Data supporting: Molecular, behavioural and morphological comparisons of sperm adaptations in a fish with alternative mating tactics

In species with alternative reproductive tactics, there is much empirical
support that sneaker males have larger testes size and greater sperm
numbers. However, support for higher sperm performance by sneakers is
inconsistent. We used the sand goby (Pomatoschistus minutus) to test
whether sperm performance differed between breeding-coloured males (small
testes, but large mucus-filled sperm-duct glands; builds a nest lined with
a sperm-containing mucus, provides care) and sneaker-morph males (no
breeding colouration, large testes, rudimentary sperm-duct glands; no
nest, no care). We compared motility (proportion motile sperm), velocity
and longevity of sperm, between the two morphs. Furthermore, we compared
gene expression of testes, and tested if the sperm-duct gland contents
affected sperm performance, and if sperm morphometrics differed between
male morphs. We found a clear difference in gene expression of testes
between the male morphs with 109 transcripts differentially expressed
between the morphs. Notably, several mucin genes were upregulated in
breeding-coloured males and two ATP-related genes were upregulated in
sneaker-morph males. There was some evidence of higher sperm velocity in
sneaker-morph males, but no difference in sperm motility. Presence of the
sperm-duct gland contents significantly increased sperm velocity, but
equally so for the two morphs. The same was true for sperm motility
although the difference was not significant. The sand goby has remarkably
long-lived sperm, with only small or no decline in motility and velocity
over time (5 min vs. 22 hours), but again, this was equally true for both
morphs. Sperm length (head, tail, total) did not differ between morphs,
and did not correlate with sperm velocity for either morph. Thus, other
than a clear difference in testes gene expression, we found only modest
differences between the two male morphs, confirming previous findings that
high sperm performance as an adaptation to sperm competition is not a
primary target of evolution.