Allozymic Variation Within and Among Mainland and Insular Populations of Oryzomys palustris and Peromyscus leucopus.

Janet L. Loxterman1, Nancy D. Moncrief2, and Raymond D. Dueser3.

1Dept. of Biology, Virginia Commonwealth Univ., Richmond, VA 23220, 2Virginia Museum of Natural History, Martinsville, VA 24112, and 3Dept. of Fisheries and Wildlife, Utah State Univ., Logan, UT 84322.

Dueser and others have studied the biogeography and ecology of mammals on the Virginia Barrier Islands since 1975. Those studies focused more on patterns of distribution and abundance, with less concentration on the biogeographic and evolutionary processes underlying these patterns. Genetic analysis of populations offers the potential to examine the relative rates of processes (e.g., colonization and extinction) that are thought to be important in island biogeography and ecology. In 1989, Moncrief began collaborating with Dueser to conduct genetic analyses of several species of rodents that inhabit the Virginia Barrier Islands and adjacent Delmarva Peninsula.

The overall objectives of this larger project include: 1) examining genetic relatedness among populations of several rodent species on the Virginia Barrier Islands; 2) comparing patterns and degrees of genetic diversity within and among insular rodent populations and those on the adjacent mainland; and 3) measuring the degree of geographical isolation of individual islands and the relative abilities of different rodents to colonize the islands.

Greater dispersal ability should lead to increased gene flow (and less genetic differentiation) among populations. Thus, for the present study we selected two species that represent relative extremes of dispersal ability. As reflected by their distribution on the Virginia Barrier Islands, the marsh rice rat (Oryzomys palustris) is a relatively good disperser across water, occurring on 19 islands, and the white-footed mouse (Peromyscus leucopus) is a relatively poor disperser, occurring on only four islands.

We used horizontal starch-gel electrophoresis to analyze allozymic variation within and among mainland and insular populations of rice rats and the white-footed mice. The sampling localities include 4 sites on the Delmarva Peninsula and 5 sites on adjacent barrier islands. Sampling localities were selected in groups to represent northern, middle, and southern portions of the mainland and adjacent barrier islands. Pairs of sites were chosen to allow comparisons of genetic distance versus geographic distance in relation to gene flow and intraspecific variation.

A total of 118 O. palustris distributed among nine sites and 96 P. leucopus distributed among seven sites was collected on the southern Delmarva Peninsula and associated Virginia barrier islands in 1989, 1990 and 1993. Sampling sites were as follows (names of sites are in bold, numerals in parentheses following each locality indicate the number of O. palustris and P. leucopus examined, respectively): VIRGINIA:Accomack Co.:Assateague Island, Chincoteague National Wildlife Refuge (10, 13); 1.5 kilometers S, 2 kilometers E Wattsville (13, 9); Cedar Island (12, 11); 3 miles N Quinby (12, 13); Parramore Island (13, 0). VIRGINIA:Northampton Co.:2 miles E Nassawadox (10, 24); Myrtle Island (25, 0); Eastern Shore of Virginia National Wildlife Refuge (ESVNWR) (10, 15); Fishermans Island National Wildlife Refuge (13, 11).

Twenty-one enzyme systems encoded by 29 presumptive gene loci were assayed from heart, liver, and kidney samples for each individual as follows: peptidase A (valyl-leucine used as substrate; PEPA, Enzyme Commission number 3.4.11), peptidase B (leucyl-glycyl-glycine used as substrate; PEPB, 3.4.11), peptidase D (phenylalanyl-proline used as substrate; PEPD,, peptidase S (valyl-leucine or leucyl-glycyl-glycine used as substrate; PEPS, 3.4.11), adenylate kinase (AK,, creatine kinase (CK,, guanine deaminase (GDA,, glutamate dehydrogenase (GLUD, 1.4.1.-), adenosine deaminase (ADA,, malic enzyme (ME,, malate dehydrogenase (MDH-1, -2,, phosphoglucose isomerase (PGI,, nucleoside phosphorylase (NP,, glutamate oxaloacetate transaminase (GOT-1, -2,, lactate dehydrogenase (LDH-1, -2,, aconitase (ACN-1,-2,, glucose-6-phosphate dehydrogenase (G6PD,, phosphoglucomutase (PGM-1, -2, -3,, 6-phosphogluconate dehydrogenase (6PGD,, isocitrate dehydrogenase (IDH-1, -2,, and mannose phosphate isomerase (MPI, The BIOSYS-1 program was used to describe and statistically analyze the results.

