Sexual conflict selects for male and female reproductive characters

Background: Strict genetic monogamy leads to sexual harmony because any tra it that decreases the fitness of one sex also decreases the fitness of the other. Any deviation from monogamy increases the potential for sexual confl ict. Conflict is further enhanced by sperm competition, and given the ubiqu ity of this phenomenon, sexual conflict is rife. In support of theory, expe rimentally enforced monogamy leads to the evolution of sexual benevolence. In contrast, with multiple mating, males evolve traits causing massive fema le fitness reductions when female evolution is restrained. Theory also pred icts increased investment in spermatogenesis when sperm competition risk is high. While this supposition has correlational supper?, cause and effect h as yet to be firmly established. Results: By enforcing monogamy or polyandry in yellow-dung-fly lines, we ha ve shown experimentally that males from polyandrous treatments evolved larg er testes. Furthermore, females from this treatment evolved larger accessor y sex glands. These glands produce a spermicidal secretion, so larger gland s could increase female ability to influence paternity. Using molecular tec hniques, we have shown that, consistent with this idea, males' success as s econd mates is reduced in females from the polyandrous treatment. Neverthel ess, males from polyandrous lines achieve higher paternity during sperm com petition, and this finding further supports the testis evolution patterns. Conclusions: These results provide direct experimental support for macroevo lutionary patterns of testis size evolution. Furthermore, we have shown tha t sperm competition selects for traits likely to be important in sexual con flicts over paternity, a result only previously demonstrated in Drosophila melanogaster.