The interplay of environmental and social factors influences balancing selection: 6Pgdh in bulb mites

Abstract Balancing selection, the process by which multiple alleles are maintained in a population, can occur through genotype by environment interactions. The polymorphism of 6Pgdh in bulb mites involves two alleles, S and F, which have opposing effect on male reproductive success. While S-bearing males have a reproductive advantage and the S allele fixes in the laboratory populations, polymorphism is regularly observed in natural populations. We hypothesise that 6Pgdh is under balancing selection in bulb mites, driven by fluctuations of temperature and sexual selection intensity. This study uses experimental evolution to track allele frequency changes over 13 generations under different temperatures (12 °C and 18 °C) and sexual selection intensities (equal and female-biased sex ratios). The results show a significant increase in F allele frequency under higher sexual selection intensity at 12 °C. This suggests that individuals with the F allele gain a reproductive advantage at lower temperatures, and 6Pgdh polymorphism is maintained through a three-way interaction genotype x abiotic (temperature) x social environment. However, in another experiment we did not find significant associations between male genotype and reproductive success across different temperatures. The complex interplay between environmental factors and social dynamics in maintaining genetic polymorphism highlights that multiple factors can interact in driving balancing selection.