Genetic effects on sperm design in the zebra finch.

Sperm design and function are important determinants of male reproductive success and are expected to be under strong selection. The way that spermatozoa phenotypes evolve is poorly understood, because there have been few studies of the quantitative genetics of sperm. Here we show, in the zebra finch Taeniopygia guttata, an extraordinary degree of inter-male variation in sperm design that is independent of sperm swimming velocity. A quantitative genetics study using data from over 900 zebra finches in a complex breeding experiment showed that sperm head, mid-piece and flagellum length are heritable, that negative genetic correlations exist between sperm traits, and that significant indirect (maternal) genetic effects exist. Selection on the zebra finch sperm phenotype may be low because sperm competition is infrequent in this species, and this, in combination with negative genetic correlations and maternal genetic effects, may account for the variation in sperm phenotype between males. These results have important implications for the evolution of sperm in other taxa.

Selection and utilization of spermatozoa in the reproductive tract of the female zebra finch Taeniopygia guttata.

The numbers and proportion of spermatozoa reaching different parts of the female reproductive tract after a single natural insemination were investigated in zebra finches Taeniopygia guttata. The number of spermatozoa transferred during a single, natural copulation was estimated by comparing the number of spermatozoa in the seminal glomera of males that had performed a single copulation with control males. The mean number of spermatozoa per ejaculate was 5.8 x 10(6) +/- 1.80 x 10(6) SEM. The mean number of spermatozoa stored in the sperm storage tubules in the uterovaginal junction following a single, natural insemination was 6027 +/- 1874, 0.104% of those inseminated. The mean number of spermatozoa reaching the infundibulum and trapped on the perivitelline layer of all eggs of the clutch after a single copulation was 45.6 +/- 9.18 and a further 36 penetrated the perivitelline layer of the ovum, i.e. 82 in total (1.4% of the spermatozoa in the sperm storage tubules and 0.001% of spermatozoa in the ejaculate). Female zebra finches that completed a natural breeding cycle with a mean of 12 copulations had 404 +/- 111 spermatozoa trapped on the perivitelline layer of all eggs of the clutch, and an estimated further 173 spermatozoa penetrated the perivitelline layer. A smaller proportion of spermatozoa was trapped on the perivitelline layer of zebra finch eggs, than in chicken or turkey eggs.