The relationships between testes asymmetry, seminiferous composition and the quality and quantity of sperm produced in a precocial bird

Abstract

Theory assumes that postcopulatory sexual selection favours increased investment into testes size, as larger testes produce ejaculates that provide a competitive advantage when rival males compete for fertilisation. However, the relationship between relative increases in testes size with the organisation of sperm-producing tissue, and how such changes influence sperm quality and quantity is not fully understood. Male Japanese quail (Coturnix japonica) originating from lines artificially selected for high female reproductive investment experienced higher fertilisation success, as well as a relatively larger left testis than males from low reproductive investment lines. The aim of this study was to determine the origin of this increased fertility from a morphological perspective. For males from both lines and in both testis, we measured the proportion and absolute amount of seminiferous tissue, as well as the quantity and quality of sperm within the sperm reserves, including sperm length and proportions that were alive and morphologically normal. The left testis of high line males had higher proportions of interstitial tissue, but not seminiferous tissue, and did not produce different sperm morphologies that could explain the enhanced fertilisation success. On the contrary, the right testis had higher proportions of seminiferous tissue and produced more alive sperm. There was no difference in sperm quantity or proportions of sperm with normal morphology in either testis of males from divergent lines. Independently of line origin, the right testis contained sperm with larger tails, total lengths, and shorter ratios of the head : tail, indicating a specialisation to produce faster sperm. Overall, we found no clear difference in the quality or quantity of sperm produced by high line males that could explain their reproductive success. However, because ejaculates are complex, interactive, multivariate traits, fertilisation success could have been influenced by other traits not measured here, such as the composition of copulatory fluids, functions of somatic cells in the testes, provisioning against oxidative stress or the regulation of sperm use by the females. Altogether, these results highlight unusual relationships between testes size with testes and sperm morphology and demonstrate that relatively larger testes do not necessarily have more seminiferous tissue or produce more, or better quality sperm.