could only be placed within the Malpighiales as part of a great polytomy, an unresolved mess of relationships at the base of the clade.

Herbarium specimen of Centroplacus glaucinus, from here. This species has a restricted range in West Africa; its closest relatives belong to the genus Bhesa in south-east Asia.


A major advance in our understanding of malpighialean phylogeny was made just recently by Xi et al. (2012), who were able to obtain a more resolved phylogenetic tree than previous studies through the use of data from a large number of genes (they also ignored the Rafflesiaceae; those guys just cause trouble). Their results suggested a division of the Malpighiales between three basal clades. The smallest of these includes relatives of the families Malpighiaceae and Chrysobalanaceae. Few members of this clade are familiar outside the tropics. Some are known for their edible fruit, such as the coco plum Chrysobalanus icaco, the nance Byrsonima crassifolia, the Barbados cherry Malpighia emarginata and the butter-nut Caryocar nuciferum. In contrast, the southern African gifblaar Dichapetalum cymosum contains toxic sodium monofluoroacetate and is regarded as a serious threat to livestock.

Small individual of the mangrove Kandelia candel, photographed by Dans.


The next clade includes families relatied to the Clusiaceae, Ochnaceae and Erythroxylaceae. The latter family is closely related to (and sometimes synonymised with) the Rhizophoraceae, a small but significant family that dominates among the tropical mangroves. The Erythroxylaceae is itself most notorious for including the coca plant Erythroxylum coca, the source of the drug cocaine*. The clusioid families include the Clusiaceae, Hypericaceae and Calophyllaceae, treated in older sources as a single family Guttiferae but currently treated as separate families owing to the paraphyly of such a grouping to the families Bonnetiaceae and Podostemaceae. The name 'Guttiferae' refers to the production of resin by many clusioids. In some species, these resins are produced in the flowers in lieu of nectar and are collected for nest-building by visiting bees. Economically significant clusioids include the mangosteen Garcinia mangostana and the St John's wort Hypericum perforatum, which has been grown commercially in some parts of the world for its supposed medicinal properties but is regarded in other parts of the world as a highly undesirable weed.

*Not raisins.

Flower of Rhizanthes infanticida, a smaller relative of Rafflesia, growing from host roots on the forest floor in Thailand, from here. Further buds are visible as reddish balls closer to the tree.


The third clade, and the largest by a considerable margin, includes such families as the Euphorbiaceae, Violaceae and Salicaceae. Noteworthy examples of this clade also include the passion fruit Passiflora edulis, and the flax plant Linum usitatissimum. The Euphorbiaceae, as alluded to above, were previously considered to include taxa more recently treated as the separate families Putranjivaceae, Phyllanthaceae, Picodendraceae and Peraceae. The Putranjivaceae were placed by Xi et al. (2012) in the Malpighiaceae-Chrysobalanaceae clade, and so are not close relatives of the Euphorbiaceae sensu stricto. The remaining families are closer, but modern authors would prefer to keep them separate as the demands of monophyly would then require the Euphorbiaceae be further enlarged to include the Linaceae and Rafflesiaceae. Nobody wants a Rafflesia in their family.

REFERENCE

Xi, Z., B. R. Ruhfel, H. Schaefer, A. M. Amorim, M. Sugumarane, K. J. Wurdack, P. K. Endress, M. L. Matthews, P. F. Stevens, S. Mathews & C. C. Davis. 2012. Phylogenomics and a posteriori data partitioning resolve the Cretaceous angiosperm radiation Malpighiales. Proceedings of the National Academy of Science of the USA 109 (43): 17519-17524.

The Range of Lotus

For today's post, I'll be focusing on the lotus. And having said that, how many of you instantly thought of something like this:


To which I can only say: you should be ashamed of yourself. That is not a lotus, that is some wierd aquatic poppy-type thing called Nelumbo nucifera. This is a lotus:

Bird's-foot trefoil Lotus corniculatus, from Lyndon's Garden.


To clarify, Lotus is a genus of over a hundred species of herbaceous legumes native mostly to Eurasia and northern Africa, with smaller numbers of species in sub-Saharan Africa and Australia (Kirkbride 1999). About forty or so species have also been assigned to this genus from the Americas (particularly western North America), but all recent analyses have agreed that the New World species are not immediately related to the Old World species (Allan & Porter 2000; Arambarri et al. 2005) and they have been reclassified as genera Hosackia, Acmispon, Ottleya and Syrmatium—we shall not speak of them again. How the name 'lotus' came to be simultaneously applied to two such different plants as pictured above, I couldn't say, but the practice goes back a long time: the elder Pliny was referring to both sweet clover and a water lily as lotus in the first century AD (Kirkbride 1999). He also used the name 'lotus' for jujubes and (possibly) pomegranates, so he evidently had a certain affection for the word.

Greater lotus Lotus uliginosus, photographed by Forest & Kim Starr.


Lotus species are commonly known as trefoils, in reference to the leaves being divided into three leaflets. As it happens, most Lotus species actually have leaves with five leaflets, but two of those are separated from the others by an extended midrib. Pea-shaped flowers are borne in small terminal clusters; these are most commonly yellow, though some species produce red flowers. A few species have gone by the vernacular name of 'bacon and eggs' in reference to their producing flowers which are a combination of the two colours. Seeds are produced in long straight pods, and the appearance of the clustered pods is responsible for another vernacular name, bird's-foot trefoil. In one group of species, commonly separated as a genus Tetragonolobus but phylogenetically nested among other Lotus (Allan & Porter 2000), the pods bear four longitudinal wings. Even excluding the New World species, the exact number of species recognised in Lotus varies between authors, primarily due to disagreement over the appropriate treatment of segregates of the more widespread and variable taxa.

Young pod and flower of asparagus pea Lotus tetragonolobus, from here.

A number of Lotus species, particularly L. corniculatus and the greater lotus L. uliginosus*, are used as pasture legumes and have become established around the world as a result. Though arguably less productive than alternative legumes such as clover, they are often able to tolerate more marginal habitats (particularly waterlogged ground). Some species do contain secondary metabolites that can produce cyanide, but concentrations are not usually high enough to be a concern. The asparagus pea Lotus tetragonolobus (also known as Tetragonolobus purpureus) is grown for more direct human consumption, with the pods being eaten before they reach maturity. The name 'asparagus pea' is supposed to refer to their flavour, but this website expressed the opinion that: "If you have an excessively moist mouth, and are looking for something to suck all the moisture out and leave you all pasty, then asparagus peas are the vegetable for you."

*There seems to be some disagreement out there about whether Lotus uliginosus should be recognised as distinct from L. pedunculatus. Kirkbride (1999) uses L. uliginosus as a distinct taxon.

REFERENCES

Allan, G. J., & J. M. Porter. 2000. Tribal delimitation and phylogenetic relationships of Loteae and Coronilleae (Faboideae: Fabaceae) with special reference to Lotus: evidence from nuclear ribosomal ITS sequences. American Journal of Botany 87 (12): 1871-1881.

