THE MYTILUS-EDULIS SPECIES COMPLEX IN SOUTHWEST ENGLAND - MULTILOCUS HETEROZYGOSITY, BACKGROUND GENOTYPE AND A FITNESS CORRELATE

The relationship between heterozygosity al five polymorphic allozyme l oci and shell length (size) was investigated in the hybridizing marine bivalve molluscs Mytilus edulis and M. galloprovincialis. Data were e xamined from high and low shore locations of two populations in southw est England to determine if differences in multi-locus heterozygosity explain fitness advantages of M. galloprovincialis-like over M. edulis -like mussels within the hybrid zone between these two taxa. Mean rank ed individual heterozygosity was consistently greater amongst M. gallo provincialis-like than M. edulis-like mussels, this difference being s ignificant in two of six tests. However, significant correlations betw een multi-locus heterozygosity and length were not observed for either mussel type in either population. In comparisons of mean length betwe en mussels of the same background genotype (i.e. between M. galloprovi ncialis-like or between M. edulis-like mussels), differences in mean l ength between phenotypes were non-significant. This indicates that fit ness differences observed between mussel types within the hybrid zone result not from heterozygosity per se (i.e. from the assayed allozyme markers themselves), but from the effect of background (parental) geno type, that is, genes which characterize differences between the mussel types and that are in linkage disequilibrium with the assayed allozym e markers.