frbiol$501 Freshwater Biology (1996) 35, 311–321 Genetic differentiation in Daphnia obtusa: a continental perspective PAUL D.N. HEBERT AND TERRIE L. FINSTON Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G 2W1 SUMMARY 1. The restricted scale of most prior studies of genetic diversity in daphniid populations provides limited information on the geographical patterning of gene frequencies. The present study addresses this gap by examining allozymic divergence in populations of the most broadly distributed daphniid in the warm temperate regions of North America, Daphnia obtusa, across its range. 2. Local populations of this species show the gene frequency differentiation typical of other daphniids. In contrast to other daphniids with broad distributions, however, further divergence is apparent at a larger geographical scale, with North American D. obtusa fragmented into three lineages showing largely allopatric distributions. The three lineages are based primarily on allele frequency shifts at three polymorphic loci and are represented by eastern, central and south-western groupings. 3. Because of this pattern of differentiation, there is no simple monotonic relationship between geographical distance and genetic divergence. Instead, local metapopulations belonging to a specific lineage show little genetic divergence over several thousand km, while marked shifts in gene frequency occur over a few hundred km in regions where different lineages are in contact. Introduction Over the past 20 years, a series of allozyme studies Crease, Lynch & Spitze (1990) compared the degree of divergence between D. pulex Leydig populations at have provided the first insights concerning the gene pool characteristics of species in the genus Daphnia two North American sites with that among local populations, and showed a marked increase in genetic (Hebert, 1987; Mort, 1991; Lynch & Spitze, 1994). This work has established the occurrence of gene frequency divergence with distance. A subsequent survey of D. pulex populations across the whole of Canada, divergence among populations in a local area (Boileau, Hebert & Schwartz, 1992). There is also some evidence however, indicated the subdivision of this species into two genetic groups whose distributional boundary that the extent of this local variation differs among species (Mort & Wolf, 1986; Mort, 1991). Few studies coincided with the prairie/forest ecotone (Hebert et al., 1993). Populations in each group showed little regional have, however, attempted to gain information on the distribution of gene frequencies at a larger geograph- genetic divergence and the earlier results (Crease et al., 1990) were a direct consequence of sampling ical scale, although it has been presumed that local genetic divergence is further amplified with increasing populations from opposite sides of this boundary. While these studies on D. pulex indicated the occur- distance (Ebert, 1994; Lynch & Spitze, 1994). Initial work on Daphnia magna Straus found no rence of broader scale spatial patterns of gene frequen- cies, there was no simple relationship between genetic evidence for an association between distance and divergence, but analysis was restricted to populations divergence and distance. Instead, populations of D. pulex showed little genetic divergence over large from a relatively small geographical area (Hebert, 1974). More comprehensive studies have so far been areas of North America, and abrupt genetic transitions at the contact zones between different groups. A conducted only in North America and Australia. © 1996 Blackwell Science Ltd 311