Arambarri, A. M., S. A. Stenglein, M. N. Colares & M. C. Novoa. 2005. Taxonomy of the New World species of Lotus (Leguminosae: Loteae). Australian Journal of Botany 53: 797-812.

Kirkbride, J. H., Jr. 1999. Lotus systematics and distribution. In: Trefoil: The Science and Technology of Lotus, pp. 1-20. Crop Science Society of America and American Society of Agronomy.

Lecantheae and/or Elatostemateae

Elatostema umbellatum var. majus, photographed by Michael Becker.


The Elatostemateae is a tribe of plants in the Urticaceae, the family that includes the nettles (Urticaceae in general have been looked at in an earlier post). Elatostemateae are mostly distinguished from other members of the Urticaceae by their generally tripartite perianth in the female flowers and their brush-like stigmas (Friis 1993). There seems to be some conflict over the appropriate name to call this group: Conn & Hadiah (2009) argued for the use of Elatostemateae, but many authors had previously called this group the Lecantheae. The argument hinges purely on a question of priority a bit too tedious to examine here.

Lecanthus peduncularis, photographed by Li-Chieh Pan.


The question may or may not be moot, anyway. Recent phylogenetic analyses of the Urticaceae have generally agreed that the Elatostemateae can be divided between two clades, one including the genus Elatostema and the other the genera Lecanthus and Pilea. The two groups are morphologically as well as molecularly distinguishable: species of Pilea and Lecanthus generally have opposite leaves, but in Elatostema one of each leaf pair has been greatly reduced (or lost) so that the leaves appear alternate. However, it is currently uncertain whether these two groups together form an exclusive clade. In the analyses by Hadiah et al. (2008), trnL-F sequence data was consistent with a monophyletic Elatostemateae, but rbcL data placed the Lecanthus-Pilea group closer to the tribe Urticeae than to Elatostema. This is not inherently unreasonable: as members of the Urticeae also have brush-like stigmas like those of Elatostemateae, the main distinction between the two tribes is the plesiomorphic absence in the latter of the stinging hairs that characterise the Urticeae.

Pilea fairchildiana, previously Sarcopilea domingensis, from Javier Francisco-Ortega.


Whether monophyletic or not, the Elatostemateae are mostly a tropical and subtropical group. They are mostly herbs and subshrubs, with a smaller number of shrub species. The centre of diversity is in the Old World; only the genus Pilea is found in the Americas. Members of the Elatostema group may be either all included in a single genus or divided between genera Elatostema, Procris and Pellionia distinguished by inflorescence characters. Pilea, the largest genus in the group, includes species in both the Old and New Worlds, of which some are commonly succulent. The Hispaniolan species Pilea fairchildiana has developed a rosulate growth form, remarkably similar to members of genera in the unrelated family Crassulaceae, such as Aeonium and Sempervivum. Until very recently, this species was considered distinctive enough to be placed in a separate genus Sarcopilea; Jestrow et al. (2012) demonstrated its more nested position. Economic usages of Lecantheae species are few: the leaves of some Pilea species, such as the artillery plants* P. microphylla and P. melastomoides, have some limited use as aromatic herbs, while some species of the Elatostema group are apparently used in Indonesia as shampoo.

*The name 'artillery plant' apparently refers to the way in while the male flowers fire forth their pollen, which is supposed to resemble gunsmoke.

REFERENCES

Conn, B. J., & J. T. Hadiah. 2008. Nomenclature of tribes within the Urticaceae. Kew Bulletin 64; 349-352.

Friis, I. 1993. Urticaceae. In: Kubitzki, K., J. G. Rohwer & V. Bittrich (eds) The Families and Genera of Vascular Plants vol. 2. Flowering Plants. Dicotyledons. Magnoliid, hamamelid and caryophylliid families pp. 612-629. Springer.

Hadiah, J. T., B. J. Conn & C. J. Quinn. 2008. Infra-familial phylogeny of Urticaceae, using chloroplast sequence data. Australian Systematic Botany 21: 375-385.

Jestrow, B., J. J. Valdés, F. Jiménez Rodríguez & J. Francisco-Ortega. 2012. Phylogenetic placement of the Dominican Republic endemic genus Sarcopilea (Urticaceae). Taxon 61 (3): 592-600.

Milk-vetches, Liquorice and Locoweeds


Flowers of the weeping broom Carmichaelia stevensonii, from here.


Far in the distant past, I commented on a review of the New Zealand brooms of the genus Carmichaelia. The New Zealand brooms were placed by Wagstaff et al. (1999) in the tribe Galegeae, which includes more than 3000 species around the world. The vast majority of these are placed in the genus Astragalus, milk-vetches, which may itself have well over 2500 species and be the largest recognised genus of flowering plant. Other significant members of the Galegeae include the locoweeds (Oxytropis) and Glycyrrhiza, a small genus of less than twenty species that nevertheless deserves praise for being the source of liquorice*.

*I am reliably informed that there are people in this world who do not appreciate the flavour of liquorice. There is simply no accounting for taste.


Flowers of Astragalus monspessulanus, from here.


However, despite its wide recognition, phylogenetic studies of recent years have been unanimous in declaring the Galegeae polyphyletic. It is true that the vast majority of galegeans remain in a clade, with only a few relatively minor genera placed elsewhere (Wojciechowski et al. 2000). The problem is that one of those 'minor genera' happens to be Galega itself, the type genus and therefore sine qua non of the tribe, which is more closely related to the chickpeas of the genus Cicer. Wojciechowski et al. (2000) circumvented this issue by recognising a clade called Hologalegina, uniting the 'Galegeae' with other tribes such as Hedysareae, Trifolieae, Fabeae, Loteae and Robinieae. Within the Hologalegina, the taxa previously assigned to the Galegeae all belong to what is called the IRLC, the 'Inverted Repeat-Lacking Clade'. The name of this clade refers to the loss of a copy of the 25 kb inverted repeat in the chloroplast genome that is otherwise found in almost all other land plants. Members of the IRLC are largely herbaceous, and many of the most commercially significant legumes (such as peas, beans, lentils, clover and alfalfa) belong to this clade.


Liquorice, Glycyrrhiza glabra, photographed by J. C. Schou. The flavour comes from the roots.


With the exception of the aforementioned Glycyrrhiza (which is another phylogenetically distant genus from other galegeans), most of the 'Galegeae' are not commercially grown. They may be commercially eradicated: species of Oxytropis get their name of 'locoweed' from their toxic effects on grazing livestock, and various species of Astragalus and Swainsona are also known for their toxicity. Some species are cultivated as garden plants, such as the weeping broom Carmichaelia stevensonii shown in the top photo (though many places still list it under the older name of Chordospartium stevensonii) and the kaka beak Clianthus puniceus. One particular galegean is widely regarded as the stuff that gardeners' dreams are made of, despite being a notoriously difficult plant to grow. My partner tells me that when he lived in Kalgoorlie as a child, tourist buses would regularly appear outside his house just so that their passengers could see this plant flowering in the front yard. I speak, of course, of Swainsona formosa, Sturt's desert pea:


Photographed by Marj Kibby.


