Failure to transfer sperm by sterile hybrids is caused by a testis’s blockage at its basal end
Visual inspection of the testes identified a region towards its basal end that looked enlarged and more oval in sterile hybrid males than fertile parentals (Figure 3). Because this structure contains individualized sperm, we refer to it as the seminal vesicle. In 5 days-old fertile males, a pinch in the testes often defines the apical end of what we identified as the seminal vesicle (i.e. mature sperm storage) with the basal end almost continuously extending into a tubular structure that we refer to as the vas deferens (Figure 3A). The basal end of the vesicle can be harder to identify in fertile males, in some a subtle torsion or pinch and the presence of a darker mass of sperm can be used as a landmark (Figure 3A) while in others the basal end is more distinguishable (Figure 3B). In both cases, the vesicle continues into a thicker tubular structure (Figures 3A and B) compared to sterile males (Figure 3C). In sterile hybrids, the seminal vesicle is more clearly distinguishable by an apical and basal pinch, and it is more oval and enlarged (Figure 3C) compared to the more elongated form found in the fertile testes. We used the landmarks described above to delimit this structure and to provide approximate measurements of its area in fertile and sterile males of different age. Analysis of variance showed significant differences in the size of the seminal vesicle between fertile parental males and sterile hybrids at five (F1,59= 148.6; P< 0.001), and ten-days of age (F1,59= 379.5; P< 0.001) (Table 3). When the analysis is partitioned by population types (Uruguay, Guadeloupe, Saint Vincent and Puerto Rico), Scheffe’s post-hoc tests showed that parental populations are significantly smaller than sterile male hybrids at all ages and not significantly different from each other. In sterile hybrids the seminal vesicle is approximately two times larger than in fertile males and grows as males ages (Table 3; Figures 3D and E).
Closer examination of the seminal vesicle and vas deferensallowed us to find some interesting structural abnormalities in the sterile hybrids that might at least partially explain how the enlargement of the seminal vesicle might affect the transfer of sperm: 1) the seminal vesicle is more vascularized in sterile males, with sperm present but displaced to the edges (Figure 4A), 2) sperm is not found in the vas deferens of sterile hybrids (passing a pinch at the base of the seminal vesicle, which is enlarged in sterile males) suggesting that the enlarged and vascularized structure impedes proper movement of sperm into the vas deferens (Figure 4B), and 3) Some level of atrophy of the vas deferens in sterile hybrid males is apparent by the observation of its irregular shape and the lack of lumen suggesting collapsing and obstruction of the duct (Figure 4C).