Chromosome studies in wild populations of D. melanogaster.

Chromosome studies of wild D. melanogaster populations from Missouri, Mississippi, Louisiana and Texas uncovered 58 inversions. Six were common and cosmopolitan; 52 were new, rare and generally endemic. In one of two Missouri populations tested, structurally heterozygous females carried significantly more sperm at capture than did the homozygotes. In both populations comparisons of wild sperms with the females carrying them indicated significant positive assortative mating and an excess production of homozygotes among the F1 progeny. Wild females structurally heterozygous in up to three major autosomal arms showed no associated nondisjunctional egg lethality; those heterozygous in all four arms produced from 0% to 24% dead eggs, suggesting the presence of intrapopulational gene modifiers of meiosis. Texas populations supported on windfall citrus fruit showed a slight but significant difference in inversion frequencies between flies breeding on oranges and those breeding on grapefruit. Within these populations inversions were not distributed at random among individuals; rather there was an observed excess of individuals carrying intermediate numbers, and a deficiency of those carrying very few or very many inversions. While there was no significant linkage disequilibrium associated with this central tendency, there was a significant interchromosomal interaction: flies carrying inversions in chromosome 2 tended not to carry them in chromosome 3, and vice versa.