LACK OF DEGENERATION OF LOCI ON THE NEO-Y CHROMOSOME OF DROSOPHILA-AMERICANA AMERICANA

The extent of genetic degeneration of the neo-Y chromosome of Drosophi la americana americana has been investigated. Three loci, coding for t he enzymes enolase, phosphoglycerate kinase and alcohol dehydrogenase, have been localized to chromosome 4 of D. a. americana, which forms t he neo-Y and neo-X chromosomes. Crosses between D. a. americana and D. virilis or D. montana showed that the loci coding for these enzymes c arry active alleles on the neo-Y chromosome in all wild-derived strain s of americana that were tested. Intercrosses between a genetically ma rked stock of virilis and strains of americana were carried out, creat ing F-3 males that were homozygous for sections of the neo-Y chromosom e. The sex ratios in the F-3 generation of the intercrosses showed tha t no lethal alleles have accumulated on any of the neo-Y chromosomes t ested. There was evidence for more minor reductions in fitness, but th is seems to be mainly caused by deleterious alleles that are specific to each strain. A similar picture was provided by examination of the s egregation ratios of two marker genes among the F-3 progeny. Overall, the data suggest that the neo-Y chromosome has undergone very little d egeneration, certainly not to the extent of having lost the functions of vital genes. This is consistent with the recent origin of the neo-Y and neo-X chromosomes, and the slow rates at which the forces that ca use Y chromosome degeneration are likely to work.