The role of fertility restoration in the maintenance of the inversion In(2L)t polymorphism in Drosophila melanogaster

In order to explain the worldwide latitudinal distribution and seasonal flu ctuations in In(2L)t frequencies in Drosophila melanogaster, fitness differ ences among In(2L)t and Standard (ST) homo-and heterokaryotypes under high- temperature conditions were determined. Viabilities were measured for high- temperature treatment started at different juvenile stages. The capacity to restore fertility after high-temperature treatment was measured for adults and juveniles. Furthermore, genetic adaptation for increased temperature r esistance for these traits was determined for strains which were reared at 33 degrees C for 10 generations. Whereas larva-pupa survival rates were hig h, highest juvenile mortalities and strongest karyotypic effects were obser ved during the pupal stage when preceding larval stages were reared at 33 d egrees C. ST karyotypes showed lowest viabilities. Although mating rate was hardly influenced, sterility was induced for females and males after high- temperature treatment of adults as well as juveniles. Subsequent transfer t o 25 degrees C, however, resulted in restored fertility in some of the indi viduals, depending on the length of the recovery period. Fertility restorat ion was significantly higher for heterokaryotype males and females. Heterok aryotype superiority for restored fertility as well as for viability was po sitively correlated with severity of the treatment. Tell generations of sel ection at 33 degrees C resulted in significant improvement of juvenile surv ival and fertility restoration for all karyotypes. These fitness components were positively correlated (r = 0.91; P < 0.001), which might suggest plei otropic effects. It is concluded that the capacity to restore fertility aft er heat stress is an important fitness component, especially with respect t o the In(2L)r polymorphism The observed heterokaryotypic superiority fits w ith the idea that the latitudinal distribution of In(2L)t frequencies is ma intained by balancing selection, with equilibrium values decreasing with la titude.