Molecular and phenotypic diversity in Chionactis occipitalis (Western Shovel-nosed Snake), with emphasis on the status of C. o. klauberi (Tucson Shovel-nosed Snake).

Abstract

Chionactis occipitalis (Western Shovel-nosed Snake) is a small colubrid snake inhabiting the arid regions of the Mojave, Sonoran, and Colorado deserts. Morphological assessments of taxonomy currently recognize four subspecies. However, these taxonomic proposals were largely based on weak morphological differentiation and inadequate geographic sampling. Our goal was to explore evolutionary relationships and boundaries among subspecies of C. occipitalis, with particular focus on individuals within the known range of C. o. klauberi (Tucson Shovelnosed snake). Population sizes and range for C. o. klauberi have declined over the last 25 years due to habitat alteration and loss prompting a petition to list this subspecies as endangered. We examined the phylogeography, population structure, and subspecific taxonomy of C. occipitalis across its geographic range with genetic analysis of 1100 bases of mitochondrial DNA sequence and reanalysis of 14 morphological characters from 1543 museum specimens. We estimated the species gene phylogeny from 81 snakes using Bayesian inference and explored possible factors influencing genetic variation using landscape genetic analyses. Phylogenetic and population genetic analyses reveal genetic isolation and independent evolutionary trajectories for two primary clades. Our data indicate that diversification between these clades has developed as a result of both historical vicariance and environmental isolating mechanisms. Thus these two clades likely comprise 'evolutionary significant units' (ESUs). Neither molecular nor morphological data are concordant with the traditional C. occipitalis subspecies taxonomy. Mitochondrial sequences suggest specimens recognized as C. o. klauberi are embedded in a larger geographic clade whose range has expanded from western Arizona populations, and these data are concordant with clinal longitudinal variation in morphology.

Comparative phylogeography reveals deep lineages and regional evolutionary hotspots in the Mojave and Sonoran Deserts

“…In addition to embayments, steep environmental gradients existing along the Sonoran/Mojave ecotone may maintain genetic structure. Previous analyses of Chionactis occipitalis found that the strongest environmental predictors of genetic divergence were elevation, temperature and desert assignment (Wood et al ., ). The Mojave and Sonoran ecoregions are defined by a combination of temperature, precipitation, vegetation and terrain features (Bailey, ; McNab et al .…”

“…), and the Mojave Desert is generally higher in elevation than the Sonoran. Consequently, regional Mojave and Sonoran Desert lineages may not only be divergent at the molecular level in neutral loci but may also be locally adapted to specific Mojave and Sonoran environmental conditions (Wood et al ., ; Wilson et al ., ).…”

“…Similarly, evidence of demographic expansion within lineages should coincide with the end of the LGM, as desert habitats expanded to maximum distributions. Work describing diversification patterns in the desert southwest may support this prediction by revealing the presence of moderately divergent mtDNA lineages between Mojave and Sonoran Deserts (Ayoub & Riechert, ; Douglas et al ., ; Wood et al ., ; Jezkova et al ., ), but whether the timing of these diversification events is consistent with Pleistocene glacial cycles is unclear. Furthermore, corresponding evidence of range expansion in lineages coinciding with the LGM remains to be tested.…”

Fuzzy Boundaries: Color and Gene Flow Patterns among Parapatric Lineages of the Western Shovel-Nosed Snake and Taxonomic Implication

“…We also included the 80 mtDNA sequences generated from our previous study ([37], Genbank numbers: EU280331 – EU280410). The total number of sequences used for mtDNA analyses was 223.…”

Evolutionary Hotspots in the Mojave Desert

“…The resulting dataset included 17 wildlife species. These included herpetofauna: Anaxyrus punctatus [37], Chionactis occipitalis (Appendix 1) [38], Crotaphytus bicinctores [39], Dipsosaurus dorsalis dorsalis (Appendix 1), Gopherus agassizii [40,41], Lichanura trivirgata [42], Plestiodon gilberti [43], Sceloporus magister [44], Uma scoparia (Appendix 1) [45], Xantusia vigilis [46]; mammals: Chaetodipus penicillatus [31], Ovis canadensis [47], Perognathus longimembris [48], Thomomys bottae [49], Xerospermophilus mohavensis [50,51]; and invertebrates: Homalonychus selenopoides [52], Homalonychus theologus [52] ( Table 1). Most of the herpetofauna, small mammals and invertebrates comprise distinct lineages endemic to the Mojave Desert [20,45,51,53].…”