In O. palustris, five loci are polymorphic: ADA, G6PD, NP, 6PGD, and PGM-3 (Table 1). Percent polymorphic loci ranges from 0 (Fishermans Island) to 14.3% (Assateague Island) with an average of 7.13%. Mean heterozygosity averages 1.5% among all nine samples and ranges from 0.3 (Fishermans Island) to 2.5% (Assateague Island). P. leucopus also is variable at five of 29 loci: G6PD, MPI, NP, PEP A, and PGM-1 (Table 2). Percent polymorphic loci ranges from 6.9 (Fishermans Island) to 17.2% (Cedar Island) with an average of 10.8%. Mean heterozygosity ranges from 0.6% (Fishermans Island) to 5.4% (Wattsville) with an average of 3.4% for all seven sites. In both species, estimates of mean heterozygosity averaged among island samples (Assateague, Cedar, and Fishermans islands, and for O. palustris Parramore and Myrtle islands as well) were lower than these estimates averaged among mainland sites (Wattsville, Quinby, Nassawadox, and ESVNWR). Mean heterozygosity for island samples of O. palustris and P. leucopus, respectively, averaged 1.2% and 2.6%, while mean heterozygosity for mainland samples averaged 1.8% and 4.0% (Tables 1 and 2).

In O. palustris, Rogers' genetic distance measures range from 0.005 between Nassawadox and Parramore Island to 0.043 between Fishermans Island and Assateague Island as well as between Cedar Island and Assateague Island (Table 3), with an average genetic distance of 0.017 for all nine samples. In P. leucopus, Rogers' genetic distance measures range from 0.011 between Wattsville and Nassawadox to 0.063 between Fishermans Island and Cedar Island (Table 3) with an average genetic distance of 0.033 for all seven samples. In terms of genetic distance versus geographic distance, no clear patterns of relationships among populations were evident for either species. However, Rogers' genetic distance values (Table 3) indicate that, in general, populations of O. palustris are more similar to each other than are populations of P. leucopus. This observation is consistent with gene flow estimates calculated from F-statistics (data not given) of Nm = 1.49 for O. palustris and Nm = 1.21 for P. leucopus and predictions based on the relative dispersal abilities of these species.

In 1993 and 1994 we sampled additional populations of these rodents as well as other species of small mammals. As part of our ongoing studies of mammalian biogeography and evolution, we will use additional (more sensitive) genetic techniques to analyze these samples, including those reported here.


