The evolutionary consequences of sperm senescence in Drosophila melanogaster
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Sperm senescence, a decline in sperm quality caused by male ageing and by sperm ageing before or after copulation, may have fitness costs manifested as infertility or lowered genetic quality of offspring. This thesis tested the distinct evolutionary roles of sperm senescence using a laboratory-adapted population of Drosophila melanogaster. We developed a practical approach to avoid confounding male age with sperm age by standardizing pre-copulatory sperm age and mating history in young and old male age groups. Applying this approach, we documented sperm senescence in D. melanogaster and discussed its potential evolutionary importance. First, ageing males declined in fitness as evidenced by the reduction in fertilization potential of their ejaculates but not by decreased offspring fitness (the ability that a fly can survive to adulthood, successfully mate and produce viable offspring). This suggests a decline in the quality or quantity of seminal fluid or spermatozoa, with no decline in the genetic quality of sperm that actually fertilized ova. Second, post-copulatory sperm senescence has significant negative impacts on offspring fitness, indicating degraded genetic integrity of the spermatozoa stored in females. In both cases, male ageing and sperm ageing had similar fitness impact on male and female offspring, different from what has been suggested by previous work. In addition, We demonstrated that female fecundity, fertility, and length of the fertile period after a single mating were positively associated with the concentration of yeast in their food, and were negatively associated with the duration of yeast restriction in their diet, which suggested that sperm storage is affected by the nutritional status of the females. By revealing the significance of sperm senescence on male and female fertilization success and the fitness of the next generation, this thesis sheds light on a number of evolutionary and applied issues, and provokes new questions for future research on sperm senescence.