REFERENCES

Wagstaff, S. J., P. B. Heenan & M. J. Sanderson. 1999. Classification, origins, and patterns of diversification in New Zealand Carmichaelinae (Fabaceae). American Journal of Botany 86 (9): 1346-1356.

Wojciechowski, M. F., M. J. Sanderson, K. P. Steele & A. Liston. 2000. Molecular phylogeny of the “temperate herbaceous tribes” of papilionoid legumes: a supertree approach. In: Herendeen, P. S., & A. Bruneau (eds). Advances in Legume Systematics vol. 9, pp. 277–298. Royal Botanic Gardens, Kew.

Cunoniaceae and Friends


The Albany pitcher plant Cephalotus follicularis of south-western Australia. The streaked colours on the inside of the lid attract insects into the pitcher; the incurved teeth around the rim stop them from climbing back out. Photo by Holger Hennern.


The plant order Cunoniales was first established in 1926 to include plants with similar flowers to the Saxifragales but that were primarily woody rather than herbaceous (Dickison, 1975). Woodiness vs. herbaceousness is no longer considered that significant a feature in plant classification (sometimes you can find both in the same genus) and the content of the order has varied between classifications*. Today, the type family Cunoniaceae is included in the order Oxalidales and a taxon "Cunoniales" is no longer used as such. However, one of the two basal clades within the Oxalidales includes the Cunoniaceae and two ex-Cunoniales genera placed in their own families, Cephalotus and Brunellia, together with the family Elaeocarpaceae (previously in its own order) (Matthews & Endress, 2002). With the notable exception of Cephalotus, the members of this clade are mostly shrubs or trees. Economically, the clade is not overly significant: some species are used for wood; some have good reputations as honey sources for bees; a few produce edible fruits but do not appear to have been systematically cultivated for them. Members of all families bear their flowers clustered into (most often cymose) inflorescences; the size of individual flowers in the inflorescences varies between species. Brunellia and Cephalotus both produce flowers with thick sepals and no petals; in the other two families, petals may be present or absent (Matthews & Endress, 2002).

*As have most flowering plant "orders". There's a reason why order-level taxa don't get much day-to-day use among botanists compared to families.


Coachwood, Ceratopetalum apetalum, a member of the Cunoniaceae from eastern Australia. Photo by Melburnian.
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The clade formed by these four families has a distinctly southern distribution in southern Africa, South and Central America, south-east Asia, Australia and New Zealand. Elaeocarpaceae are absent from continental Africa but are present in Madagascar. Also, while currently absent from India, they have been recorded from the fossil record there (Crayn et al., 2006). The Cunoniaceae include about 300 species, half in the genus Weinmannia, whereas the Elaeocarpaceae include about 600 species. Molecular studies have shown that an Australian radiation of dry-habitat shrubs previously regarded as the family Tremandraceae is in fact a subclade of the otherwise mostly rainforest-inhabiting Elaeocarpaceae (Crayn et al., 2006). Interestingly, the molecular dating study by Crayn et al. (2006) suggests that the 'Tremandraceae' developed scleromorphy (a suite of adaptations such as hardened leaves that are usually associated with arid habitats) some time before the Australian continent developed its current arid climate. While this may seem counter-intuitive, it is worth pointing out that the fossil record supports the same thing in the evolution of the genus Banksia (Mast & Givnish, 2002). It has been suggested that scleromorphy in these groups was therefore not originally an adaptation for arid living, but for growing in the poor soils of the Palaeogene Australian rainforest.


Prima donna, Elaeocarpus reticulatus. Fringed petals are characteristic of a number of species of Elaeocarpaceae. Elaeocarpaceae flowers are also adapted for buzz-pollination, where anthers do not release their pollen until vibrated by the wingbeats of their pollinating bees. Photo from Kate's Photo Diary.


The oddest member of this clade, however, has to be the Albany pitcher plant, Cephalotus follicularis. Restricted to the south-west corner of Western Australia, this plant bears a superficial resemblance to pitcher plants from other parts of the world, the mostly Indomalayan Nepenthaceae and the North American Sarraceniaceae. All three of these families have evolved the pitcher morphology independently, and can in fact be placed in separate subclasses: Cephalotus in the Rosidae, the Nepenthaceae in the Caryophyllidae and the Sarraceniaceae in the Asteridae. In the Nepenthaceae, the pitchers are developed from trendrils at the ends of the leaves; in the other two families, the entire leaves form the pitchers growing from a ground-level rhizome. In Cephalotus, only the outer leaves of an individual plant are developed into pitchers; the inner leaves remain flat and simple.

REFERENCES

Crayn, D. M., M. Rossetto & D. J. Maynard. 2006. Molecular phylogeny and dating reveals an Oligo-Miocene radiation of dry-adapted shrubs (former Tremandraceae) from rainforest tree progenitors (Elaeocarpaceae) in Australia. American Journal of Botany 93 (9): 1328-1342.

Dickison, W. C. 1975. Studies on the floral anatomy of the Cunoniaceae. American Journal of Botany 62 (5): 433-447.

Mast, A. R., & T. J. Givnish. 2002. Historical biogeography and the origin of stomatal distributions in Banksia and Dryandra (Proteaceae) based on their cpDNA phylogeny. American Journal of Botany 89 (8): 1311-1323.

Matthews, M. L., & P. K. Endress. 2002. Comparative floral structure and systematics in Oxalidales (Oxalidaceae, Connaraceae, Brunelliaceae, Cephalotaceae, Cunoniaceae, Elaeocarpaceae, Tremandraceae). Botanical Journal of the Linnean Society 140 (4): 321-381.

A Simple Stream Life

In a footnote to a recent post, I made the offhand comment that the strangest flowering plants were to be found among the Podostemoideae. Today I'd like to introduce you to them.


Unidentified Podostemaceae (perhaps Podostemon) collected from a fast-flowing stream in Venezuela. Photo by Kevin Nixon. (And yes, this is the identity of Name the Bug #17.)


Podostemaceae is a family of about 270 species of freshwater aquatic plants found mostly in tropical parts of the world. The family is divided into three subfamilies, Podostemoideae, Tristichoideae and the monogeneric Weddellina. The main difference between Tristichoideae and Podostemoideae is that the flowers of the latter developed encased in a sack-like covering called a spathella. Weddellina resembles tristichoids in lacking a spathella but some of the finer features of its flower anatomy are more like podostemoids, to which phylogenetic analysis indicates it is the sister taxon (Kita & Kato 2001).

