Hybrid dysgenesis in Drosophila melanogaster: a possible explanation in terms of spatial organization of chromosomes.

Male recombination and female sterility, two aspects of hybrid dysgenesis in D. melanogaster, have been studied in crosses between a locally collected wild population and laboratory strains. Dysgenesis occurs in the F1 hybrid of such crosses only if the wild type is used as male parent and the laboratory strain as female, suggesting an interaction between genotype and cytoplasm. However the results from further crosses are difficult to interpret in terms of a conventional genotype-cytoplasm model, and suggest that for dysgenesis to occur it is necessary that the wild-type chromosomes be contributed by the male parent. Furthermore, receipt of any of the three major wild-type chromosomes in crosses to laboratory females is sufficient to cause hybrid dysgenesis. A model in terms of spatial organization of chromosomes is put forward to explain these results. It is postulated that (1) normal nuclear functioning requires a definite spatial organization of chromosomes, which is presumably achieved by chromosome-membrane associations, (2) chromosomes are inherited from the female parent with spatial ordering preserved, i.e. membranes and associated chromosomes are handed on directly from the female parent, (3) spatial ordering is not necessarily preserved in male gametes, and paternally derived chromosomes carry information enabling them to become correctly organized within the zygote nucleus, and (4) hybrid dysgenesis results when the chromosome(s) from the male of one strain lack the information to become correctly organized in the nucleus of a second strain. The model seems to explain all aspects of the results, and offers the possibility that the present system may yield information on the genetics of membrane development and other aspects of spatial organization in the normal nucleus.