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
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
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.
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.
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].
Frog chain © UC Irvine; phylogeny & text material © 2007 by Steven M. Carr