How Many Species of Rattlesnakes Are There in the Crotalus durissus Species Group (Serpents: Crotalidae)?

Abstract

Members of the Crotalus durissus species group are amongst the largest species of rattlesnakes and are of strong medical importance. The taxonomy of the group is convoluted, and the line of what is considered a species, subspecies or populations is hard to define. A recent study split one of the members of the group (C. culminatus) into three species, the nominal species, as well as C. ehecatl and C. mictlantecuhtli, based on genetic and morphological data. Here we reanalyze previously published mitochondrial and nuclear data, as well as additional sequences for two mitochondrial genes from members of the Crotalus durissus group, to test if isolation by distance (and not speciation) could be responsible for the genetic variation observed in previous studies. Our results show that the genetic differences between some of the species in the group (Crotalus simus – C. durissus and C. culminatus – C. ehecatl) are lower than intra-specific genetic distances. The only nuclear gene at hand for the C. durissus group (oocyte maturation factor Mos, c-mos) is not phylogenetically informative and cannot distinguish between many of the species in the genus, thus it is of little support when it comes to taxonomic decisions. We find low divergence at the mitochondrial level between these two species pairs, and at least in one of the species pairs we find a pattern of isolation-by-distance (IBD). Additionally, we performed a newly developed species delimitation analysis, DELINEATE, which supports the validity of C. mictlantecuhtli, but not C. ehecatl. These results suggest that the molecular differences at the mitochondrial level in members of the Crotalus durissus species group could be the result of IBD and not necessarily due to speciation events. We believe that additional sampling as well as additional molecular data is necessary to clarify species limits in the Crotalus durissus species group.