Deeply Diverged but Morphologically Conserved Lineages in Tornier's Cat Snake (Crotaphopeltis tornieri) of the Eastern Arc Mountains

Abstract

The Eastern Arc Mountain (EAM) forests in Tanzania have remarkably high endemism. Closely-related forest-adapted species are found isolated on different “sky islands” testifying to allopatry as a major driver for speciation in this region. However, some species defy this pattern. Tornier's cat snake (Crotaphopeltis tornieri) occupies most of the isolated mountain rainforest, despite presumably not being able to move across the arid savannah landscape that separates them. To test contrasting hypotheses of recent dispersal vs morphological conservatism we examined scale characters of 218 C. tornieri individuals and sequenced 80 full mitochondrial genomes covering populations from eight mountain blocks across the EAM and Southern Highlands of Tanzania (SHT). The morphological examination revealed no differentiation between populations except the Usambara Mountain populations showing significant differences in some scale characters. This was in stark contrast to the genetic analyses showing very high divergence between mountain populations. On average the mitochondrial genome showed > 12% genetic differentiation with cytB and COI showing interpopulation distances of up to 28.5% and 15.1%, respectively. Both Bayesian coalescent and maximum-likelihood based phylogenies, uncovered a highly distinct clade structure in C. tornieri defined by the mountains. Divergence times were estimated at c. 21 million years for the split between the EAM and SHT populations and 5.4–1.4 millions years for population splits within EAM. Our results point towards old isolation events but with a highly conserved morphology resulting in just one recognized species. By including presumed outgroups of C. degeni and C. hotamboeia in the phylogeny we found C. tornieri to be paraphyletic. These results have implications for understanding evolution in the EAM and warrant a revision of the number of species in this genus.