The Herpetological Journal

pp.13-17

Authors: H. R. Kobel, B. Barandun And Ch. H. Thlébaud

Abstract: Based on morphological, biochemical and karyological characters, the genus Xenopus can be divided into two main groups (subgenera), Silurana and Xenopus, and the latter into five subgroups. The relationships among these five subgroups are less clear. Since all except one species are allopolyploid (tetra-, octo- and dodecaploid), they are, by definition, not monophyletic. In principle, sequence data would permit unravelling of these complex relationships, provided that all duplicated genes were conserved. However, that is not the case: redundant genetic information tends to become lost, interrupting phylogenetic lines of descent of the genes. Since the mitochondrial genome is inherited in a purely matrilinear manner, problems linked to polyploidy are seemingly avoided. However, this character is not monophyletic either. At least at the start of an allopolyploid speciation, mitochondria of both parental species can be present though one or the other type eventually becomes extinct. Which one is conserved is probably random. Nevertheless, it may be interesting to compare the phylogeny of mitochondria to species trees based on nuclear characters. We sequenced about 600 bp of mitochondrial 12s and 16s rRNA genes of the diploid X. tropical is, of most tetraploid species, and of the octoploid X. wittei. Trees obtained with Neighbor Joining, Maximum Likelihood and Maximum Parsimony methods essentially confirm the tropicalis, laevis and muelleri groups and subgroups, whereas thefraseri subgroup is less well defined. Mitochondria of X. clivii and X. largeni, members of the muelleri and the laevis subgroup respectively, show only a low bootstrap score when connected to any subgroup, thus forming a polytomy of several speci es. Divergence of the same sequence between Rana catesbeiana and R. temporaria, for which immunological and zoogeographic considerations suggest a possible age of roughly 30-40 Ma, was used for tentative calibration of the Xenopus mitochondrial tree. This calibration is necessary for comparison with other phylogenetic data on this genus.