Overdominance of a Muscle Protein (Mp-1) Locus in the Japanese Oyster, Crassostrea gigas (Ostreidae)

Abstract

A microgeographical population genetic study of the Japanese oyster, Crassostrea gigas, revealed a case of overdominance for a muscle protein (Mp-1) locus. This locus consists of a two allele polymorphism. A study of the relative "fitness" of the three genotypes (Mp-1105/105, Mp-1105/100, and Mp-1100/100) revealed a heterozygote superiority in the segment of the oyster population at the lowest tidal level (+ 1.2 m). In segments of the population at higher tidal heights (i.e. + 1.8 and + 2.4 m), there appears to be a selective advantage for Mp-1105 allele since the highest "fitness" values belong to the Mp-1105/105 genotype. Since oysters do not appear at the field station lower than 1.2 tidal m, it could not be determined if the Mp-1100 allele had a selective advantage at tidal levels lower than 1.2 m. However, the Mp-1105 allele possibly exhibits a selective advantage in the subtropical marine environment of southern Japan, while the Mp-1100 allele is favored in the temperate marine environment of northern Japan. This suggests perhaps that a micro- and macrogeographical cline in gene frequency exists for the Mp-1 locus due to a thermal gradient. Key words: Japanese oyster, Crassostrea gigas, population genetic study, overdominance, heterozygote superiority, Mp-1 locus, micro- and macrogeographical cline, thermal gradient, tidal height, latitude