Genetic Variation in Cottonmouth 59 



DISCUSSION 



The average heterozygosity value of 1.6 reported here for Virginia 

 populations of A. piscivorus is less than that of 4.1% - 8.3% reported for 

 Thamnophis sirtalis by Sattler and Guttman (1976), and the 9.2% and 

 7.7% reported for Thamnophis proximus and Thamnophis sauritus, 

 respectively, by Gartside et al. (1977). It is interesting to note the 

 extremely low heterozygosity in the Sea Shore State Park population 

 (0.9%). Virginia populations of A. piscivorus appear to exhibit less vari- 

 ation than other species of snakes. Two specimens of the Florida Cot- 

 tonmouth, Agkistrodon piscivorus conanti, were found to be almost 

 identical to Virginia specimens based on electrophoretic analysis of the 

 same loci. Only the presence of a unique allele at the Got-1 locus distin- 

 guished it from Virginia A. piscivorus. Since there is so little variation 

 between these two subspecies, the low amount of variation and hetero- 

 zygosity reported in this study may be typical for the species. Thus, A. 

 piscivorus may have had a rather conservative biochemical evolution, if 

 we can draw inferences from data at hand. 



The highest level of heterozygosity was observed in the population 

 at False Cape State Park (2.9%), while the lowest was seen in the Sea 

 Shore State Park sample (0.9%). This population is unique in that hous- 

 ing developments separate it from other populations. The surrounding 

 area is one of the most rapidly developing areas in the United States. 

 Although the park once supported high-density populations of cotton- 

 mouths, the species has become extremely rare there in recent years. 

 Despite extensive field efforts over a two-year period, only two speci- 

 mens were collected. This apparent decline may be an effect of the 

 extensive droughts this area has suffered during the past few years. 



Nei's (1972) index of genetic similarity reveals all populations are 

 extremely close genetically. Even the lowest pairing value (.964), obtained 

 in comparing Sea Shore State Park with the Newport News population, 

 indicates a very high degree of genetic similarity among all Virginia 

 populations sampled. Although all Virginia populations of A. piscivorus 

 sampled are very similar genetically, the Hopewell, Gum Swamp, and 

 Newport News populations show the highest values for genetic similar- 

 ity. All three displayed a very high frequency for Xdh-l a , which occurred 

 in much lower frequencies in the other populations and was absent from 

 the Sea Shore State Park samples. While the Newport News population 

 shared a slightly higher identity with the Gum Swamp population than 

 with the Hopewell population (.99880 vs .99826), this extremely small 

 difference can be explained by the very low frequency of Got-2 a (.111) 

 and higher frequency of Xdh-l a (.944) in the Hopewell population. Both 

 of these loci appear to be heading towards complete fixation of alleles, 

 either by selection or by drift, in this isolated population. Such patterns 



