The costs and benefits of genetic heterogeneity in resistance against parasites in social insects

The occurrence of polygyny and polyandry in social insects has long puzzled evolutionary biologists. If cooperation requires genetic relatedness, how do we explain the occurrence and maintenance of mechanisms that reduce the degree of relatedness among colony members? A much-discussed hypothesis states that genetically diverse colonies are more resistant to parasitism than homogenous colonies because genetic diversity reduces the spread of a disease within a colony. However, as we will argue in this note, a necessary condition for the parasite hypothesis is that genetically heterogeneous colonies have a larger suite of parasites that are capable of infecting them. This implicit relationship is important because it implies that even if the cost per infection is reduced, this may not be sufficient to offset the increased rate of acquiring infections. The advantages of genetic heterogeneity as a defense against parasites thus may not be as big as commonly thought.