Sperm-pathogen interactions and the evolution of ejaculate antimicrobial defences in passerine birds

Clinical studies in humans have long reported an association between bacterial infections and male infertility. The potential implications of bacteria for male fertility in wild animals, however, has been largely neglected. This project demonstrated that the common bacteria, E. coli, can significantly reduce sperm motile performance in house sparrows. Further, using state-of-the-art genetic methods, we found that the ejaculates of male house sparrows harbour complex and diverse bacterial communities, including a number of potentially pathogenic species. Work undertaken in this project also confirmed that the ejaculates of wild birds exhibit antibacterial activity, and that males with higher antibacterial activity in their ejaculates also have higher sperm quality. As such, this work provides support for the idea that male birds have evolved antibacterial defenses in their ejaculate to protect sperm from bacterial-induced damage and reductions in sperm performance and thus protect male fertility. In addition, this project uncovered the molecular basis of antibacterial activity in avian ejaculates, finding a number of proteins that function in antibacterial defense and immunity in both sperm and seminal fluid (i.e. the non-sperm component of the male ejaculate). Furthermore, this work showed that these proteins can have critical consequences for the fertilisation process and may contribute to fertilisation barriers between closely related species, thus contributing to the process of speciation.

The project has generated new knowledge of the effects of bacteria on sperm quality and the implications of these effects for male reproductive health and fertility. Additionally, the project has provided new information on the molecular basis of immune defenses in male semen. The potential impacts of these findings are diverse. Most notably, knowledge generated by this research will likely be of interest to those studying evolutionary medicine and reproductive microbiomes and male infertility in humans. The knowledge generated from this research may provide conceptual and methodological advances to the researchers. Thus, there is significant potential for cross-fertilization of research ideas and methods among research fields. This project has also contributed to the training and education of a number of researchers, including 3 undergraduate researchers, a masters student (who is now pursuing a PhD in the Netherlands), a PhD student, and a research technician.

Organisms are constantly exposed to pathogens, and the ability of individuals to combat microbial attack is an important component of fitness. Sperm cells are not immune to microbial exposure; sperm encounter bacteria in the testes and ejaculate, during m ating and in the female reproductive tract. Bacteria can cause reductions in sperm quality and compromise male fertility. Moreover, ejaculate-borne bacteria can be transferred during mating (i.e. sexually transmitted diseases; STDs), with negative consequ ences for female fertility. Ejaculate-borne bacteria are thus predicted to generate intense selection for the evolution of antibacterial substances in seminal fluid that minimise bacterial-induced sperm defects and limit the transmission of STDs. However, little is known about the evolutionary consequences of ejaculate-borne bacteria and STDs in non-human taxa. This project brings together expertise in avian ecology, sperm biology, microbiology, immunology and proteomics to investigate the role of bacteri a in the evolution of avian seminal fluid and examine the consequences of STDs for the evolution of male secondary sexual traits. Our aims are to: 1) determine the consequences of ejaculate-borne bacteria for male sperm quality and paternity success, 2) t est the hypothesis that male traits reflect a male's current ejaculate bacterial load or antibacterial capacity, 3) characterise the immunity proteome of seminal fluid and test the functionality of antimicrobial substances, 4) examine the relationship bet ween ejaculate and general immunity, 5) quantify the impact of positive selection on immunity components of the seminal fluid proteome, and 6) examine the role of bacteria-driven ejaculate evolution in ejaculate-female interactions. As such, the proposed study will offer novel insight into pathogen-mediated selection and ejaculate evolution, as well as bacterial-induced fertility problems in domestic species and endangered species in conservation programs.