COMPONENTS OF HYBRID DYSGENESIS IN A WILD POPULATION OF DROSOPHILA MELANOGASTER

Hybrid dysgenesis is a condition foand in certain interstrain hybrids of Drosophila melanogrrster caused by the interaction of chromosomal and cytoplasmic factors. Germ line abnormalities, including sterility, high mutability and male recombination, appear in the affected individuals. There are at least two distinct systems of hybrid dysgenesis. We examined a Wisconsin wild populetion in two consecutive years to determine the distribution of the chromosomal P factor and the extrachromosomal M cytotype that together cause one kind of hybrid dysgonic sterility. The P factor was found to be very common in the populetion, with all three major chromosomes heing polymorphic for it. This polymorphism was strongly correlated with variability for male recombination elements, suggesting that these two traits are part of the same system of hybrid dysgenesis. There was a slight tendency for the P factor to be lost in lines taken from this population and inbred in the laboratory for many generations. A large-scale search for the M cytotype, which causes susceptibility to the P factor, showed that it is present in the papulation at only very low frequencies. Further evidence that the population is mostly immune to the action of the P factor was our finding of a general lack of dysgenic sterility in the wild flies themselves. However, we were able to isolate several wild strains that consistently showed the M cytotype. In some cases, the frequency of the M cytotype could be maintained in these lines, but it could not usually be increased by artificial sclection. Some possible consequences of hybrid dysgenesis for the evolutional y Siology of Drosophila are suggested.