GENETIC VARIATION AND ISLAND BIOGEOGRAPHY OF MAINLAND AND INSULAR POPULATIONS OF MARSH RICE RATS ORYZOMYS PALUSTRIS AND WHITE-FOOTED MICE PEROMYSCUS LEUCOPUS.

Nancy D. Moncrief1 and Raymond D. Dueser2. 1Virginia Museum of Natural History, Martinsville, VA 24112, and 2Department of Fisheries and Wildlife, Utah State University, Logan, UT 84322.

This ongoing study is designed to investigate the biogeographic and intraspecific evolutionary relationships of small mammals inhabiting the Virginia barrier islands and adjacent Delmarva Peninsula. Representative pairs of island and mainland sites have been chosen for northern, middle, and southern segments of the Virginia portion of the Delmarva Peninsula and adjacent barrier islands. These island-mainland pairs allow comparisons of genetic distance versus geographic distance in relation to gene flow and genetic variation.

In 1989 and 1990 we collected marsh rice rats (Oryzomys palustris) and white-footed mice (Peromyscus leucopus) from four sites on the Delmarva Peninsula and five adjacent islands ( Table 1, sites 1-9) for mitochondrial DNA (mtDNA) analysis by Takahiro Asami (Tachikawa College of Tokyo). The islands included: Myrtle, Fisherman, Cedar, Parramore, and Assateague. Peromyscus leucopus does not occur on Myrtle and Parramore Islands. The four sites on the Delmarva Peninsula were Quinby, Eastern Shore of Virginia National Wildlife Refuge (Mainland South), Wattsville (Mainland North), and Nassawadox.

In the summer of 1993, we began allozyme analyses (with Janet Loxterman, Virginia Commonwealth University) of the same individual rice rats and white- footed mice that were collected in 1989-90, with the exception of Peromyscus from Cedar Island (samples of which were not available). Also in summer and fall 1993, we collected additional samples of small mammals from sites 1-4 and 6-9 (Table 1) as well as several southern islands (sites 10-18; Table 1).

Preliminary results from both mtDNA and allozyme analyses indicate that the rice rat exhibits less differentiation among populations than does the white-footed mouse. Differences in intraspecific variability may be related to the dispersal ability of these two species. We hypothesize that the rice rat is a relatively good disperser among islands and adjacent mainland sites, whereas the white-footed mouse is a relatively poor disperser across water barriers.


Table 1.-- Sampling sites, species, and number of individuals
collected for genetic analyses of small mammals.  All individuals
collected for this study are presently housed as voucher specimens in
the Department of Mammals, Virginia Museum of Natural History.

                                                              
                                                             NUMBER 
SITE #    SITE NAME                SPECIES             OF INDIVIDUALS

                                                       1993   1989/90

 1   3 mi. N Quinby             Peromyscus leucopus       5
                                Oryzomys palustris                 12
 2   Myrtle Island              O. palustris             11        18
                                Microtus pennsylvanicus   1
 3   Eastern Shore of Virginia  P. leucopus              15
     National Wildlife Refuge   Blarina                   2
                                O. palustris                       10
 4   Fisherman Island           O. palustris              3        10
                                P. leucopus               9         2
 5   Cedar Island               P. leucopus                        11
                                O. palustris                       22
 6   1.5 km S 2 km E of         P. leucopus               3         7
     Wattsville                 Pitymys pinetorum         1   
                                O. palustris                       24
 7   Assateague Island          O. palustris              5         5
                                Cryptotis parva           1
                                Peromyscus leucopus                13
 8   Parramore Island           M. pennsylvanicus        11
                                O. palustris             10         5
 9   2 mi. E  Nassawadox        P. leucopus              14        10    
                                O. palustris              3         7
                                C. parva                  2 
 10  Smith Island               Mus musculus              2
                                O. palustris             20
 11  Chimney Pole Island        O. palustris              2
 12  Revel Island               M. pennsylvanicus         3
 13  Sandy Island               O. palustris              1
 14  Ship Shoal Island          O. palustris             11
                                M. pennsylvanicus         5
 15  Wreck Island               O. palustris             11
 16  Mink Island                O. palustris             16    
 17  8 mi. S 1 1/2 E            Blarina                   2
         of Cheriton            O. palustris              4
                                Lasiurus borealis         1
 18  Mockhorn Island            O. palustris             20