Podostems specialise in living in fast-moving, temporary streams and waterfalls that become dry for part of the year, usually on rocky surfaces. Many podostem species are known for having ridiculously small distributions, often restricted to a single waterway (more on that later). Podostems differ from all other aquatic flowering plants in lacking aerenchyma, the gas-filled tissue I mentioned in reference to duckweeds. The primary podostem morphology involves spreading, usually flattened 'roots' that give rise to branching 'shoots' that in turn produce 'leaves' (Rutishauser 1997). However, all parts are photosynthetic and these structures probably do not correspond directly to comparable structures in other plants. The plumule and the radicle, the initial shoots in a normal germinating seed that develop into the stem and the root, respectively, are absent in most podostems (Sehgal et al. 2002). Instead, the germinating plant body develops as a lateral outgrowth of the hypocotyl, the stem that would normally support the plumule. The plant attaches itself to the rock by small rootlets growing from the 'roots' or by disk-shaped holdfasts. Either the rootlets attach themselves in pre-existing films of adherent bacteria (Jäger-Zürn & Grubert 2000) or they may secrete their own sticky mucilage (Sehgal et al. 2002). In the Indian Dalzellia zeylanica the distinction between 'roots' and 'shoots' has disappeared entirely; instead, the plant grows as a crustose thallus bearing both rootlets and 'leaves'. 'Leaves' of podostems are varied in appearance from large compound structures up to two metres long to minute scales or bundles of filaments. Species with larger leaves often have hairs, prickles or warts covering their surface.


The South American podostem Rhyncholacis penicillata in flower. Photo by Berrucomons.


The flowers of podostems, whether covered by a spathella or not, are very variable. Inflorescences of Mourera fluviatilis (the species with the two-metre leaves) can be up to 64 cm tall including the spike with up to ninety flowers. Other species (including all Tristichoideae) may bear only a single flower on a short stem. Depending on the species, podostem flowers may be wind-pollinated, insect-pollinated or self-pollinated. Podostem seeds are tiny, wind-dispersed and lack endosperm (podostems lack the double fertilisation of most flowering plants). 1g of weight may contain over a million seeds. Fruits vary greatly in size between species - Mourera fluviatilis may produce fruits containing up to 2400 seeds each while Farmeria metzgerioides produces only two seeds per fruit. In the case of the latter, the fruit doesn't break open to release the seeds; instead, the seeds germinate in place over the remains of their parent.


Podostemon in the dry season: desiccated thalli holding maturing fruits. Photo by Renato Goldenberg.


Much speculation has been conducted on why so many podostems have restricted distributions. Some have implied that many podostem 'species' may turn out to be ecological variants of more widespread species; however, multiple podostem species may be found growing in a single habitat. Others have suggested that podostems are somehow under less selective pressure morphologically than terrestrial plants, allowing a higher rate of mutational drift; however, this proposal remains untested. Interestingly, the molecular phylogenetic analysis of Kita & Kato (2001) found that the highly modified Dalzellia zeylanica was closely related to the morphologically conservative Indotristicha ramosissima. Indeed, the genetic distance between the two was little greater than that between separate populations recognised as the single species Tristicha trifaria, unusual among podostems in being found in both Africa and the Americas. This would suggest that podostems are indeed capable of rapid morphological changes—the only question is how?

REFERENCES

Jäger-Zürn, I., & M. Grubert. 2000. Podostemaceae depend on sticky biofilms with respect to attachment to rocks in waterfalls. International Journal of Plant Sciences 161 (4): 599-607.

Kita, Y., & M. Kato. 2001. Infrafamilial phylogeny of the aquatic angiosperm Podostemaceae inferred from the nucleotide sequences of the matK gene. Plant Biology 3 (2): 156-163.

Rutishauser, R. 1997. Structural and developmental diversity in Podostemaceae (river-weeds). Aquatic Botany 57: 29-70.

Sehgal, A., M. Sethi & H. Y. Mohan Ram. 2002. Origin, structure, and interpretation of the thallus in Hydrobryopsis sessilis (Podostemaceae). International Journal of Plant Sciences 163 (6): 891-905.

A South American Paradox (Taxon of the Week: Sellocharis)


Aspects of Sellocharis paradoxa (Papilionaceae) as illustrated by Polhill (1976): "1, habit; 2, node with stem cut away to show leaf-arrangement; 3, flower; 4, calyx, opened out; 5, standard; 6, wing; 7, keel; 8, stamens, spread out; 9, anthers; 10, pistil; 11, same with ovary-wall cut away to show ovules".


A brief entry today, because Taxon of the Week this week is a bit of a mystery. The southern Brazilian leguminous subshrub Sellocharis paradoxa was first described in 1889, but for a very long time was known solely from the original isotypes*. It has only been rediscovered in scrubby grasslands and rockfields of the Brazilian state of Rio Grande do Sul within the last ten years or so (Conterato et al., 2007). Without having seen the original description, I can't tell you for certain what earned Sellocharis the name of 'paradoxa', but I suspect it was probably the unique arrangement of its leaves. As you can see in the figure above**, S. paradoxa has its leaves arranged in regular whorls of five to seven. The individual "leaves" are more similar to the leaflets of other leguminous plants, and Polhill (1976) tentatively suggested that that might be what they were - that instead of having whorls of six leaves, S. paradoxa might have only a single leaf that had lost its basal stalk so completely that it had merged with the main stem.

*For the non-botanists among you, "isotypes" are two or more type specimens that have been taken from the same original individual, such as two branches from a single tree.

**Now possibly the only depiction of Sellocharis available freely online. Not that I'm bragging or anything (especially considering I just scanned it out of the original book).

As befits its unusual morphology, the relationships of Sellocharis paradoxa are similarly mysterious. The most similar genus is Anarthrophyllum, a genus of Andean 'cushion plants' in which the stipules of the often trifoliate leaves surround the stem, often forming a sheath, and most authors seem to have assumed a relationship between the two genera. Flower morphology and the presence of α-pyridone alkaloids in Anarthrophyllum suggest a position in the Genisteae, the tribe including brooms, gorse and lupins, which I described in a previous post. Within the Genisteae, the flowers of Anarthrophyllum and Sellocharis are most similar to those of the basal Argyrolobium group. A large-scale molecular analysis of Papilionaceae placed Anarthrophyllum as sister to the clade of Lupinus and Genistinae (Wojciechowski et al., 2004), which is consistent with the previously suggested position of the Argyrolobium group (Ainouche et al., 2003) though no other members of the group were included in the later analysis. However, Polhill (1976) noted that, if one interprets the 'leaves' of Sellocharis as leaflets of a single divided leaf, then they bear a certain resemblance to the leaves of lupins and may indicate a relationship to that genus instead.

