The Dermacentor Ticks

Pacific Coast tick Dermacentor occidentalis, copyright Jerry Kirkhart.

Among the ticks of most concern to humans are species of the genus Dermacentor. This genus of about forty known species is widely distributed in Africa, Eurasia and the Americas. Examples include the meadow tick D. reticulatus in Europe, and the wood tick D. variabilis and Rocky Mountain wood tick D. andersoni in North America. They are parasites of mammals, including both generalist and more host-specific species; records of Dermacentor individuals from reptiles and even carpenter bees (Goddard & Bircham 2010) presumably represent incidental and/or accidental associations. Species of Dermacentor are responsible for the spread of bacteria causing diseases such as Rocky Mountain spotted fever (which, despite sounding like a 1950s dance craze, is presumably not much fun), Q fever and tularemia. The ticks can also be more directly hazardous, as their bites inject a toxin that can cause tick paralysis.

Distinguishing features of Dermacentor species relative to other ticks include a rectangular base to the capitulum, relatively short, broad palps, well-developed eyes and the presence of festoons (impressed divisions of the posterior margin of the body) (Keirans 2009). Most are ornate—that is, marked on the dorsum with contrasting pale patterns—with the notable exception of the tropical horse tick D. nitens of the Americas (until recently, often treated as forming its own genus Anocentor). The function of such markings is unknown though suggestions include environmental protection, warning predators of distastefulness, or sexual signalling.

Meadow tick Dermacentor reticulatus, copyright Ferran Turmo Gort.

The majority of Dermacentor species have a three-host life cycle, dropping off the host between each of the life stages of larva, nymph and adult, and seeking out a new host after moulting. However, at least two New World species, the aforementioned D. nitens and the winter tick D. albipictus (a parasite of deer), are one-host ticks that remain on their original host between instars. In general, Dermacentor species are more resilient to dry climates than many other tick species. Individual species can differ in their climate tolerance, however. In North America, the geographical divide between D. variabilis in the east of the continent and D. andersoni in the west seems to be driven by the need for the latter of drier conditions (Yoder et al. 2007). Older instars also tend to be hardier than younger. Females of the ornate sheep tick D. marginatus, a European species, leave their host after gorging at the beginning of winter and then wait for more amoenable spring conditions before laying their delicate eggs (Dörr & Gothe 2001).

Higher relationships within the genus do not appear to have been extensively studied. A preliminary molecular phylogeny of hard ticks has suggested the possibility of a basal division between Afrotropical, Eurasian and New World lineages (Barker & Murrell 2004). Comparison with related tick genera raises the possibility of an Afrotropical origin for Dermacentor, though the genus has only a relictual presence in that continent now. However, with only a handful of species subjected to broad phylogenetic analysis to date, further testing is demanded. Does the continental divide hold true? Do the one-host species form a single clade within the genus? Inquiring minds wish to know.

REFERENCES

Barker, S. C., & A. Murrell. 2004. Systematics and evolution of ticks with a list of valid genus and species names. Parasitology 129: S15–S36.

Dörr, B., & R. Gothe. 2001. Cold-hardiness of Dermacentor marginatus (Acari: Ixodidae). Experimental and Applied Acarology 25: 151–169.

Goddard, J., & L. Bircham. 2010. Parasitism of the carpenter bee, Xylocopa virginica (L.) (Hymenoptera: Apidae), by larval Dermacentor variabilis (Say) (Acari: Ixodidae). Systematic and Applied Acarology 15: 195–196.

Keirans, J. E. 2009. Order Ixodida. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology 3^rd ed. pp. 111–123. Texas Tech University Press.

Yoder, J. A., D. R. Buchan, N. F. Ferrari & J. L. Tank. 2007. Dehydration tolerance of the Rocky Mountain wood tick, Dermacentor andersoni Stiles (Acari: Ixodidae), matches preference for a dry environment. International Journal of Acarology 33 (2): 173–180.

Shadow of the Palaeoniscoids

Palaeoniscum freieslebeni, copyright James St. John.

Depending how you cut it, the ray-finned fishes (Actinopterygii) are arguably the most diverse group of vertebrates in the modern fauna. They are the dominant vertebrates in all aquatic environments, they encompass an enormous array of species, and they have evolved a bewildering assemblage of morphologies. But despite their current pre-eminence, the early evolution of actinopterygians remains rather understudied. The earliest actinopterygians appear in the fossil record in the Late Silurian/Early Devonian but, until fairly recently, the majority of Palaeozoic ray-finned fishes have often been lumped into a catch-all holding tank, the 'Palaeonisciformes'. This was a vague assemblage of fishes united by plesiomorphic features such as ganoid scales (heavy, bony scales with an outer layer of enamel, also found in modern gars and sturgeons), a single dorsal fin and a heterocercal tail (with the upper arm of the tail fin longer than the lower). The key genus of the group, the Permian Palaeoniscum, had a fusiform (or torpedo-shaped) body; at first glance, it would not have looked dissimilar to a modern herring. However, it lacked the mobile jaw structure of modern teleost fishes, with the maxilla and preopercular bones being fixed together. As such, it would have lacked the modern fish's capacity for suction feeding (Lauder 1980). Prey capture by Palaeoniscum would have been a simple smash-and-grab affair. Palaeoniscoid fishes remained a component of both marine and freshwater faunas until the end of the Cretaceous before being entirely supplanted by modern teleost radiations such as the ostariophysans and percomorphs.

Reconstruction of Acrolepis gigas, copyright DiBgd.

