THE FERTILITY EFFECTS OF PERICENTRIC INVERSIONS IN DROSOPHILA-MELANOGASTER

Heterozygotes for pericentric inversions are expected to be semisteril e because recombination in the inverted region produces aneuploid game tes. Newly arising pericentric inversions should therefore be quickly eliminated from populations by natural selection. The occasional polym orphism for such inversions and their fixation among closely related s pecies have supported the idea that genetic drift in very small popula tions can overcome natural selection in the wild. We studied the effec t of 7 second-chromosome and 30 third-chromosome pericentric inversion s on the fertility of heterokaryotypic Drosophila melanogaster females . Surprisingly, fertility was not significantly reduced in many cases, even when the inversion was quite large. This lack of underdominance is almost certainly due to suppressed recombination in inversion heter ozygotes, a phenomenon previously observed in Drosophila. In the large sample of third-chromosome inversions, the degree of underdominance d epends far more on the position of breakpoints than on the inversion's length. Analysis of these positions shows that this chromosome has a pair of ''sensitive sites'' near cytological divisions 68 and 92: thes e sites appear to reduce recombination in a heterozygous inversion who se breakpoints are nearby. There may also be ''sensitive sites'' near divisions 31 and 49 on the second chromosome. Such sites may be import ant in initiating synapsis. Because many pericentric inversions do not reduce the fertility of heterozyotes, we conclude that the observed f ixation or polymorphism of such rearrangements in nature does not impl y genetic drift in very small populations.