Sexual selection plays an important role in the evolution of animals. Today we already know that it takes place not only before copulation (precopulatory sexual selection), but also after copulation. This type of sexual selection is called postcopulatory sexual selection, and occurs mainly in promiscuous species, where females mate with multiple males. In this case, sperm competition occurs in the female reproductive tract. To increase the likelihood of their reproductive success, males began to develop surprisingly diverse sperm adaptations at the morphological, physiological, or behavioural levels. These adaptations often affect sperm velocity (motility), which is a key factor for successful egg fertilization. However, the result of reproductive success can also be influenced by females, who may prefer sperm with a certain phenotype in the process of cryptic female choice, and thus, for example, obtain better genes for offspring. In some species, females even can have the ability to sort and store sperm in specialized organs in which the sperms are nourished for some time, and then used to fertilize the egg. Even in this case, the storage of sperm is often affected by sperm morphology. Due to these mechanisms of postcopulatory sexual selection, sperm are under strong selection pressure, which can...

Sperm competition is a widely-recognized and powerful selective force. Male accessory glands are organs found across animal taxa that can influence sperm performance, and thus may be selected for in competitive contexts. In fishes, these organs are in fact rare, but display great diversity in form and function across species. Although the accessory gland is known to play a role in mate attraction, parental care, fertilization, or post-copulatory competition in a few select species, the role of this organ remains a mystery in most species. Many fishes with accessory glands also exhibit alternative reproductive tactics (ARTs), which add an extra layer of complexity to how species respond to sperm competition. Because males of different ARTs typically experience different levels of sperm competition risk, it’s possible they may differentially invest in accessory glands to overcome this competition. In this thesis, I used the plainfin midshipman fish (Porichthys notatus), a species with both ARTs and an accessory gland, to experimentally investigate the role of the accessory gland in sperm competition and uncover how this organ may differ between ARTs. Over a two-year period, I studied tactic-specific investment in the accessory gland in fish from the beaches of British Columbia. I also examined the effects of seminal fluid, produced in part by the accessory gland, on sperm performance and morphology. I found that males adopting the ‘guarder’ male tactic invested more in one region (the lobules) of this organ, while males adopting the smaller ‘sneaker’ male tactic invested more the other region of the gland (the nodes). Using data collected over five years, I found that guarder males also invested more in their whole accessory glands. Additionally, I report that sperm swam faster in the presence of seminal fluid, and seminal fluid increased sperm head size in both male tactics and increased midpiece size in guarder males. These results suggest that the plainfin midshipman accessory gland may have dual functions, one of which may be to aid sperm competitive ability through enhancements in swimming speed and potentially more successful sperm morphology. Taken together, the results of my thesis improve our knowledge of the role of non-sperm components like seminal fluid and the accessory gland in sperm competition, and demonstrate how species with ARTs can have varying physiological responses to such competition. Only a handful of studies have considered the effects of seminal fluid on sperm performance. By examining sperm competition in a more biologically relevant way and incorporating the effects of a little-studied organ that impacts sperm competition, we should be able to more generally and accurately appreciate the dynamics of post-copulatory competition and fertilization. / Thesis / Master of Science (MSc)