56 Donald A. Merkle 



are essentially isolated by "urban sprawl". Thus, there appear to be a 

 number of potential barriers to gene flow in populations of this species 

 in Virginia. 



Several other species of snakes whose northern limits are essentially 

 restricted to the Coastal Plain of southeastern Virginia also occur on the 

 peninsula between the James and York rivers. They include the Timber 

 Rattlesnake, Crotalus horridus; the Brown Water Snake, Nerodia taxis- 

 pilota; the Redbelly Water Snake, Nerodia erythrogaster erythrogaster, 

 and the Glossy Crayfish Snake, Regina rigida rigida (Linzey and Clif- 

 ford 1981). 



The distribution of such populations is open to two interpretations. 

 The first is to assume that populations occurring north of the James 

 River were established by animals that crossed the river in the same area 

 despite its width (approximately 6 km). The second hypothesis is to 

 assume that all of these species once occurred throughout the Coastal 

 Plain of southeastern Virginia. This would enable colonizers to invade 

 areas north of the James by crossing in the general vicinity of the Fall 

 Line, where the river is a less formidable barrier. Subsequent elimina- 

 tion of intervening populations would then produce the distributional 

 patterns currently observed. The potential isolation of populations 

 could result in genetic differentiation due to lack of gene flow. 



In this study, I used electrophoresis to examine genetic variation in 

 several populations of A. p. piscivorus to determine: 1) levels of genetic 

 variation that might accompany potential isolation; and 2) if patterns of 

 genetic variation reveal any insights into the origin of populations of A. 

 piscivorus north of the James River. 



MATERIALS AND METHODS 



Specimens collected in the field from May 1980 until September 

 1982 were returned to the laboratory and sacrificed by freezing. Attempts 

 were made to collect specimens from every drainage system in Virginia 

 where the species has been recorded. Localities where snakes used for 

 this study were collected (Fig. 1) are: 1) Newport News Reservoir, New- 

 port News; 2) Appomattox River near Hopewell; 3) Sea Shore State 

 Park, Virginia Beach; 4) Gum Swamp near North Landing River, Vir- 

 ginia Beach; 5) Northwest River, Chesapeake; and 6) False Cape State 

 Park, Virginia Beach. 



Prior to electrophoresis, specimens were thawed and extracts of 

 soluble proteins prepared by homogenizing samples of heart, liver, and 

 skeletal muscle with equal volumes of 2% 2-phenoxyethanol. Homoge- 

 nates were centrifuged at 4,000 G for 30 minutes. Supernatants were 

 then decanted and used for electrophoresis. 



