HYBRID DYSGENESIS IN DROSOPHILA-MELANOGASTER - INFLUENCE OF TEMPERATURE ON CYTOTYPE DETERMINATION IN THE P-M SYSTEM

In D. melanogaster, the P-M system of hybrid dysgenesis is a syndrome of germ line abnormalities, including temperature dependent gonadal dysgenesis (GD sterility), high rates of mutation and male recombination, which occurs in some interstrain hybrids but only from 1 of the 2 crosses. In the P-M system, hybrid dysgenesis results from interaction between chromosomally transposable elements of the P element family and a particular extrachromosomal state referred to as the M cytotype. Cytotype (M or P) is known to be determined by the absence or presence of chromosomal factors, but principally with limited cytoplasmic transmission. In a series of experiments in which F1 hybrid females from various P and M strains were submitted to different preadult and aging temperature treatments, it was found that the cytotype switch is strongly temperature-dependent in the F1 females from M.female. .times. P.male. but not in the reciprocal cross. In the F1 females from the former cross, a strong M cytotype occurs at a low developmental temperature (18.degree. C) and a weak M cytotype occurs at a high developmental temperature (26.5.degree. C). A high aging temperature applied after a low developmental temperature switches the cytotype from M to P and reciprocally, a low aging temperature applied after a high developmental temperature switches the cytotype from P to M. This thermo-reversibility of the extrachromosomal state exists only in the F1 females from M mother, but not in the F1 females from P mothers; this dissymmetrical behavior is discussed in relation to the mechanism proposed by O''Hare and Rubin, which explains cytotype determination by a positive feedback of the regulator of the P transposase on its own level of activity.