Selection for class II Mhc heterozygosity by parasites in subterranean mole rats

Mhc organization and polymorphism have previously been studied²⁶ in the four chromosomal species of the Spalax ehrenbergi superspecies in Israel, serologically, and at the DNA, RFLP and sequence levels of class I and class II genes. Here we demonstrate that the observed heterozygosity of Mhc class II genes Pα₁ with 11 alleles, and Qβ, with at least 14 alleles, is positively and significantly correlated with infectivities of ectoparasites (gamasid mites)¹⁷ and endoparasites (helminths)¹⁸. Mhc heterozygosity is highest in the most infected area, which is in the most humid-warm region of the superspecies range, or where two zoogeographic regions overlap. We conclude that the evolutionary forces responsible for the Mhc class II two-gene polymorphisms include selection for increased heterozygosity as a defense strategy against ecto- and endoparasite infections.