mitochondrial DNA clarifies
karyotypic evolution within the genus Xenopus
comprises two clades. The first (the Silurana group)
tropicalis, which has retained the ancestral diploid
number for pipid
also seen in the related genus Hymenochirus), and X.
epitropicalis, which has
undergone polyploidization (2n=40).
The second clade (the Xenopus group) comprises the
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
and X. borealis are sister species, the octaploid
X. vestitus is most closely related to X. clivii,
X. ruwenzoriensis is most closely related to X.
The allo- or autopolyploid origins of various species were examined by Carr et al. (1986). 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 morphologically more similar to borealis or muelleri, suggests an allopolyploid origin. Finally, ruwenzoriensis is a cryptic species that is almost indistinguishable from fraseri, which suggests an origin by allo-triploidy, possibly with laevis.
[The top figure shows a neighbor-joining
analysis of 1429bp from the mitochondrial Cytochrome b,
subunit I, & 12S rDNA genes; numbers associated with each
indicate the occurrence of each clade in 1000 bootstrap
results are obtained with parsimony & likelihood
bottom figure shows mating by pelvic amplexus. The
smaller male frog
approaches the larger female from behind and above, and grasps
in front of her pelvis. This places their genital openings in
proximity, and allows him to fertilize her eggs as they are
Reproductive behavior is inducible in the lab by injecting
males and females with
gonadotropic hormones, which stimulates oogenesis in females
and spermatogenesis in
males. Under lab conditions, male frogs are not choosy:
the third frog in the chain has grasped the second, a male].