Xenopus phylogeny revised: six new species

Genetics, Morphology, Advertisement Calls, and Historical Records Distinguish Six New Polyploid Species of African Clawed Frog (Xenopus, Pipidae) from West and Central Africa.

Evans BJ, Carter TF, Greenbaum E, Gvozdík V, Kelley DB, McLaughlin PJ, Pauwels OS, Portik DM, Stanley EL, Tinsley RC, Tobias ML, Blackburn DC.

PLoS One. 2015 Dec 16;10(12):e0142823. doi: 10.1371/journal.pone.0142823. eCollection 2015.

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     Highlights and Summary contributed to Xenbase by Ben Evans.

Highlights

•    Half a dozen new species of the African clawed frog are described, and one more is added back to the list of known species. These discoveries increase the number of known clawed frog species from 22 to 29—a 30% increase.

•    These new species were distinguished from others using multiple sources of information, including DNA, recordings of vocalizations, CT scanning of internal anatomy, and chromosome analysis.

•    Like most African clawed frogs, all of these new species are polyploid, meaning their DNA doubled during evolution, giving them a redundant copy of every gene.  Five of these species are tetraploid because their genomes were duplicated once, and two are dodecaploid, because their genomes were duplicated a remarkable three times.

•    Several taxonomic (nomenclatural) updates are proposed including downgrading “Silurana” from a genus to a subgenus within the genus “Xenopus”, and substantially restricting the application of the names “X. fraseri” and “X. epitropicalis”; most previous studies that use these two species names actually refer to the new species “X. parafraseri” and “X. mellotropicalis”, respectively.

Summary

Because African clawed frogs are so widely used as a model organism for biological research, one might think we had already pinned down the number of species in this group, and other aspects of their diversity such as where they live and how they are related to one another. This study by a team of researchers from Canada, the USA, the UK, Czech Republic, and Belgium suggests otherwise.

A total of six new species are described and a seventh is resurrected from synonymy with X. tropicalis based on hundreds of individuals collected in their native habitats in West Africa, and using information from DNA, vocalization, karyotypes, and CT scans of internal anatomy.

From the perspective of experimental biology, this main utility of this study is to help standardize the names of the species with which we work. This includes downgrading “Silurana” to a subgenus of “Xenopus”, and clarifying the small distributions of the species names “X. fraseri” and “X. epitropicalis”. Most previous references to these species actually referring to the new species X. parafraseri and X. mellotropicalis, respectively. The identification and resurrection of X. calcaratus from synonymy with X. tropicalis probably does not affect most X. tropicalis colonies because X. calcaratus has a small range (Bioko Island and a small portion of the northwest coast of Cameroon) and is therefore unlikely to have been admixed with laboratory colonies of X. tropicalis.

In describing species diversity of African clawed frogs, this article makes several other contributions including: (1) estimating evolutionary relationships among known species (Fig. 3), (2) describing and illustrating internal and external morphological differences between subgenus Silurana and subgenus Xenopus, including differences in cranio-facial anatomy (Figs 4, 5, 11, 12), and (3) highlighting aspects of the unique evolutionary history of these frogs such as (i) the existence of as yet unidentified tetraploid ancestral species (‘lost ancestors’) whose genomes comprise a portion of extant dodecaploid species, (iii) variation in male vocalizations (Fig. 8), and (ii) high species diversity in West Africa (Fig. 10).

This is an open access article distributed under the terms of the Creative Commons Attribution License.