Linkage disequilibrium in natural and experimental populations of Drosophila melanogaster.

Abstract

We have studied linkage disequilibrium in Drosophila melanogaster in two samples from a wild population and in four large laboratory populations derived from the wild samples. We have assayed four polymorphic enzyme loci, fairly closely linked in the third chromosome: Sod Est-6, Pgm, and Odh. The assay method used allows us to identify the allele associations separately in each of the two homologous chromosomes from each male sampled. We have detected significant linkage disequilibrium between two loci in 16.7% of the cases in the wild samples and in 27.8% of the cases in the experimental populations, considerably more than would be expected by chance alone. We have also found three-locus disequilibria in more instances than would be expected by chance. Some disequilibria present in the wild samples disappear in the experimental populations derived from them, but new ones appear over the generations. The effective population sizes required to generate the observed disequilibria by randomness range from 40 to more than 60,000 individuals in the natural population, depending on which locus pair is considered, and from 100 to more than 60,000 in the experimental populations. These population sizes are unrealistic; the fact that different locus-pairs yield disparate estimates within the same population argues against the likelihood that the disequilibria may have arisen as a consequence of population bottlenecks. Migration, or population mixing, cannot be excluded as the process generating the disequilibria in the wild samples, but can in the experimental populations. We conclude that linkage disequilibrium in these populations is most likely due to natural selection acting on the allozymes, or on loci very tightly linked to them.

The effect of superoxide dismutase alleles on aging inDrosophila

“…As shown in Table 1, the S allele is significantly more frequent in O populations, averaging about 25 %, with no S alleles detected in the B population samples. Indeed, this average frequency of S in the O populations is greater than has been observed before for this allele in natural populations of & Ayala, 1988;Singh, Hickey & David, 1982 and unpub, data). While these samples suggest that the B populations entirely lack the S allele, further sampling of these populations has produced a few instances where flies from B populations carry the S allele heterozygously.…”

Two modes of balancing selection in Drosophila melanogaster: overcompensation and overdominance.

“…The experimental strains are derived from a natural population of D. melanogaster, "El Rio Vineyard," San Joaquin County, California. The frequencies of the S and F alleles in this population are about 0.140 and 0.860, respectively, which frequencies remain approximately constant from year to year (e.g., SMIT-MCBRIDE, MOYA and AYALA 1988, for two samples I-year apart). The standing adult population is large throughout the year, so that numerous D. melanogaster flies can be collected on warm days.…”

Comparisons of Genetic Variability and Genome Structure Among Mosquito Strains Selected for Refractoriness to a Malaria Parasite

“…Severson and Kassner (1995) than 10 cM apart (Langley et al 1977(Langley et al , 1978. Linkage disequilibrium is more likely to be found for closely linked genes, usually in genome regions less than 100 kb (e.g., Aquadro et al 1992;Langley et al 1988;Macpherson et al 1990;Schaeffer and Miller 1993;Smit-McBride et al 1988), or for functionally related genes (e.g., Baker 1975;Bech-Hansen et al 1983;Van der Loo et al 1987). In a large, random-mating population, linkage disequilibrium should be nearly zero without epistatlc selection or other genetic mechanisms.…”

“…Linkage disequilibrium may result from various genetic mechanisms other than epistatic selection, such as low recombination rate, genetic drift, population subdivision, migration, and hitchhiking among the linked genes (Hedrick et al 1978;Hill and Robertson 1968;Lewinton 1974;Nei 1987;Ohta and Kimura 1969). The rationale for epistatic selection is that natural selection may favor particular combinations of alleles at different loci (coadapted gene complex), therefore the frequencies of these allelic combinations in the gametes are higher than expected from the random combinations of their frequencies (Smit-McBride et al 1988). Population subdivision and migration are not applicable to our mosquito populations.…”