An Integrative Approach to Species Delimitation in the Massasauga Rattlesnake (Sistrurus Catenatus) with an Emphasis on the Western Massasauga, S.C. Tergeminus, and Desert Massasauga, S.C. Edwardsii in Texas
The subspecies concept was originally introduced as a means to explain geographic variation in species with subspecific boundaries normally being designated by morphological variation. Because a growing wealth of studies have shown that these morphologically defined subspecies are often not reflective of true evolutionary history, it is important to reassess subspecific boundaries. Subspecific designations have conservational consequence with regards to management practices. I reassessed the subspecific designations of the massasauga rattlesnake, S. catenatus, using both ecological niche modeling and molecular phylogenetic techniques. The ecological niche modeling determined the western and desert massasauga, S. c. tergeminus and S. c. edwardsii occupy completely distinct niches. This is evidence that these two subspecies represent evolutionary divergent lineages. There is no obvious isolating geographical boundary, but other studies have shown that strong local adaptation to environmental gradients can cause ecological divergence in parapatric populations in ectotherms. My genetic data provide differential results dependent upon type of DNA, mitochondrial vs nuclearThe mitchondial DNA sequences showed an eastern clade consisting entire of the eastern massasauga, S. c. catenatus, and western clade consisting of both the western and desert massasauga, S. c. tergeminus and S. c. edwardsii. Mitochondrial DNA also show strong evidence that the eastern massasauga should be elevated to its own species, which is consistent with previous studies (Kubatko et al. 2011; Ryberg et al. 2014). Within the western clade using mtDNA there is only slight differentiation between S. c. tergeminus and S. c. edwardsii. The nuclear DNA showed only very little differentiation between all three subspecies. I feel this is an artifact of recent divergence within S. catenatus and that the mtDNA, which has much higher mutation rates, is a better matric for assessing the phylogenetic relationship within this species. This study provides evidence that S. c. catenatus should be elevated to the sole member of the species of S. catenatus. The other two subspecies, S. c. tergeminus and S. c. edwardsii, reflect divergent evolutionary lineages however should be separated into their own species, S. tergeminus, and renamed S. t. tergeminus and S. t. edwardsii respectively. Keeping the western and desert massasaugas as separate subspecies has conservational impacts as they need to be treated as biological separate management units.
Date of publication
Hein, Steven R., "An Integrative Approach to Species Delimitation in the Massasauga Rattlesnake (Sistrurus Catenatus) with an Emphasis on the Western Massasauga, S.C. Tergeminus, and Desert Massasauga, S.C. Edwardsii in Texas" (2015). Biology Theses. Paper 29.