Molecular systematics of African 20-chromosome toads (Anura: Bufonidae)

Intrageneric lineages and the historical biogeography of toads, genus Bufo, are poorly resolved due to their conservative morphology, their highly conserved karyotypes (typically 2N = 22), and erratic patterns of interspecific hybridisation. Here, we use mitochondrial and nuclear DNA sequence data to reconstruct relationships of the 20-chromosome toads, a major component of the African bufonid fauna. Mitochondrial 12S and 16S sequences from 29 species revealed two independent transitions between phenotypically distinct savannah and forest adapted forms. Analyses of mitochondrial 12S, 16S, and ND2, along with nuclear ACTC and Rhodopsin sequences from 12 species greatly increased bootstrap, and likelihood support for internal branches including a basal split into two pan-African 20-chromosome clades. Hybridisation is a weak indicator of phylogenetic relationship as it occurs across these deeply divergent clades, between Bufo rangeri and Bufo gutturalis. These analyses suggest a secondary reversion to 22-chromosomes in Bufo pardalis, within the 20-chromosome group, although we could not reject an alternative hypothesis that this lineage forms a sister to all 2N = 20 toads. Other informally recognised 22-chromosome groups form independent phylogenetic lineages outside the 20-chromosome group, such as the Angusticeps and Vertebralis divisions. Bufo lindneri, from the Taitanus division, is closely related to Stephopaedes anotis, and these species should be considered congeners.