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Abstract:
Mate choice, based on secondary sexual traits is a common phenomenon in many animal species. These traits are exhibited by one of the partners, usually the male. If the traits reflect the ability of a mating partner to contribute direct or indirect benefits to the offspring, individuals (usually females) are able to increase their fitness by choosing these traits. Parasite load and parasite resistance are only two aspects of an animal that can be signalled by such traits. To investigate the effect of parasites and the MHC (Major Histocompatibility Complex) genetic background that is important for parasite resistance, on different male traits, I used the three-spined stickleback (Gasterosteus aculeatus) as a model organism. In most cases, invasion of a host by a parasite is followed by a reaction of the host’s’ immune system. Distinct parasite species have a different impact on their host which might result in different physiological effects. In Chapter I it was shown, in a controlled infection experiment, that two nematode parasites (Camallanus lacustris and Anguillicola crassus) differed in prevalence in their stickleback host. The corresponding immune reaction of the fish was also found to be different, which generally could result in alterations of male traits. The immune reaction of the host does not only depend on parasite species, but also on its own immuno-genetic background. The specific, adaptive immune system in vertebrates is represented by MHC alleles (class I and class IIB). Do male traits also signal direct information about the MHC background of the fish irrespective of parasite load and do females base their mating decision on such traits? In Chapter II two sexually selected male traits, the red breeding colouration and nest quality were correlated to MHC allele number. We found that males with intermediate number of MHC class II alleles built nests that should be more attractive for females, while redness correlated positively with number of MHC I alleles. Visual traits such as red colouration or nest quality seem to reflect parasite resistance of male sticklebacks. Laboratory experiments showed that females also choose mating partners for odour cues that are related to the MHC genetic background. If visual and olfactory cues do not carry the same
information, e.g. different aspects of a males’ condition, this raises the question of female preference in situations where visual and olfactory traits are available at the same time. Through mate choice experiments (Chapter III) it was shown that, in this situation, females seem to use a combination of visual and olfactory traits. While they choose the redder male in the visual test and the best fitting one in the olfactory test, no choice for either the one or the other was found when both choice situations were combined. The females then preferred males for their overall condition, which was not reflected solely by either of the male traits investigated. In order to test whether the results from my laboratory experiments are applicable to the natural situation in the field, outdoor enclosures were stocked with fish of known MHC background (Chapter IV). Parenthood analysis was performed for eggs collected from the males’ nests. Reproductive success was estimated for males and females in combination with measurements of male traits and parasite load. Since it was not possible to identify a certain male trait the females always preferred, choice for a combination of traits also seems most probable in the field.
