Evidence for selection at multiple allozyme loci across a mussel hybrid zone.

The frequency and strength with which selection shapes patterns of genetic variation is unknown. Whereas all loci should be roughly equally affected by demography and population history, selected loci may exhibit increased or decreased genetic differentiation relative to neutral loci (Cavalli-Sforza 1966). Thus, one way to test for selection on a specific category of loci is to compare geographic differentiation of this particular category relative to a category presumed to be neutrally evolving (McDonald 1994; McDonald, Verrelli, and Geyer 1996). A number of recent studies have followed this approach, in particular comparing patterns of genetic differentiation at allozyme to nonallozyme loci. Where discordances between allozymes and other nuclear markers have been reported, less genetic partitioning has been observed for allozymes, consistent with balancing selection reducing differences among geographical populations (e.g., Karl and Avise 1992; Pogson, Mesa, and Boutilier 1995; Latta and Mitton 1997). Here, we report the reverse situation; geographic partitioning is greater at allozymes relative to nonallozyme loci across a mussel hybrid zone. Northern Hemisphere populations of the mussels Mytilus trossulus and Mytilus edulis are distinguished by nearly fixed allozyme allele frequency differences and some morphometric differences (McDonald, Seed, and Koehn 1991). Mytilus edulis is found throughout the northern Atlantic, whereas M. trossulus is found in three disjunct regions: the Pacific, Atlantic North America, and the Baltic Sea. A northern European hybrid zone between the Atlantic M. edulis and the Baltic Sea M. trossulus has been well described, based on allozyme surveys. A relatively sharp transition in mussel allozyme allele frequencies (over 100 km) occurs at the mouth of the Baltic with edulis-like alleles predominating in the Atlantic populations and trossulus-like alleles predominating in the Baltic. This pattern is particularly distinct for Aap, Est-D, Gpi, Lap, Mpi, and Pgm (Theison 1978; Bulnheim and Gosling 1988; Varvio, Koehn, and Väinölä 1988; Väinölä and Hvilsom 1991), and these loci, with the exception of Lap, are diagnostic between M. edulis and M. trossulus throughout the Northern Hemisphere (Varvio, Koehn, and Väinölä 1988; McDonald, Seed, and Koehn 1991). Rapid clinal transitions in allele frequencies, in combination with significant linkage disequilibria (particularly involving Gpi, Lap, and Pgm), have supported the idea that this hybrid zone represents