The core concept of 'Palaeonisciformes' has united fishes with a fusiform body shape like Palaeoniscum; depending on the author, more divergent contemporary fishes such as the deep-body platysomoids might be combined in the same order or treated separately. By modern standards, former 'Palaeonisciformes' probably combine stem-actinopterygians, stem-chondrosteans, stem-holosteans and possibly even stem-teleosts. As such, the term Palaeonisciformes has tended to fall out of favour, though the less formal 'palaeoniscoid' remains a useful descriptor. Nevertheless, the exact phylogenetic position of many palaeoniscoid taxa remains unestablished. Part of this is due to a lack of observable detail: though those heavy ganoid scales preserve well, they effectively cover up internal skeletal features. Many palaeoniscoids are preserved as compression fossils, effectively not much more than intriguing silhouettes. However, part of the problem is simple neglect. Palaeoniscoids are not rare fossils; in some formations, they may be the dominant part of the fauna by a large margin. They certainly deserve a closer look.

REFERENCE

Lauder, G. V., Jr. 1980. Evolution of the feeding mechanism in primitive actinopterygian fishes: a functional anatomical analysis of Polypterus, Lepisosteus, and Amia. Journal of Morphology 163: 283–317.

Herbs of Dragons and Worms

Preparing for this post has inspired me to some low-key experimentation. When it came time to assign myself its topic, I landed on the plant genus Artemisia. This is the genus that, among others, includes the culinary herb tarragon, Artemisia dracunculus. Which got me thinking that I wasn't sure if I'd ever actually eaten tarragon. I asked Christopher if he was familiar with it; he responded that all he knew about tarragon was that you had to consume it in the 1970s. Without access to a functioning Delorean, I did the next best thing and prepared a dish of tarragon chicken myself. The verdict: very tasty, though I could appreciate why tarragon might have a reputation for being somewhat difficult as it had a light flavour that I could imagine being easily overwhelmed.

Tarragon Artemisia dracunculus, copyright Cillas.

Tarragon is not the only species of Artemisia of significance to humans. This genus of composite-flowered plants comprsises over five hundred species and subspecies of herbs and small shrubs. The greatest diversity is found in arid and semi-arid regions of the Northern Hemisphere temperate zone (Sanz et al. 2008). The genus is characterised by its distinctive pollen with surface spinules reduced or absent. This pollen type is associated with the wind pollination typical of the genus, though some species do exhibit features such as sticky pollen and colourful flower-heads associated with insect visitation (Hayat et al. 2009). The flower-heads or capitula (a reminder that the 'flowers' of composite plants such as daisies and thistles actually represent a fusion of multiple flowers) of Artemisia are either disciform, with an outer circle of reduced ray florets surrounding the inner disc florets, or discoid, with disc florets only. In disciform capitula, the outer limb of the ray florets is reduced to a membranous vestige, not readily visible without minute examination. The ray florets are female whereas the disc florets are ancestrally hermaphroditic (more on that shortly). In discoid capitula, where the ray florets have been lost, all florets are uniformly hermaphroditic.

Mugwort Artemisia vulgaris, copyright Christian Fischer.

Historically, there has been some variation in the classification of Artemisia but a popular system divides the genus between five subgenera. A phylogenetic analysis of Artemisia and related genera by Sanz et al. (2008) found that the genus as currently recognised is not monophyletic, with a handful of small related genera being embedded within the clade. Time will tell whether this inconsistency is resolved by subdividing Artemisia or simply rolling in these smaller segregates, but for the purposes of this post they can be simply set aside. The subgenus Dracunculus, including tarragon and related species, falls in the sister clade to all other Artemisia. As well as being united by molecular data, members of this clade are distinguished by disciform capitula in which the central disc florets have become functionally male (female organs have been rendered sterile).

Wormwoood Artemisia absinthium, copyright AfroBrazilian.

The second clade encompasses the subgenera Artemisia and Absinthium, with disciform capitula, and Seriphidium and Tridentatae, with discoid capitula. Not all authors have supported the distinction of Artemisia and Absinthium, and Sanz et al. identify both as non-monophyletic, both to each other and to the discoid subgenera. Because of their similar flower-heads, most authors have presumed a close relationship between the Eurasian Seriphidium and the North American Tridentatae (commonly known as sagebrushes). Some have even suggested the former to be ancestral to the latter. However, Sanz et al.'s results questioned such a relationship, instead placing the Tridentatae species in a clade that encompassed all the North American representatives of the Artemisia group.

As well as the aforementioned tarragon, economically significant representatives of Artemisia include wormwood A. absinthium, best known these days as the flavouring agent of absinthe (though historically it has also been used for more innocuous concoctions). Mugworts (A. vulgaris and related species) have also been used for culinary and medicinal purposes. Sagebrushes are a dominant component of the vegetation in much of the Great Basin region of North America, providing crucial habitat for much of the region's wildlife. Artemisia species have shaped the lives of many of their co-habitants, both animal and human.

REFERENCES

Hayat, M. Q., M. Ashraf, M. A. Khan, T. Mahmood, M. Ahmad & S. Jabeen. 2009. Phylogeny of Artemisia L.: recent developments. African Journal of Biotechnology 8 (11): 2423–2428.

Sanz, M., R. Vilatersana, O. Hidalgo, N. Garcia-Jacas, A. Susanna, G. M. Schneeweiss & J. Vallès. 2008. Molecular phylogeny and evolution of floral characters of Artemisia and allies (Anthemideae, Asteraceae): evidence from nrDNA ETS and ITS sequences. Taxon 57 (1): 66–78.