Indirect effects of selection for maternal investment on fertility traits in Japanese quail (Coturnix japonica).

The sexes are defined by divergent reproductive strategies, and selection often acts in a sex-specific manner. However, because males and females share an autosomal genome, selection acting on one sex is likely to have fitness consequences on the opposite sex. The primary aim of my PhD was to examine how female-specific selection influences male fertility traits, including testis structure, sperm function, and seminal fluid protein abundance, to better understand the co-evolution of male and female reproductive traits. I investigate this question in Japanese quail (Coturnix japonica) that have been artificially selected for divergent female investment in reproduction (measured as egg size relative to body size). A positive genetic correlation between female reproductive investment and male fertilisation success had previously been demonstrated in this study system, but the underlying mechanisms driving this correlation were unknown. I compared male fertility traits including testis structure, sperm form and function, and seminal fluid composition in males from lines where female reproductive investment was either increased or relaxed. A secondary aim of my PhD was to assess whether females selected to invest more in egg size face trade-offs with other aspects of egg quality, such as eggshell structure and integrity. Both aspects of my work have direct applications to commercial animal breeding, as well as broader implications for our understanding of the causes of variation in individual fertility traits and reproductive success.