Table 1. Allelic frequencies of polymorphic loci in Oryzomys palustris

        1                                               Samples                                                       
        1           2           3           4           5           6           7           8           9             
        Quinby      ESVNWR      Fishermans  Cedar ID    Wattsville  Assateague  Nassawadox  Parramore   Myrtle ID     
                                ID                                  ID                      ID                        
    n   12          10          13          12          13          10          10          13          25            
G6PD    A           A           A           A           A           A(0.85)     A           A           A             
PGM3    A           A           A           A(0.92)     A           A           A           A           A(0.98)       
                                            B(0.08)                                                     B(0.02)       
6PGD    A(0.96)     A(0.85)     A           A           A(0.96)     A(0.55)     A           A           A(0.92)       
        B(0.04)     B(0.15)                             B(0.04)     B(0.45)                             B(0.08)       
NP      A(0.83)     A(0.94)     A           A           A(0.84)     A(0.85)     A           A           A(0.88)       
        B(0.08)                                         B(0.08)                                         B(0.08)       
        C(0.08)     C(0.06)                             C(0.08)     C(0.15)                             C(0.04)       
ADA     A(0.88)     A(0.85)     A(0.96)     A(0.88)     A(0.54)     A(0.50)     A(0.55)     A(0.69)     A(0.80)       
        B(0.12)     B(0.15)     B(0.04)     B(0.12)     B(0.46)     B(0.50)     B(0.45)     B(0.31)     B(0.20)       
H       0.024       0.018       0.003       0.009       0.019       0.025       0.011       0.005       0.019         
Hexp    0.022       0.023       0.003       0.014       0.031       0.057       0.019       0.016       0.026         
P       7.1         10.7        0           7.1         7.1         14.3        3.6         3.6         10.7          


Table 2. Allelic frequencies of polymorphic loci in Peromyscus leucopus

         1           2           3           4           5           6           7             
         Quinby      ESVNWR      Fishermans  Cedar ID    Wattsville  Assateague  Nassawadox    
     n   13          15          11          11          9           13          24            
G6PD     A(0.96)     A           A           A(0.59)     A           A           A(0.98)       
         B(0.04)                             B(0.41)                             B(0.02)       
PGM1     A           A(0.60)     A           A(0.73)     A(0.89)     A(0.96)     A(0.96)       
                                             B(0.27)                 B(0.04)                   
                     C(0.40)                             C(0.11)                 C(0.04)       
PEP A    A(0.92)     A           A           A(0.64)     A(0.94)     A(0.85)     A             
         B(0.08)                             B(0.36)     B(0.06)     B(0.15)                   
MPI      A(0.85)     A(0.80)     A(0.91)     A(0.73)     A(0.56)     A(0.85)     A(0.53)       
         B(0.15)     B(0.03)                 B(0.23)     B(0.39)                 B(0.33)       
                     C(0.03)     C(0.09)     C(0.04)     C(0.05)     C(0.15)     C(0.14)       
NP       A(0.42)     A(0.33)     A(0.82)     A(0.36)     A(0.56)     A(0.35)     A(0.58)       
         B(0.50)     B(0.67)                 B(0.64)     B(0.28)     B(0.65)     B(0.19)       
         C(0.08)                 C(0.18)                 C(0.16)                 C(0.23)       
H        0.042       0.030       0.006       0.041       0.054       0.032       0.034         
Hexp     0.037       0.045       0.017       0.080       0.052       0.038       0.045         
P        10.3        10.3        6.9         17.2        13.8        10.3        6.9           


Table 3. Rogers' genetic distance measures for Oryzomys palustris (above diagonal) and Peromyscus leucopus (below diagonal). Peromyscus leucopus does not occur on Parramore and Myrtle islands.

 Population      1         2         3         4         5         6         7         8         9           
1 Quinby         X        0.008     0.010     0.010     0.013     0.036     0.018     0.013     0.006        
2 ESVNWR        0.027      X        0.011     0.011     0.018     0.032     0.018     0.013     0.008        
3 Fishermans    0.024     0.039      X        0.006     0.021     0.043     0.015     0.010     0.013        
4 Cedar ID      0.040     0.046     0.063      X        0.021     0.043     0.015     0.010     0.012        
5 Wattsville    0.022     0.035     0.028     0.049      X        0.024     0.007     0.012     0.013        
6 Assateague    0.015     0.024     0.029     0.037     0.030      X        0.029     0.034     0.034        
7 Nassawadox    0.024     0.038     0.022     0.054     0.011     0.032      X        0.005     0.016        
8 Parramore ID   X         X         X         X         X         X         X         X        0.011        
9 Myrtle ID      X         X         X         X         X         X         X         X        X