Studies on interactions among Xenopus taxa using comparative osteology and other methods : an evolutionary perspective
The past thirty years have seen a tripling in the number of Xenopus species recognized. The
complexp atterno f speciesd istributiond emonstrateadc rossth es ub-Sahararna nger eflectse cological
constraintsi mposedb y climate conditionst hroughoutt he Pleistocenea, nd highlights a numbero f
areas of specific biogeographic interest.
Although previous application of comparative osteology as a method for phylogenetic
investigation at the species level in Anura has been limited, the approach, adopted in this study, has
proven to be highly informative in its application to specific taxonomic studies.
An overview of osteologicalc haractersin the type specieso f the genusX enopusi s presented
for the first time and forms the basis for subsequent osteology-based species descriptions. Results
implicate two tetraploids, X 1. victorianus and X fraseri in the hybrid origin of an octoploid species
pair, X. wittei and X. vestitus in the Central African highlands. Existing ambiguity surrounding the
taxonomic distinction between two cryptic taxa, also living in these highlands, XL victorianus and X
1. bunyoniensis is considered. Osteologically, the two are distinct.
Corroboration for the sub-division of X. muelleri into eastern and western forms is provided
by studies on comparative osteology and mating-call, amongst others. The taxonomic significance of
thesef indings is overshadowedh owever,b y evidenceo f widespreadh ybridization and introgression
with distantly relatedX . 1.l aevis in the southeasternp art of the X. muelleri range.
Although only two species are currently known from Ethiopia, evidence of morphology
indicates that museum collections of X. clivii comprise at least two distinct forms, one appearing to be
intermediate between the two Xenopus sub-genera. Furthermore, osteology reveals that X largeni
also straddles this taxonomic boundary, casting doubt on the robustness of this phylogenetic division.
Results are consistent with recent findings from molecular-based phylogenies, qualifying the use of
comparative osteology as an informative method for investigation of evolutionary interactions