A genistean position for Sellocharis and Anarthrophyllum is still not un-problematic. As described in the previous post, the Genisteae is a primarily Old World lineage. Lupinus is the only other genus of Genisteae found in the Americas, and as it is found in both the Old and New Worlds it could be a later invader of the latter. Nevertheless, other genera of the Argyrolobium group are found in southern Africa, and the ancestors of Sellocharis may have come from there as other organisms are known to have done (the ancestors of the New World monkeys being perhaps the most famous example). Also potentially problematic is that the number and morphology of chromosomes in Sellocharis is very distinct from those of any other Genisteae; however, the authors who described Sellocharis' karyotype (Conterato et al., 2007) only referred to its differences from Genisteae without comparing it to members of other tribes. Hopefully, now that Sellocharis paradoxa has been re-found, more progress can be made on establishing just what it is.

REFERENCES

Ainouche, A., R. J. Bayer, P. Cubas & M.-T. Misset. 2003. Phylogenetic relationships within tribe Genisteae (Papilionoideae) with special reference to genus Ulex. In Advances in Legume Systematics part 10, Higher Level Systematics (B. B. Klitgaard & A. Bruneau, eds.) pp. 239-252. Royal Botanic Gardens: Kew.

Conterato, I. F., S. T. Sfoggia Miotto & M. T. Schifino-Wittman. 2007. Chromosome number, karyotype, and taxonomic considerations on the enigmatic Sellocharis paradoxa Taubert (Leguminosae, Papilionoideae, Genisteae). Botanical Journal of the Linnean Society 155 (2): 223-226.

Polhill, R. M. 1976. Genisteae (Adans.) Benth. and related tribes (Leguminosae). Botanical Systematics 1: 143 - 368.

Wojciechowski, M. F., M. Lavin & M. J. Sanderson. 2004. A phylogeny of legumes (Leguminosae) based on analysis of the plastid matK gene resolves many well-supported subclades within the family. American Journal of Botany 91: 1846-1862.

In a bunch, in a bunch!


Cytisus scoparius, a widespread broom species, and the most widespread as an invasive species. Photo by Paul Slichter.


After the dummy-spit of the last post, let's get on to something a little more comforting. This is, I think, the perfect time for a botany post to calm the nerves. Which is lucky, because the Taxon of the Week post this week is on Genisteae.

Genisteae is the tribe of about 450 species of mostly Holarctic leguminous plants that includes brooms*, gorse and lupins**. The first of these are the Palaearctic brooms - in an earlier post, I wrote about the New Zealand brooms, which belong to a different legume tribe, the Galegeae, and whose broom-like appearance is convergent with that of the Genisteae brooms. However, a number of species of Genisteae have been transported by humans to many temperate regions around the world (including New Zealand), and many are familiar weed species outside their native ranges.

*The shrubs, obviously, not the things you sweep with. Though brooms the plants were often used for the making of brooms the implements, which is probably the origin of the name of one or the other.

**Monty Python fans are permitted to start humming now.


Argyrolobium zanonii, a Mediterranean species of the possibly polyphyletic genus Argyrolobium that is one of best contenders for inclusion in the Genisteae. Photo from here.


Phylogenetically, Genisteae can be divided into three well-divided groups (Ainouche et al., 2003). The probably paraphyletic Argyrolobium group includes five genera of mostly southern African and South American plants that lie outside the clade formed by the Palaearctic members of Genisteae. Previously members of this group were included in the sister tribe Crotalarieae, and their position remains unsettled. In particular, it has been suggested that Argyrolobium itself may be polyphyletic, with some species belonging to Crotalarieae and others to Genisteae. With the Argyrolobium group included, the Genisteae are characterised by a basically two-lipped calyx with a trifid lower lip, and the presence of quinolizidine alkaloids of a-pyridone type (chemical characters have proven to be very useful in plant systematics, with many groups characterised by the production of particular secondary metabolites).


Lupinus polyphyllus, a lupin species native to western North America. Photo from Wikimedia.


The genus Lupinus (the lupins) form a distinct group from the remainder of the Palaearctic Genisteae. Lupinus is by far the largest genus of Genisteae, including about half the species and the only genus to have made it to the New World, where it is found in both North and South America. Most lupins are readily distinguished from other Genisteae by their palmate, divided leaves. While the flowers are not particularly edible (despite the rumoured possibilities of lupin soup, roast lupin, steamed lupin, braised lupin in lupin sauce, lupin in the basket with sauted lupins, lupin meringue pie, lupin sorbet...), the beans of some species are eaten pickled (raw beans generally contain toxic alkaloids) in Mediterranean areas or Latin America. Lupins have been widely grown as stock feed and as ornamentals, and include many of the aforementioned weed species.


Valley in Victoria (Australia) overrun by the vile Ulex europaeus. Photo by Kate Blood.


The remaining genera of Genisteae form the subtribe Genistinae. Diversity of Genistinae is centred around the Mediterranean, with the three best-known genera being the brooms in Genista and Cytisus, and gorse in Ulex (Ulex and the closely related, possibly synonymous, genus Stauracanthus have been placed in a separate subtribe Ulicinae, but Ainouche et al., 2003, demonstrated that these genera fell within Genistinae). A number of other broom genera are recognised, but classifications may differ on which genera are recognised as including which species. The Genistinae also includes the tree genus Laburnum (Käss & Wink, 1997). Members of the Genistinae are characterised by adaptations for arid habitats such as very small or absent leaves and photosynthetic stems. Ulex species have the leaves modified into long spines. The species Ulex europaeus (commonly called simply "gorse") was introduced into New Zealand for use in hedges, a role which it apparently fulfils admirably in Britain. Unfortunately, the imported gorse plants found the New Zealand climate much more to their liking than that of their native Britain, and are currently one of New Zealand's most widespread and visible weed species. Yours truly has many unwelcome memories of hot summer days spent grubbing up gorse plants.

REFERENCES

Ainouche, A., R. J. Bayer, P. Cubas & M.-T. Misset. 2003. Phylogenetic relationships within tribe Genisteae (Papilionoideae) with special reference to genus Ulex. In Advances in Legume Systematics part 10, Higher Level Systematics (B. B. Klitgaard & A. Bruneau, eds.) pp. 239-252. Royal Botanic Gardens: Kew.

Käss, E., & M. Wink. 1997. Phylogenetic relationships in the Papilionoideae (family Leguminosae) based on nucleotide sequences of cpDNA (rbcL) and ncDNA (ITS 1 and 2). Molecular Phylogenetics and Evolution 8 (1): 65-88.

Nettle, Where Is Thy Sting?


Our highlight taxon for this week is the Urticaceae, the so-called nettle family. I say "so-called" because, as with many plant families, the supposed representative member is not necessarily that representative at all, but just happens to be the best-known temperate member of a mostly tropical family. Urticaceae includes about 2500 species in a bit under 80 genera found world-wide (the image above, from here, shows the North American species Laportea canadensis). Members encompass all growth habits from herbs to lianas to trees, though tree species are relatively few (depending on whether or not the tropical tree genus Cecropia is included in Urticaceae or in a family of its own). They are wind-pollinated, and like other wind-pollinated plants the flowers are quite small and reduced. The stinging hairs for which the nettles are so well-known are actually restricted to a single tribe of Urticaceae, the Urticeae (also known as Urereae - Hadiah et al., 2003), most members of which belong to one of the two large genera Urtica and Urera.

