Sex, parasites and resistance--an evolutionary approach.

Immune systems are among the most diverse biological systems. An evolutionary arms race between hosts and rapidly evolving pathogens is supposed to be a reason for this diversity, and might explain why most eukaryotic hosts and parasites reproduce sexually. In this review, I will focus on possible benefits of sexual reproduction in hosts and parasites, using a model system consisting of a tapeworm and its two intermediate hosts, copepods and sticklebacks. We found that the hermaphroditic tapeworms can increase their infection success by reproducing sexually with a partner (outcrossing), instead of reproducing alone. The defence system of the copepods provides highly specific discrimination of antigenic characteristics of the tapeworms. This supports the finding that tapeworms benefit from outcrossing, but contradicts the conventional notion that the immune system of invertebrates, in contrast to vertebrates, is not able to react with specificity. Finally, sticklebacks seem to benefit from optimal diversity in their specific immune system. Previous studies showed that female sticklebacks prefer mates, which sire offspring with an optimal diversity in the MHC (genes involved in antigen presentation). We now found that these individuals suffer less from tapeworm infection. Furthermore, they are able to reduce the expression of an unspecific immune trait, thereby possibly avoiding harmful side effects of a highly activated, unspecific immune system.