Molecular genetic variation in the centromeric region of the X chromosome in three Drosophila ananassae populations. II. The Om(1D) locus.

Restriction-site and sequence-length polymorphism in the Om(1D) locus region on the X chromosome in Drosophila ananassae was investigated for three natural populations (from Burma, India, and Brazil), by using hexanucleotide-recognizing restriction enzymes. The estimates of average heterozygosity per nucleotide (pi) were 0.0085, 0.0043, and 0.0004 for the Burma, India, and Brazil populations, respectively, and the average frequencies of insertions/deletions were 0.078, 0.054, and 0.007/chromosome/kb. While the pi values at this locus are similar to the estimates obtained from other euchromatic loci in D. ananassae or in other Drosophila species, the frequencies of insertions/deletions are much higher than those previously reported from Drosophila. The exceptionally high frequencies of length polymorphisms in the Burmese sample and, to a lesser extent, in the Indian sample indicate that the hypermutability of Om(1D), caused by the frequent insertion of the transposable element tom, may be due to locus-specific rather than to tom element-specific properties. The low level of nucleotide variation in the Brazilian population seems to be due to a recent bottleneck of population size. This population was apparently founded in recent years by a small number of individuals and has been relatively isolated ever since.