Triploid oysters were induced using cytochalasin B upon retention of either the first (meiosis I
triploids) or the second (meiosis II triploids) polar body in embryos from a single cohort derived
from mixed parentage. Allozyme and microsatellite assays enabled the confirmation of both
parentage and triploidy status in each oyster. Comparison of meiosis I triploids, meiosis II
triploids and diploid siblings established that improved physiological performance in triploids was
associated with increased allelic variation, rather than with the quantitative dosage effects of ploidy
status. An unidentified maternal influence also interacted with genotype. Among full sibs, allelic
variation measured as multi-locus enzyme heterozygosity accounted for up to 42% of the variance
in physiological performance; significant positive influences were identified upon feeding rate,
absorption efficiency, net energy balance and growth efficiency (= net energy balance ÷ energy
absorbed). Whilst allelic variation was greater in both meiosis I and meiosis II triploids than in
diploid siblings, both allelic variation and net energy balance were highest in triploids induced
at meiosis I. This suggests that it may be preferable to induce triploidy by blocking meiosis
I, rather than meiosis II as has traditionally been undertaken during commercial breeding
programmes.