Sexual selection in the ant Leptothorax gredleri

L. gredleri is an ant species that is mating by female calling. Virgin queens attract males with a droplet of a sexual pheromone in the vicinity of their maternal nest. Both sexes are expected to be choosy because females store the semen in their spermatheca throughout their whole lives and do not remate after this initial mating period. Males are short-lived and survive only as sperm in the spermathecae of queens. Furthermore, males possess only a fixed amount of sperm, since their testes degenerate by the time they reach sexual maturity. Mating is therefore an important event that results in a lifetime investment for both sexes. Haplodiploidy and complementary single-locus sex allocation bears the risk of producing sterile diploid males, especially with sib-mating. After the mating, queens have to found a new colony or they may return into their maternal nest, where they might have the possibility to inherit the colony. In any case, queens generally have to invest first into the production of workers and produce sexual offspring only later. This constellation restricts the possibilities for the evolution of sexual conflict and antagonistic coevolution. Males should avoid harming their mates, because this has a direct negative impact on their own reproductive success. Thus, rapidly evolving accessory gland proteins, as observed in Drosophila, are not expected for L. gredleri. The studies showed that males, workers and virgin queens have a distinct cuticular hydrocarbon pattern. Like workers, males show additionally a colony specific pattern which could be used by queens to avoid mating with nestmates. Although mating experiments showed that mating occurred significantly more often between foreign individuals than between nestmates, discrimination was only weak and mating between nestmates was not a rare event. After the mating, queens change their cuticular hydrocarbon pattern within only 30 min. This might explain why additional mating of queens was mainly observed only few minutes after their first copulation. The change of the hydrocarbon pattern was not caused by males marking females during copulation, since it was not detectable directly after the mating but is produced by the queens afterwards. The pattern shifted towards a lower relative amount of alkenes and branched alkanes and a higher relative amount of linear alkanes. This could help queens to be less conspicuous for males and also for nestmates when re-adoption into their maternal nests occurs. Males could discriminate in this way against already mated queens to invest more efficiently their sperm in virgins. During copulation, sperm is transferred with a gelatinous substance that might serve as spermatophore. The ejaculate is placed into the bursa copulatrix with the sperm cells in the tip of the spermatophore at the entrance of the spermathecal duct. Sperm cells have to migrate into the spermatheca which takes approximately 3 hours, because at the end of the spermathecal duct a very narrow constriction restricts the amount of sperm that passes. The sperm cells therefore have to pass this section virtually one by one. Whether this enables the queens to exert cryptic female choice and whether the spermatophore also serves as mating plug for males are questions for subsequent studies. Two-dimensional SDS page investigations of the variability of accessory gland protein patterns in L. gredleri revealed that accessory gland proteins are rather conserved compared to thorax protein pattern variability. Also when compared to some few samples of closely related ant species, only marginal differences could be observed between the accessory gland protein patterns. Accessory gland proteins seem to be rather conserved, contrary to observations of Drosophila, where antagonistic coevolution might be the reason for rapid evolutionary change and a resulting high variability. A representational difference analysis (RDA) of the gene expression in male accessory glands of L. gredleri resulted in 20 unique sequences (US). Searches in nr databases revealed that nine of these sequences did not significantly match to any known sequences of another species. Whether these represent rapidly evolving genes has to be checked. However, one sequence was already recorded to be differentially expressed in Drosophila females after mating. This sequence was annotated also in the context of immune defence and is therefore expected to be a conserved gene and crucial for insect reproduction. Altogether, the results of this thesis provide further support for the notion that sexual selection in L. gredleri ants might be dominated rather by sexual cooperation than sexual conflict. However, research on sexual selection in ants is still in its infancy. Although sexual conflict was not discovered in this work on L. gredleri, a final proof for sexual cooperation requires also further verification.