Relationship-wise, the Angiosperm Phylogeny Group classification (Angiosperm Phylogeny Group, 2003) includes Urticaceae within the Rosales. However, phylogenetic analyses have established the monophyly of a smaller sub-group of the Rosales including Urticaceae, Moraceae (the mulberry and fig family), Ulmaceae (the elm family) and Cannabaceae (cannabis and hops) that corresponds to the Urticales of previous classifications (Hadiah et al., 2003). Relationships within the Urticaceae are a little more unsettled, a situation not helped by the fact that the family seems to have received relatively little monographic treatment since the work of Weddell in the mid-1800s. Monro (2006), in a molecular analysis centred on the largest genus of Urticaceae, Pilea, suggested that the five tribes originally established by Weddell fell into two clades, one containing the tribes Boehmerieae, Parietarieae and Forskohleae, and the other containing the Lecantheae and Urticeae, though the genus Myriocarpa (previously in Boehmerieae) fell into the Lecantheae-Urticeae clade. Monro (2006) also supported the inclusion of the genus Poikilospermum with the Urticaceae, specifically within the Urticeae (though Poikilospermum lacks stinging hairs). Other authors have included Poikilospermum in the Cecropiaceae, but Monro (2006) placed Cecropia distant from Poikilospermum in a trichotomy with the two Urticaceae clades (so unresolved as to whether or not it should be included within Urticaceae).



Economically, the Urticaceae are not a significant family. Species of Boehmeria, particularly B. nivea (shown above in an image from here), have been used for many years as a source of the fibre known as ramie. Ramie cloth was one of the textiles used for wrapping mummies in the Egyptian pre-dynastic period, but before mechanised methods became available extraction of fibres required a laborious process of repeated soaking and scraping. Industrial-scale production of ramie did not become practical until about the mid-1900s (Cook, 1984).

Some species of nettles of the genus Urtica are collected as edible herbs - the venom from the stinging hairs is destroyed by heat, so cooked nettles are quite harmless. An Indian species, Urtica tuberosa, also produces edible tubers. According Botanical.com, nettles can also be used to produce a fibre in a similar manner to ramie - however, the inferiority of nettle fibre to linen resulted in the decline in its use (though it did have something of a renaissance during the height of shortages in the Second World War). For the most part, nettle stings are more of an irritant than a significant medical issue, though some species are significantly more toxic than others. The New Zealand Urtica ferox (ongaonga or tree nettle, shown below in an image from Trek Nature) is a shrub of up to five metres in height that produces a strong enough sting to sometimes be fatal. At least one traditional chant of the Ngati Kahungunu iwi indicates that the ongaonga, along with other spined plants, was originally planted by Kupe, the discoverer of New Zealand, to protect the new land (Cowan, 1930):



Nau mai, e Tama,
Ki te tai ao nei.
Kia whakangungua koe
Ki te kahikatoa.
Ki te tumatakuru.
Ki te tara-ongaonga;
Na tairo rawa
Nahau e Kupe
I waiho i te ao nei.


Thou'lt be a powerful shield against
The weapons of the world;
The sharp and deadly spears,
The pricking darts and stings
That fill the foeman's armoury;
Thou'lt conquer e'en the barriers
Which Kupe the explorer raised
To guard this new-found land.


I haven't been able to find anything to support the rather more dramatic version recounted in Wikipedia that claims he placed these obstacles to evade the men whose wives he had stolen, so (unfortunately) I suspect this version to be a little dubious.

REFERENCES

Angiosperm Phylogeny Group (APG). 2003. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II. Botanical Journal of the Linnean Society 141: 399-436.

Cook, J. G. 1984. Handbook of Textile Fibres. Woodhead Publishing.

Cowan, J. 1930. The Maori: Yesterday and To-day. Whitcombe and Tombs.

Hadiah, J. T., C. J. Quinn & B. J. Conn. 2003. Phylogeny of Elatostema (Urticaceae) using chloroplast DNA data. Telopea 10(1): 235-246.

Monro, A. K. 2006. The revision of species-rich genera: a phylogenetic framework for the strategic revision of Pilea (Urticaceae) based on cpDNA, nrDNA, and morphology. American Journal of Botany 93 (3): 426-441.

Taxon of the Week: Cotoneaster


This week's highlight taxon is going to be a fairly cursory affair, for reasons that I think should be obvious. Indeed, for the same equally obvious reasons, posting is going to be fairly intermittent here over the next few weeks. There's Christmas coming, and then Jack and I are off to Adelaide for a couple of weeks (if anyone has recommendations for things to do in Adelaide, feel free to pass them on). Anywho...

Cotoneaster is a genus of shrubs (occassionally small trees) native to the Palaearctic, but also present in other temperate areas as represented by weed species. As shown pretty well in the photo of a Cotoneaster frigidus specimen above (from Wikipedia), Cotoneaster is part of the Rosaceae and closely related to such genera as Crataegus (hawthorns) and Pyracantha (firethorns). Depending on how you look at it, Cotoneaster is a smallish genus or a rather large genus. The problem is that a number of polyploid lines of Cotoneaster are avid indulgers in apomixis, the asexual production of seed (plant parthenogenesis). The resulting shortage of genetic recombination means that there is almost no theoretical limit to how minutely the 'species' can be divided, and a whole host of 'microspecies' of perhaps dubious practicality potentially can be (and often have been) identified. The situation is even further complicated in many plants such as Cotoneaster that are not obligate apomicts - Cotoneaster flowers remain attractive to wasps, and a very low level of sexual reproduction may still occur. Most authors, though, continue to define their Cotoneaster 'species' fairly broadly, in a victory for pragmatism over precision.

Reference Review: Brooms of New Zealand


Heenan, P. B. 1996. A taxonomic revision of Carmichaelia (Fabaceae - Galegeae) in New Zealand (part II). New Zealand Journal of Botany 34: 157-177.

I have a suspicion that New Zealand does not have one of the most diverse floras by world standards overall. However, one can't help noticing that within the New Zealand flora, certain genera seem to make up disproportionate numbers of species. Genera such as Hebe (ermmm... Veronica?) and Coprosma seem to have undergone speciation explosions and can be found almost anywhere you'd care to go. With a little over twenty species, the New Zealand brooms of the genus Carmichaelia (C. flagelliformis shown above, from Wikipedia) are not quite in Hebe league, but still represent a quite respectable little radiation. Except for a single species on Lord Howe Island between New Zealand and Australia, the Carmichaelia species are restricted to New Zealand. In the past a number of smaller genera of New Zealand brooms were recognised in addition to Carmichaelia, but phylogenetic analysis indicates that these groups are nested within Carmichaelia (Heenan, 1998), so they have no all been synonymised with the larger genus. Unfortunately, a significant number of Carmichaelia species are endangered or vulnerable, as they seem to favour habitats that are prone to human disturbance.

