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Biochemical studies on the expression of overdominance at the phosphoglucomutase-2 locus in the Pacific oyster, Crassostrea gigas (Thunberg) Pogson, Grant H.


Numerous studies have documented significant associations between multiple-locus heterozygosity and fitness-related traits in natural populations, but the explanations for these patterns remain unknown. The objective of the present study was to examine the merits of the overdominance hypothesis as the mechanism responsible for a positive correlation between adult body weight and heterozygosity involving the phosphoglucomutase-2 (Pgm-2) locus in the Pacific oyster, Crassostrea gigas. The kinetic and structural properties of seven Pgm-2 genotypes were examined over physiological ranges of temperature and pH. Significant differences were detected between Pgm-2 genotypes in a variety of enzymic parameters, but these were largely confined to genotypes possessing the Pgm-2-92 allele, and heterozygotes displayed strict intermediacy for all functional and structural properties examined. The expression of marginal overdominance at the Pgm-2 locus was considered unlikely because of the limited scope of the observed variation between allozymes, and its incompatibility with allelic frequencies in natural populations. The three most common heterozygotes at the Pgm-2 locus displayed the extremely unusual property of overdominant enzyme activities. The magnitude of this overdominance was similar in the mantle and adductor muscle tissues, and was consistently observed in population samples from two intertidal positions in three different seasons. A physiological impact of the Pgm-2 polymorphism was demonstrated on the metabolism of glycogen, the biochemical pathway in which PGM functions. Pgm-2 genotypes exhibited different concentrations of glycogen in their mantle, but not their adductor muscle tissues, -which were directly associated with variation in their PGM activity levels. It was suggested that Pgm-2 genotype-dependent enzyme activity variation may affect rates of glycogen synthesis by a partitioning effect at the glucose-6-phosphate branch point. Non-random associations were detected between the PGM activities of Pgm-2 genotypic groups and the activities of adjacent glycogen synthesis pathway enzymes, but none that could clearly account for the differing glycogen concentrations observed between genotypes. The expression of overdominance for PGM activity, and its impact on mantle glycogen levels, provided direct evidence favoring the overdominance explanation as the cause of the larger body weights of heterozygotes at the Pgm-2 locus in Crassostrea gigas.

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