Abstract
Spermatogenesis is a costly process that is expected to be under selection to maximise sperm quantity and quality. Testis size is often regarded as a proxy measure of sperm investment, implicitly overlooking the quantitative assessment of spermatogenesis. An enhanced understanding of testicular function, beyond testis size, may reveal further sexual traits involved in sperm quantity and quality. Here, we first estimated the inter-male variation in testicular function and sperm traits in red deer across the breeding and non-breeding seasons. Then, we analysed the relationships between the testis mass, eight parameters of spermatogenic function, and seven parameters of sperm quality. Our findings revealed that the Sertoli cell number and function parameters vary greatly between red deer males, and that spermatogenic activity co-varies with testis mass and sperm quality across the breeding and non-breeding seasons. For the first time in a seasonal breeder, we found that not only is the Sertoli cell number important in determining testis mass (r = 0.619, p = 0.007 and r = 0.248, p = 0.047 for the Sertoli cell number assessed by histology and cytology, respectively), but also sperm function (r = 0.703, p = 0.002 and r = 0.328, p = 0.012 for the Sertoli cell number assessed by histology and cytology, respectively). Testicular histology also revealed that a high Sertoli cell number per tubular cross-section is associated with high sperm production (r = 0.600, p = 0.009). Sperm production and function were also positively correlated (r = 0.384, p = 0.004), suggesting that these traits co-vary to maximise sperm fertilisation ability in red deer. In conclusion, our findings contribute to the understanding of the dynamics of spermatogenesis, and reveal new insights into the role of testicular function and the Sertoli cell number on testis size and sperm quality in red deer.
Highlights
Spermatogenesis is a complex developmental process by which diploid spermatogonia generate haploid spermatozoa through a series of cyclic and highly coordinated events known as spermatocytogenesis, meiosis, spermiogenesis, and spermiation [1]
To the best of our knowledge, this is the first report to show that the Sertoli cell number and spermatogenic competence covary with testis mass and sperm quality throughout different phases of gonadal activity in red deer
Consistent with what was reported in other species, our findings support that the Sertoli cell number is important in determining testis size and sperm production in red deer
Summary
Spermatogenesis is a complex developmental process by which diploid spermatogonia generate haploid spermatozoa through a series of cyclic and highly coordinated events known as spermatocytogenesis, meiosis, spermiogenesis, and spermiation [1]. Whereas spermatogonia act as a reservoir for the succeeding bouts of spermatogenic activity [3], Sertoli cells have the role of supporting, nursing, and regulating germ-cell function, permitting them to migrate from basal to apical positions as spermatogenesis proceeds [4]. Sertoli cells play a central role in the development of a functional testis because without their physical and metabolic support, germ-cell differentiation, meiosis, and transformation into spermatozoa would not occur [5, 6]. Rajak et al [9] found that, in adult bulls, the Sertoli cell index was positively associated with ejaculate sperm concentration, mass activity, individual motility, viability, and membrane integrity. The relationship between the Sertoli cell traits and sperm quality has so far received no empirical testing in seasonal breeders, these are valuable models for the study of seasonal regulation of testis function [10]
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