Details
- Autor(en) / Beteiligte
- Titel
- Ecological genetics of the salt marsh snake Nerodia clarkii
- Ort / Verlag
- ProQuest Dissertations & Theses
- Erscheinungsjahr
- 2001
- Quelle
- ProQuest Dissertations & Theses A&I
- Beschreibungen/Notizen
- The salt marsh snake, Nerodia clarkii, is the only North American snake species restricted to brackish water habitats and has evolved unique physiological and behavioral specializations associated with a saline environment. Three subspecies exist and occur in narrow bands of coastal salt marsh from southern Texas to the middle Gulf coast of Florida (N. c. clarkii), mangrove swamp throughout southern Florida, the Florida Keys, and northern Cuba (N. c. compressicauda), and on the middle Atlantic coast of Florida (N. c. taeniata). Two of the three subspecies (N. c. compressicauda and N. c. taeniata) are threatened by progressive destruction of coastal wetland habitat. A total of 390 bp of the mitochondrial DNA (mtDNA) control region of 49 individuals from the N. clarkii subspecies, N. fasciata subspecies, and N. taxispilota (as an outgroup) were sequenced in order to elucidate the relationship among the salt marsh snakes and to better understand the evolutionary history of the group. In addition, population analyses of microsatellite variation at four loci were used to understand how genetic diversity is distributed within and among populations of the mangrove salt marsh snake, N. c. compressicauda . Differences were observed between the present study and previous studies of N. clarkii. First, conventional taxonomic units were not supported. Differences between saltwater N. clarkii and freshwater N. fasciata were not significant (AMOVA, P = 0.81). Lawson et al. (1991) reported differences among three allozyme loci that supported the existence of a salt marsh group of Nerodia (i.e. N. clarkii) whereas the data presented do not support this conclusion. No diagnostic allozyme alleles characterize salt marsh snakes and no significant partitioning of allozyme variation existed between N. clarkii and N. fasciata (AMOVA, P = 0.11). The distribution of mtDNA control region haplotypes also did not support the monophyly of a salt marsh group. In addition, these data did not support the genetic differentiation of a N. c. taeniata subspecies. Second, the presence of a single origin of saltwater tolerance is refuted by the data presented in this study. Low rates of dispersal as indicated by the microsatellite data along with the lack of phylogeographic structure produced by the mtDNA sequence analysis suggest very recent connections between freshwater and saltwater populations and rapid shifts in coloration within these saltwater snake populations. Significant population subdivision within both freshwater and saltwater populations was indicated with mtDNA, allozyme, and microsatellite data (P < 0.05) but differentiation between freshwater and saltwater groups was not significant. Much of the taxonomic controversy surrounding the salt marsh snakes of the southeastern United States during the last century has been focused on the level of ecotypic variation in color patterns and physiological adaptations to a saline environment. The high degree of color variability among these populations along with concerns over the use of coloration as a taxonomic marker leaves saltwater tolerance as the sole remaining phenotypic character differentiating this group.