Variable sperm size and motility activation in the pipefish, Syngnathus abaster; adaptations to paternal care or environmental plasticity?

Like seahorses, some of the closely-related pipefish species (Family Syngnathidae) incubate their eggs within a male brood pouch. This has contributed to considerable confusion about sperm transfer mechanisms to the eggs; some authors have reported that ejaculates are released directly into water before they reach the eggs, while others have suggested that eggs are fertilised using spermatozoa deposited directly into the brood pouch via an internal sperm duct. Here we present anatomical evidence from the freshwater pipefish, Syngnathus abaster, showing not only that direct sperm deposition into the pouch is impossible, but that spermatozoa must somehow travel a significant distance (>4 mm) outside the body of the male, to reach and fertilise eggs in the pouch. We have also used several putative sperm-activating solutions to identify the type of environment most conducive to sperm activation. Spermatozoa released from the testis were active for a brief period (<5 min) in water or 150 mm saline, but showed prolonged (>25 min) motility in ovarian fluid. This suggests that spermatozoa are released into a mixture of ovarian fluid and eggs while the male and female are in close contact. Our data also suggest that the fertilisation mechanism is highly efficient (sperm : egg ratio <200 : 1) even though this pipefish species produces dimorphic spermatozoa (with long and short flagellae). The shorter (<40 microm) morphotypes were not capable of motility activation, and are therefore probably incapable of fertilisation. If so, the sperm : egg ratio reported here would represent an overestimate.

Molecular basis of sex and reproductive status in breeding zebrafish

The zebrafish ( Danio rerio) is used extensively as a model species for studies on vertebrate development and for assessing chemical effects on reproduction. Despite this, the molecular mechanisms controlling zebrafish reproduction are poorly understood. We analyzed the transcriptomic profiles of the gonads of individual zebrafish, using a 17k oligonucleotide microarray, to define the molecular basis of sex and reproductive status in sexually mature fish. The gonadal transcriptome differed substantially between sexes. Among the genes overexpressed in females, 11 biological processes were overrepresented including mitochondrion organization and biogenesis, and cell growth and/or maintenance. Among the genes overexpressed in males, six biological processes were overrepresented including protein biosynthesis and protein metabolism. Analysis of the expression of gene families known to be involved in reproduction identified a number of genes differentially expressed between ovaries and testes including a number of sox genes and genes belonging to the insulin-like growth factor and the activin-inhibin pathways. Real-time quantitative PCR confirmed the expression profiles for nine of the most differentially expressed genes and indicated that many transcripts are likely to be switched off in one of the sexes in the gonads of adult fish. Significant differences were seen between the gonad transcriptomes of individual reproductively active females reflecting their stage of maturation, whereas the testis transcriptomes were remarkably similar between individuals. In summary, we have identified molecular processes associated with (gonadal) sex specificity in breeding zebrafish and established a strong relationship between individual ovarian transcriptomes and reproductive status in females.

Wild intersex roach (Rutilus rutilus) have reduced fertility

Endocrine-disrupting chemicals, known to be present in the environment, have great potential for interfering with reproductive health in wildlife and humans. There is, however, little direct evidence that endocrine disruption has adversely affected fertility in any organism. In freshwater and estuarine fish species, for example, although a widespread incidence of intersex has been reported, it is not yet known if intersexuality influences reproductive success. The purpose of this study was, therefore, to determine gamete quality in wild intersex roach (Rutilus rutilus) by assessing sperm characteristics, fertilization success, and ability to produce viable offspring. The results clearly demonstrate that gamete production is reduced in intersex roach. A significantly lower proportion of moderately or severely feminized fish (17.4% and 33.3%, respectively) were able to release milt compared with normal male fish from contaminated rivers (in which 97.6% of the males were able to release milt), reference male fish (97.7%), or less severely feminized intersex fish (experiment 1: 85.8%, experiment 2: 97%). Intersex fish that did produce milt produced up to 50% less (in terms of volume per gram of testis weight) than did histologically normal male fish. Moreover, sperm motility (percentage of motile sperm and curvilinear velocity) and the ability of sperm to successfully fertilize eggs and produce viable offspring were all reduced in intersex fish compared with normal male fish. Male gamete quality (assessed using sperm motility, sperm density, and fertilization success) was negatively correlated with the degree of feminization in intersex fish (r = -0.603; P < 0.001) and was markedly reduced in severely feminized intersex fish by as much as 50% in terms of motility and 75% in terms of fertilization success when compared with either less severely feminized intersex fish or unaffected male fish. This is the first evidence documenting a relationship between the morphological effects (e.g., intersex) of endocrine disruption and the reproductive capabilities of any wild vertebrate. The results suggest that mixtures of endocrine-disrupting substances discharged into the aquatic environment could pose a threat to male reproductive health.