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).