Temporal genetic heterogeneity within a developing mussel (Mytilus trossulus and M-edulis) assemblage

The genetic structure of mussel recruits on a rocky shore in Nova Scotia wa s measured after a rare ice-scouring event which completely removed the int ertidal community. Genotype frequencies were measured at three enzyme loci, phosphoglucose mutase (PGM), aminopeptidase-I (product of the leucine amin opeptidase (LAP) locus), and mannose phosphate isomerase (MPI), using cellu lose acetate gel electrophoresis. The developing mussel assemblage was a mi xture of Mytilus trossulus, M. edulis and their hybrids, with a greater pro portion of the former species. Mussel settlers were collected for two years to examine whether genetic heterogeneity existed within and between cohort s of settlers. Settlement of mussels of both species began in April or May and continued into January. Temporal genetic heterogeneity was observed amo ng groups of settlers, resulting from both variations in the relative propo rtions of the two species and from genetic heterogeneity within M. trossulu s. Allele frequencies of mussel cohorts were followed from settlement to te n months post-settlement to investigate the possibility of species-specific selection and of genotype-specific selection within M. trossulus. Temporal genetic heterogeneity primarily was attributed to changes in the proportio ns of each species, indicating that post-settlement processes were species- specific. Early post-settlement changes were inconsistent, but later change s were clearly directional, resulting in decreased proportions of M. edulis in larger mussels in both years. There was some evidence of genotype-speci fic post-settlement mortality within ill. trossulus, particularly during th e early post-settlement period. Our results indicate that temporally variab le settlement patterns and postsettlement selection interacted to produce t emporal genetic heterogeneity in this mussel assemblage.