Inbreeding depression and multiple regions showing heterozygote advantage in Drosophila melanogaster exposed to stress.

Recent studies that reveal a correlation between heterozygosity and fitness in natural populations have rekindled interest in whether balancing selection is widespread or an evolutionary oddity. We therefore quantified heterozygote advantage at 12 microsatellite markers in both inbred and outbred crosses of Drosophila grown under different forms of environmental stress. As expected, inbreeding depression reduces fitness relative to the outbred controls. In addition, many loci exhibit heterozygote advantage over and above any effect due to inbreeding, with approximately 30% of markers showing an effect in any given culture condition and approximately 75% of markers showing an effect in at least one of the four culture conditions. To explore the extent of linkage disequilibrium surrounding these loci we further typed four new markers close to each of the three strongest hits. We find a pattern where the extent of heterozygote excess tends to decline to nonsignificance within around 1.5 megabases (Mb) either side of the original hit. Crude extrapolation suggests 12 genes or regions experience detectable levels of heterozygote advantage in any one condition and as many as 25 overall. Thus, balancing selection is widespread and is likely to play an important role in maintaining genetic variability.