Prior to Heenan's work in the mid-1990s, Carmichaelia taxonomy was a bit of a mess. More than fifty species had been named at one time or another, mostly from individual collections. Heenan seems to have conducted something of a slash-and-burn, reducing the number of species from over fifty to seventeen (Heenan, 1995, 1996). When studied at more of a population level, some of the Carmichaelia species turned out to be decidedly variable - the extreme being Carmichaelia australis, which swallowed up some twenty-five synonyms, showing noticeable variation in seed and seed-pod size, shape, colour and growth habit. In the latter case, Heenan demonstrated that this represented true intra-specific variation rather than lumping of different species under one name by examining a single population in Canterbury along a transect of 150 metres, and showing that variation in seed-pod size and shape in the one population was as great as in the population as a whole. One population of C. australis had previously been described as a separate species due to its spreading growth habit, rather than erect as in other populations - however, when plants from this population were grown in cultivation they adopted the erect habit of other C. australis, indicating that the habit found in the wild was due to environment.


Variation in Carmichaelia australis seed-pods from a single population in Port Hills, Canterbury (from Heenan, 1996).


The biogeographical implications of Heenan's work are rather interesting. While C. australis, for instance, is found almost throughout New Zealand, other species have exceedingly restricted ranges. I've spoken before (see here and here) on the phenomenon of restricted soil types resulting in equally restricted plant species, and Carmichaelia provides more examples. The prostrate C. appressa, for instance, is restricted to sandy soils and dunes on the Kaitorete Spit in Canterbury. Heenan suggests that C. appressa may represent a segregate from C. australis (from which it differs only in growth habit and the colour of its cladodes [photosynthetic stems]) that has adapted to the different soil, and phylogenetic analysis (Heenan, 1998) does place C. appressa as the sister to C. australis. Heenan also records two other populations from different localities in Canterbury that are very similar to C. appressa, but refrains from actually assigning them to that species, alluding to the possibility that these may prove to be independent segregates from C. australis that have convergently developed the characters of C. appressa in adapting to similar habitat.

Carmichaelia hollowayi (shown at left in a photo by John Barkla from New Zealand Plant Conservation Network), in contrast, is restricted to limestone soils, and is known from only three outcrops of the Otekaike limestone in northern Otago, with very low numbers at each site. Heenan (1996) recorded that the largest of the three populations numbered about 45 individuals, while the smallest contained only three. Seedlings or young plants were not found at any site, and appeared to be excluded by introduced pasture grasses and weeds.

Still, some aspects of Heenan (1996) just scream out for further research. I've already mentioned the C. appressa conundrum. Another species, C. odorata, exhibits what Heenan interprets as a clinal variation across its distribution from north to south, but an uninhabited band through central Nelson divides its distribution in half. Are the northern and southern populations taxonomically distinct? Heenan points out that while the range of variation in each population is distinct, some overlap occurs.

I've also already referred to changes in growth habit due to environment in C. australis. Carmichaelia petriei also shows variation in growth habit, with both erect and prostrate forms in the wild. Cuttings of the prostrate form taken into cultivation develop the erect habit, arguing against taxonomic distinction and suggesting the prostrate habit is environmentally induced. However, the same area in the wild may contain both prostrate and erect individuals, so what is going on? Has the prostrate form only evolved recently, and potentially-prostrate individuals have not yet displaced obligately-erect individuals from prostrate-favouring habitat?

Finally, I can't help feeling that a re-division may still occur of C. australis. While some features such as pod size and shape have been shown to vary within individual populations, others such as cladode form do still vary geographically. Heenan refers to research by Purdie (1984) that identified geographical variation in flavonoid chemistry in what would become C. australis (but then represented multiple species). This is interesting, as flavonoid chemistry has some taxonomic significance in other plants (e. g. Bayly, 2001). However, Purdie did not provide voucher specimens of the species he used, reducing the taxonomic usefulness of his results (the variation did not fully match up with species boundaries as recognised at the time). One more example of the extreme importance of providing voucher specimens in any ecological study!

REFERENCES

Bayly, M. J., P. J. Garnock-Jones, K. A. Mitchell, K. R. Markham & P. J. Brownsey. 2001. Description and flavonoid chemistry of Hebe calcicola (Scrophulariaceae), a new species from north-west Nelson, New Zealand. New Zealand Journal of Botany 39: 55-67.

Heenan, P. B. 1995. A taxonomic revision of Carmichaelia (Fabaceae – Galegeae) in New Zealand (part I). New Zealand Journal of Botany 33: 455-475.

Heenan, P. B. 1998. Phylogenetic analysis of the Carmichaelia complex, Clianthus, and Swainsona (Fabaceae), from Australia and New Zealand. New Zealand Journal of Botany 36: 21-40.

Purdie, A. W. 1983. Some flavonoid components of Carmichaelia (Papilionaceae) — a chemotaxonomic survey. New Zealand Journal of Botany 22: 7-14.

Taxon of This Week: Not All Violets are Violet


And in a double whammy for the day, I'll head straight into this week's highlight taxa, meaning the Dactylopodolidae (I can't help it - I love the name!) lose their seat faster than an Italian government. So let's welcome in the plant family Violaceae!

Violaceae are a medium-sized family (about 800 or more species), about half of which belong to the single genus Viola (violets, pansies and small string instruments) (shown in the image above from Wikipedia. Most Violaceae are herbs, but a few are woody shrubs, trees or lianes - for instance, the South American tree Leonia triandra reaches 25m in height (see here). In fact, I get the impression that, taken genus by genus, there are actually more woody genera of Violaceae than herbaceous ones, and it is only the high diversity of the mostly herbaceous Viola that skews the ratio. Phylogenetically Violaceae are members of the rosid order Malpighiales that I've had cause to mention before as containing the gigantic-flowered holoparasite Rafflesia.


Violaceae don't appear to include anything as remarkable as Rafflesia (at least as far as I know), but they are certainly not devoid of interest. Many species of Viola produce cleistogamous flowers, i. e. the flowers never open and fertilise themselves. Often (as in Viola pubescens, shown here in a picture from Wikipedia) both cleistogamous and open flowers are produced (Culley & Wolfe, 2001), thus achieving the best of both options - the greater genetic variability obtained through outcrossing, as opposed to the more guaranteed success in setting seed of cleistogamy.

