Thitipan Meemongkolkiat, Jane Allison, Frank Seebacher, Julianne Lim, Chanpen Chanchao, and Benjamin P. Oldroyd
In honeybees there are three alleles of cytosolic malate dehydrogenase: F, M and S. Allele frequencies are correlated with environmental temperature, suggesting that the alleles have temperature-dependent fitness benefits. We determined the enzymic activity of each allele across a range of temperatures in vitro. The F and S alleles have higher activity and are less sensitive to high temperatures than the M allele, which loses activity after incubation at temperatures found in the thorax of foraging bees in hot climates. Next, we predicted the protein structure of each allele and used molecular dynamics simulations to investigate their molecular flexibility. The M allele is more flexible than the S and F alleles at 50°C, suggesting a plausible explanation for M’s loss of activity at high temperatures, and has the greatest structural flexibility at 15°C, suggesting that it can retain some enzyme activity at cooler temperatures. MM bees recovered from two hours of cold narcosis significantly better than all other genotypes. Combined, these results explain clinal variation in MDH allele frequencies in the honeybee at the molecular level.