Genetic studies: Across North America - Across Lake Erie
Genetic patterns among Walleye (Sander vitreus) populations across the Laurentian Great Lakes
Carol A. Stepien, Rachel N. Lohner, and Douglas J. Murphy
Abstract: Many Great Lakes walleye spawning populations crashed by the mid-twentieth century and although some recovered, other historic groups were lost. The genetic delineation of remaining spawning groups is of considerable conservation management importance. We analyze allelic variation at 10 nuclear microsatellite loci to test for population relationships among 690 walleye in 19 spawning groups across the Great Lakes. Results reveal considerable divergence among most spawning groups and population areas, with almost no gene flow among lakes. Some intra-lake gene flow among selected spawning locations occurs along the southern shore of western and central Lake Erie, as well as between closely spaced groups in Lake St. Clair and Georgian Bay. The greatest genetic demarcation divides Great Lakes walleye population groups between the upper versus lower Lakes. The second most prominent division separates Lakes Erie from Ontario populations, the third distinguishes Lake Superior, and the fourth delineates riverine spawning groups in the eastern basin of Lake Erie. These broad scale patterns appear to reflect the signatures of two or more separate glacial refugia, as well as their contemporary divergence through spawning site specificity. Although walleye may travel from lake to lake, the genetic data show that the vast majority return to their natal sites to spawn. In Lake Erie, the genetic composition of eastern riverine spawning groups differs despite ample opportunity for gene flow. Although some genetic variation in walleye may have been obscured by anthropogenic activities, many distinct native population stocks remain whose maintenance should be of key conservation importance.
Fig. 1. Sampling locations of walleye spawning sites in the Laurentian Great Lakes. Latitude and longitude coordinates of these sites are given in Table 1. Bars denote primary genetic break divisions among population groups using the Manni et al. (2004a,b) BARRIER approach, designated with Roman numerals from greatest to less pronounced (I to VII).
Fig. 2. Mantel test of the pairwise relationship between genetic divergence and geographic distance expressed as FST/(1-FST) and the natural logarithm of their nearest water pathway in kilometers. The linear regression (b) and the correlation coefficients are b = 0.0009 and r = 0.3451, respectively. P = 0.003**, R2 = 0.1191. The equation is y = 0.0083x + 0.00089.
Fig. 3. Neighbor-joining tree (Saitou and Nei 1987; constructed in PHYLIP) showing relationships among major geographic sites tested for walleye based on Nei’s (1972) genetic distances. The tree calculated from Cavalli-Sforza and Edwards’ (1967) chord distances was similar in topology and resolution, and thus is not shown. Tree is “rooted” with the walleye’s sister species, the sauger Sander canadensis. Values at nodes denote relative support from 1000 bootstrap iterations.
Genetic studies: Across North America Across Lake Erie
