QUANTITATIVE AND MOLECULAR-GENETIC ANALYSES OF HETEROSIS IN BIVALVE MOLLUSKS

Associations of allozyme-heterozygosity with growth and its physiologi cal underpinnings have been well documented for bivalve molluscs. The associations are correlational, however, derived almost entirely from studies of wild-caught juveniles or adults. Such studies cannot resolv e alternative genetic explanations of heterosis. Four experimental app roaches have recently been made to this problem; (1) a correlational s tudy contrasting allozyme and presumably selectively neutral nuclear D NA polymorphisms; (2) detailed studies of allozyme inheritance in fami lies; (3) a study contrasting the performance of meiosis-I and meiosis -II triploids with diploids and (4) a classical quantitative genetic s tudy of the performance of hybrids produced by crosses among inbred li nes. The last approach has uncovered remarkable heterosis in growth an d its physiological components, both for the larval and juvenile or ad ult stages, and has implicated epistasis as a significant cause of thi s heterosis. More importantly, this approach now permits dissection of heterosis into quantitative trait loci (QTL) mapped by the co-segrega tion of allozyme and nuclear DNA markers with growth phenotypes in the F-2 hybrid and backcross generations.