GENETIC-BASIS OF PHOTOPERIODIC CONTROL OF SUMMER AND WINTER DIAPAUSE IN GEOGRAPHIC ECOTYPES OF THE RICE STEM MAGGOT, CHLOROPS ORYZAE

Chlorops oryzae Matsumura (Diptera: Chloropidae) is bivoltine in north ern Japan but trivoltine in the southern part of the country. Larvae o f the bivoltine strain (Akita, 39 degrees N) develop without delay und er L14:D10 but enter summer diapause in the mature larval stage under L15:D9. In contrast, larvae of the trivoltine strain (Aichi, 35 degree s N) develop quickly under L15:D9 but enter summer diapause under L14: D10. The genetic basis of their different summer-diapause behavior was analyzed by reciprocal crossing between the two ecotypes and backcros sing to the hybrids. Although a polygenic system seems to be involved, there is a single gene on the X-chromosome exerting a major effect on summer diapause. The northern bivoltine ecotype has a longer critical photoperiod for the induction of winter diapause in the first larval stage and remains in diapause longer than the southern trivoltine ecot ype. The differences in the critical photoperiod and the winter diapau se duration between the two ecotypes seem to be controlled by a polyge nic system but again a sex-linked or closely linked gene (or genes) pl ays a major role. The two geographic ecotypes differ significantly in summer- and winter-diapause traits, both of which are characterized by sex-linked inheritance. Despite this, no apparent sterility or inviab ility in hybrids between the ecotypes occurs.