SIZE-DEPENDENT, SPATIAL AND TEMPORAL GENETIC-VARIATION AT A LEUCINE AMINOPEPTIDASE (LAP) LOCUS AMONG BLUE MUSSEL (MYTILUS-GALLOPROVINCIALIS) POPULATIONS ALONG A SALINITY GRADIENT

Biochemical genetic variation at a leucine aminopeptidase (LAP) locus is related to salinity variation in several marine bivalve molluscs. T his paper details an investigation of the Long Island Sound model of L AP selection (LAP genotype-dependent mortality occurring among newly s ettled Mytilus edulis mussels) carried out in 1997 among three M. gall oprovincialis mussel populations along the salinity gradient of Wellin gton Harbour, New Zealand. Significant LAP genotypic heterogeneity was observed at the LAP locus between small (<25 mm shell length) and lar ge (> 25 mm shell length) M. galloprovincialis from Petone and Eastbou rne (the two sites experiencing the greatest salinity variation), wher eas genotypic heterogeneity was not significantly different between sm all and large mussels from Seatoun (the site experiencing the least sa linity variation). The Lap(3) allele decreased in frequency and the La p(4) allele increased in frequency at Petone and Eastbourne, whereas t he Lap(3) and Lap(4) allele frequencies remained effectively constant at Seatoun. Both these findings are consistent with the Long Island So und model of selection. At all three locations, the Lap(3.3) genotype decreased in frequency From small to large mussels, whereas the Lap(3, 4) genotype increased in frequency from small to large mussels. All ot her LAP genotypes occurred at low frequencies (< 0.10) at all three lo cations and showed no evidence of frequency change from small to large -size mussels nor evidence of clinal change among the three locations. These genotype frequency data possibly indicate that the Lap(3,3) gen otype is at a selective disadvantage compared to the Lap(3,4) genotype at all three locations, and that this selective disadvantage is relat ed to the extent of salinity variation which exists at each location. Further investigation is required before it can be determined if the L ong Island Sound model of selection best describes the size-dependent and location-specific changes in LAP allele and genotype frequencies a long this salinity gradient. Comparison of the population genetic stru cture at the LAP locus in 1995 and in 1997 revealed a profound change from heterozygote excesses to heterozygote deficiencies for all three M. galloprovincialis populations. The reason for the change is unknown , but the change indicates that population genetic structure at the LA P locus is highly variable in time, but consistent in spacer among the se M. galloprovincialis populations.