Genetic architecture of the egg size–number trade-off in the Pacific oyster, Crassostrea gigas

The trade-off between egg size and number is a fundamental concept in life-history theory, yet its genetic underpinnings remain poorly characterized in marine invertebrates. In this study, we investigated the genetic architecture of reproductive traits in the Pacific oyster (Crassostrea gigas) using quantitative genetics and genome-wide association analyses. Reproductive traits were measured in 650 offspring, and 220 ripe females were randomly selected for genotyping followed by genome-wide association analysis. We found negative phenotypic and genetic correlations between egg size and egg number. Notably, two variants in the 5' UTR of the TRIM71 locus exhibited opposite allelic effects on the two traits, suggesting a candidate regulatory locus contributing to variation in reproductive allocation. By integrating GWAS with stage-specific transcriptomic profiles, we prioritized candidate genes associated with each trait and identified distinct temporal expression patterns: genes linked to egg size were more highly expressed during early developmental stages, whereas those associated with egg number showed elevated expression during gametogenesis and gonadal ripening. These findings reveal that the egg size-number trade-off in C. gigas is associated with both allelic variation at specific loci and temporally separated expression programs during reproductive development, providing novel insights into the molecular basis of reproductive allocation in marine invertebrates.