Molecular Phylogeny of Xenopus frogs (Pipidae):
Speciation by auto- & alloploidy

    Analysis of mitochondrial DNA clarifies karyotypic evolution within the genus Xenopus (Pipidae). Xenopus comprises two clades. The first (the Silurana group) comprises X. tropicalis, which has retained the ancestral diploid number for pipid frogs (2n=20, also seen in the related genus Hymenochirus), and X. epitropicalis, which has undergone tetraploidization (2n=40). The second clade (the Xenopus group) comprises the remaining species of the genus, and has undergone an independent polyploidization, such that all species share a modified tetraploid (2n=36) karyotype or euploid multiples thereof. Within this clade, tetraploid species X. muelleri and X. borealis are sister species, the octaploid (2n=72) X. vestitus is most closely related to X. clivii, and the dodecaploid (2n=108) X. ruwenzoriensis is most closely related to X. fraseri.

    The allo- or autopolyploid origins of various species are examined. The strong morphological resemblance of epitropicalis and ruwenzoriensis to their respective sister species indicates that both the maternally-inherited mtDNA and the dominant nuclear alleles influencing morphology are derived from the same species, possibly by autopolyploidy. On the other hand, the close mtDNA affinities of vestitus to clivii, which is otherwise morphologically more similar to borealis or muelleri, suggests an allopolyploid origin.

    [The top figure shows a neighbor-joining analysis of 1429bp from the mitochondrial Cytochrome b, Cytochrome Oxidase subunit I, & 12S rDNA genes; numbers associated with each branch point indicate the occurrence of each clade in 1000 bootstrap replications. Similar results are obtained with parsimony & likelihood analyses.

    The bottom figure shows pelvic amplexus mating. The smaller male frog approaches the larger female from behind and above, and grasps her around her pelvis. This places their genital openings in close proximity, and allows him to fertilize her eggs as they are extruded. Reproductive behaviour is inducible in the lab by injecting males and females with hormones, which stimulates oogenesis in females and spermatogenesis in males. Under lab conditions, male frogs are not particularly choosy: the third frog in the chain has grasped the second, a male].