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The aim of the work presented in this thesis is to investigate how costly adaptations promoted by sexual selection affect fertility and offspring quality through changes in germline maintenance. Germline maintenance, comprising mechanisms maintaining DNA-integrity and homeostasis within germ cells, is known to be costly and, therefore, may trade-off with other costly reproductive traits that are under sexual selection. However, sexual selection may also act on condition dependent traits that reflect the overall genetic quality of its bearer, in which case sexual selection for high quality mates may lead to improved germline maintenance. Using experimental evolution lines of the seed beetle Callosobruchus maculatus, evolving under three different mating regimes that manipulated the opportunity for sexual and natural selection, I show evidence indicating that sexual selection can lead to improved germline maintenance through selection on condition dependent traits. However, I also found evidence for the alternative hypothesis, suggesting that when sexual selection is much stronger than natural selection it may lead to excessive investment into mating traits that trade-off with and reduce germline maintenance. We present an RNA expression analysis suggesting 18 candidate genes responding to DNA-damage and sociosexual interactions that may be involved in trade-offs between sexual selection and germline maintenance. I also found that the fertility of males and females that evolved under intense sexual selection was more sensitive to heat stress, and male sensitivity of fertility to heat stress was genetically correlated to sperm competitive ability. This suggests a trade-off between male postcopulatory reproductive success and the thermal sensitivity of fertility. The increased sensitivity to heat was also reflected in the fertility of females, suggesting that female heat tolerance may have evolved via genetic correlations with sexually selected male reproductive traits. The work presented in this thesis shows that sexual selection indeed affects germline maintenance. Sexual selection can increase germline maintenance through selection on condition dependent traits. But at the same time, traits under sexual selection can trade off with aspects of germline maintenance. If traits evolved under sexual selection and only weak constraints by natural selection, evolved allocation shifts in response to sexual selection can lead to deleterious repercussions when stressful environmental conditions increase demands on germline maintenance. The results presented in this thesis highlight important aspects of how sexual selection affects condition dependent germline maintenance with significant implications for the maintenance of genetic variation, adaptive processes, and mate choice processes in species under sexual selection.