Section 7
Chapter 6,051

Over compensation at an enzyme locus in drosophila pseudoobscura

Tosic, M.; Ayala, F.J.

Genetical Research 36(1): 57-68


Accession: 006050738

The hypothesis of overcompensation (i.e., whether polymorphic populations exploit limiting environmental resources better than populations uniform for a single genotype) was studied. Overcomposition is an ecological consequence of some models of frequency-dependent selection. Whether overdominance exists at the Mdh-2 (malate dehydrogenase-2) locus in D. pseudoobscura was investigated. Two types of experimental populations were established: low-variability populations, in which all flies in a culture were offspring from only 2 laboratory strains and high-variability populations, in which the flies in a culture were derived from 20 different strains. The overall degree of individual heterozygosity is the same in both types of populations. Three kinds of populations with respect to the Mdh-2 locus are established within each type; 2 are homozygous for either the 100 or the 112 allele, the 3rd is heterozygous. A 4th kind of population exists among the high-variability populations; namely, populations in which all 3 Mdh-2 genotypes are present. The experiments are done at 2 densities; 1 quasioptimal, the other highly competitive. Populations with high overall levels of genetic variation consistently produce more flies than low-variability populations. The differences are significant at the low, but not at the high, density. Populations polymorphic for the Mdh-2 locus generally produce more flies than populations having only 1 Mdh-2 genotype. At high density, the Mdh-2 polymorphic populations have greater productivity than populations with anyone of the 3 genotypes, and the differences are statistically significant when the polymorphic populations are compared with either one of the 2 homozygotes or with the average of all 3 genotypes. In brief, overcompensatory effects.sbd.which may account for frequency-dependent selection.sbd.are observed in the experiment and may be a common phenomenon in nature. Populations in which all individuals are heterozygous at the Mdh-2 locus produce in every case more flies than populations with only homozygotes for one or the other allele. The superiority of the heterozygotes is statistically significant for all comparisons at low density, but at high density it is statistically significant for the comparison with the homozygote rarer in nature and only in low variability populations.

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