Also worth a mention are the ten or so species of Viola endemic to Hawaii, which are unique among the genus in their combination of woody stems (present in a few other species) and flowers borne in inflorescences (as opposed to singly in all other Viola). [The picture at left from the Hawaiian Plants website of Gerald Carr shows Viola chamissoniana var. tracheliifolia.] These distinctive features have lead to the suggestion that the Hawaiian species are quite basal in the genus (possibly relicts) and that they are related to basal South American Viola species that also have woody stems. However, a molecular study by Ballard & Sytsma (2000) indicates that, far from being an ancient group, the Hawaiian violets represent a single quite recent colonisation, not from South America, but from the Arctic! The sister taxon of the Hawaiian violets is the herbaceous Viola langsdorffii, found in the American Arctic and Japan. As circumstantial support for this result, Ballard and Sytsma pointed out the large numbers of migratory birds passing Hawaii from their breeding grounds in the Arctic, and that at least two Hawaiian birds appear to have recent Arctic origins - the goose Branta sandvicensis (from B. canadensis and the duck Anas wyvilliana (from A. platyrhynchos).

REFERENCES

Ballard, H. E., Jr & K. J. Sytsma. 2000. Evolution and biogeography of the woody Hawaiian violets (Viola, Violaceae): Arctic origins, ancestry and bird dispersal. Evolution 54 (5): 1521-1532.

Culley, T. M., & A. D. Wolfe. 2001. Population genetic structure of the cleistogamous plant species Viola pubescens Aiton (Violaceae), as indicated by allozyme and ISSR molecular markers. Heredity 86 (5): 545-556.

The fall of Rafflesiales

I have to make a confession - I probably won't be covering a huge number of plants (in the sense of embryophytes) on this blog. This is, I admit, a pretty huge omission. After all, plants are a pretty significant chunk of phylospace, and pretty damn influential in shaping our environment. But compared to animals, plants are scary and complicated and I just don't know all that much about them. That said, I thought I should rectify my previous deficiencies by giving some plants the time of day. I'm not going to completely break with precedent, though - I'm looking at another group of parasites, the "Rafflesiales".

Until very recently, Rafflesiales was an order of holoparasites (i.e. they derive all their nutrition from their host, as opposed to hemiparasites such as mistletoes that still have leaves and produce some of their own nutrients) on roots of other flowering plants. Their main claim to fame lies in the genus Rafflesia, well-known as producers of the largest flowers in the world - Wikipedia gives the largest as over 100 cm in diametre and up to 10 kg in weight. I would be interested to know why they produce such ridiculously huge flowers. There just doesn't seem to be much point. When not flowering or fruiting, Rafflesiales are pretty much invisible, as they are otherwise entirely contained within the host.

I say "until very recently" because the Rafflesiales has, of recent years, fallen apart. This is not particularly surprising. Parasitic taxa of all varieties often show a great reduction in complexity, as organs related to nutrient gathering, production and processing lose their function - the host supplies all those things ready-made. Internal parasites are particularly notable in this regard. No need for a protective dermis of your own when you're safely contained within another organism's. No need for eyes, ears, nostrils - what are you going to see, hear, smell (that you would want to smell, at least)? Pretty much the only thing that the devoted endoparasite needs to do for itself is reproduce, and so many become little more than balls of gonad.*

*Seriously, no pun intended. I didn't even realise I'd written that at first.

Reduction in complexity often means loss of the characters that unite a parasitic clade with its non-parasitic sister group. And simplified characters come to resemble each other despite their different origins. Hence the previous unification of the families of 'Rafflesiales', and their sometimes-suggested connection with the Balanophorales, another group of reduced root-parasites (James Reveal seems to have phrased it best, though I'm not sure he meant quite the same thing as I do - "wherever go the Rafflesiales so go the Balanophorales").

The first molecular study that broke apart the Rafflesiales was Barkman et al. (2004). Barkman et al. looked at two genera of Rafflesiaceae (Rafflesia and Rhizanthes) as well as Mitrastema, another genus previously included in Rafflesiales. The two Rafflesiaceae clustered together, and appeared closely related to the Malpighiales, a large order of the flowering plant subclass Rosidae. In contrast, Mitrastema appeared as a member of the Ericales, in the subclass Asteridae. The remaining two families, Apodanthaceae and Cytinaceae, were added to the investigation by Nickrent et al. (2004). Cytinaceae was associated with Malvales (Rosidae). Apodanthaceae differed in its placement within Rosidae depending on which gene was examined, but it was never close to Rafflesiaceae (they may yet be related to Cytinaceae).

Nickrent et al. found that the peculiarities of 'rafflesialean' molecular evolution interfered with phylogenetic resolution. All the 'Rafflesiales' showed whacking great branch lengths, and when analysed under maximum parsimony, which is rather vulnerable to long-branch attraction (the tendency of long branches to randomly attach to each other), the Rafflesiales appeared as a near-monophlyetic clade. In one of the genes used by Nickrent et al., atp1, the Apodanthaceae clustered with Leguminosae, the host family of one of the Apodanthaceae. Because this result was in conflict with those from other genes, Nickrent et al. suggested that it represented lateral gene transfer from the host to the parasite. The occurrence of lateral (aka horizontal) gene transfer in eukaryotes is a subject of much debate, but it has been demonstrated to occur at least occasionally in flowering plants (Bergthorsson et al., 2003).

Most recently, Davis et al. (2007) investigated the specific position of Rafflesiaceae proper within Malpighiales, and found it to be actually nested within the Euphorbiaceae. This is a decidedly unexpected result, as Euphorbiaceae as a whole produce relatively minute flowers. Davis et al.'s result was unlikely to be simply long-branch attraction, because the other cluster on their tree to show accelerated evolutionary results was not Euphorbiaceae but the branch including Clusiaceae and Podostemaceae (the latter are aquatic plants that also show extreme reduction in complexity, to the point where they barely resemble flowering plants at all)*. I'm waiting in anticipation for Davis et al.'s results to be tested further.

*In recent years, there has been an increasingly distressing trend for articles to appear in high-impact journals with only the briefest of summaries of results in the actual printed journal, with the greater proportion of hard data, methods, etc. buried in online supplementary material (where it generally becomes inacessible after a couple of years). Davis et al. (2007) is actually one of the most extreme examples of this, with the printed article only a single page, with the supplementary info 424 pages long (fairness does compel me to admit that 415 pages of that are sequence alignments).

REFERENCES

Barkman, T. J., S.-H. Lim, K. Mat Salleh & J. Nais. 2004. Mitochondrial DNA sequences reveal the photosynthetic relatives of Rafflesia, the world's largest flower. Proceedings of the National Academy of Sciences of the USA 101 (3): 787-792.

Bergthorsson, U., K. L. Adams, B. Thomason & J. D. Palmer. 2003. Widespread horizontal transfer of mitochondrial genes in flowering plants. Nature 424: 197-201.

Davis, C. C., M. Latvis, D. L. Nickrent, K. J. Wurdack & D. A. Baum. 2007. Floral gigantism in Rafflesiaceae. Science 315: 1812.

Nickrent, D. L., A. Blarer, Y.-L. Qiu, R. Vidal-Russell & F. E. Anderson. 2004. Phylogenetic inference in Rafflesiales: the influence of rate heterogeneity and horizontal gene transfer.
BMC Evolutionary Biology 4: 40. (online here).