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.
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.