Chromosomal inversion polymorphism in natural populations of Drosophila ruberrima

The standard map of the giant polytene chromosome of Drosophila ruberrima, one of the members of the immigrans species subgroup of the immigrans speci es group, is constructed according to the primitive gene arrangement or sta ndard sequence (ST) for each chromosome. A pure line with homozygous chromo somes was established. Chromosomal inversions can be identified easily by c rossing a male fly with the pure line and checking the F-1 with the aid of this standard map. Inversion polymorphism, both kinds and frequencies, of t his species is high. The Taiwan population is at Hardy-Weinberg equilibrium , but the Yunnan population shows an excess of homozygotes which is probabl y affected by natural selection. The geographic pattern of chromosomal inve rsion polymorphism in natural populations reveals that the marginal island population, Taiwan, is less variable than the central mainland population, Yunnan, mainland China. Little genetic differentiation among subpopulations within Taiwan indicates that extensive gene flow exists. In contrast, mode rate genetic differentiation was observed between the Taiwan and Yunnan pop ulations.