Magellanic penguins have a high level of variation in genes associated with the ability to fight infectious disease, but a recent study found that the mechanism the penguins use to ensure that diversity is far from black and white. A recent study published in the Journal of Heredity tested whether the significant diversity in the Major Histocompatibility Complex (MHC) genome region observed in these birds is attributable to mate choice or genetic selection based on disease exposure. Surprisingly, they found no direct evidence for disassortative mating based on the genotypes of the breeding pairs. Incidence of shared alleles between males and females in breeding pairs was not significantly different from what would be expected by chance.
But heterozygosity was found to be associated with increased fitness of adults, as heterozygous females hatched significantly more eggs and fledged significantly more chicks than homozygous females (in fact, none of the homozygous females that hatched eggs actually fledged any chicks). This finding suggests that a mechanism for balancing selection is at work in maintaining MHC diversity, even if it is not promoted by disassortative mating.