|The implications of heterozygosity in the scallop Placopecten magellanicus (Gmelin)|
Volckaert, F.A.M.J. (1988). The implications of heterozygosity in the scallop Placopecten magellanicus (Gmelin). PhD Thesis. Dalhousie University: Halifax. 113 pp.
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|Document type: Dissertation|
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- Volckaert, F.A.M.J., more
In many instances, variation in phenotypic traits has been correlated with electrophoretic heterozygosity of plants, vertebrates and invertebrates. Enhanced growth of selected heterozygous molluscs has been attributed to lower energetic requirements for basal metabolism. I investigated the correlation between single- and multiple-locus heterozygosity and phenotype of the juvenile scallop Placopecten magellanicus (Gmelin). Two questions were being asked: do heterozygous scallops grow faster and which metabolic processes are affected by the degree of heterozygosity? No significant correlation was observed between genotype (scored at 6 polymorphic loci) and growth in six different samples of juvenile scallops, all belonging to the same time series. This result and published records indicate that the allozyme heterozygosity of pectinids does not influence growth to the same degree as in the bivalves Mytilus edulis, Crassostrea virginica and Mulinia lateralis. A decrease in heterozygote deficiency with age suggests selective mortality in scallops between the ages of 2 months and 13 months. The metabolism of 13 month old scallops switched from the catabolite carbohydrate (i.e. glycogen) to protein when they were starved for 4 weeks. Allozyme genotype had no measurable effect on oxygen uptake, excretion rate, O:N ratio and carbohydrate content, either under routine or basal metabolic conditions. Two hours after induction of muscle contractions in 20 month old scallops, the end-product octopine reached a concentration of 4 µmol.(g wet muscle weight)-1. Multiple-locus heterozygosity was positively correlated with octopine accumulation in the phasic part of the adductor muscle. Octopine dehydrogenase activity was on average higher among ODH homozygotes than among heterozygotes. In summary, multiple-locus heterozygosity is correlated with selected traits related to functional anaerobiosis. A model integrates the above mentioned results with the hypotheses of ''associative overdominance" , of a "balanced energy pathway" and of "energetic efficiency". I argue that energy savings due to heterozygosity are used for enhanced "activity" (such as feeding and swimming) in freely moving molluscs and for enhanced "growth" (such as somatic growth in juveniles, gonadal growth in adults and resistance to starvation) in sessile molluscs.