Chromosome Inversion Polymorphisms in DROSOPHILA MELANOGASTER. I. Latitudinal Clines and Associations between Inversions in Australasian Populations.

Nineteen Australasian populations of Drosophila melanogaster have been screened for chromosome inversion polymorphisms. All 15 of the inversion types found are paracentric and autosomal, but only four of these, one on each of the major autosome arms, are common and cosmopolitan. North-south clines occur, with the frequencies of all four of the common cosmopolitan inversions increasing toward the equator. These clines in the Southern Hemisphere mirror north-south clines in the Northern Hemisphere, where the frequencies of all four of the common cosmopolitan inversions again increase towards the equator.-While few of the Australasian populations show significant disequilibrium between linked common cosmopolitan inversions, those that do invariably have excesses of coupling gametes, which is consistent with other reports. We also find nonrandom associations between the two major autosomes, with the northern populations in Australasia (those with high inversion frequencies) tending to be deficient in gametes with common cosmopolitan inversions on both major autosomes, while the southern populations in Australasia (low inversion frequencies) tend to have an excess of this class of gametes.-The clines and the nonrandom associations between the two major autosomes are best interpreted in terms of selection operating to maintain the common cosmopolitan inversion polymorphisms in natural populations of D. melanogaster.