Sperm competition, but not major histocompatibility divergence, drives differential fertilization success between alternative reproductive tactics in Chinook salmon

Exposure to exogenous egg cortisol does not rescue juvenile Chinook salmon body size, condition, or survival from the effects of elevated water temperatures

Climate change is leading to altered temperature regimes which are impacting aquatic life, particularly for ectothermic fish. The impacts of environmental stress can be translated across generations through maternally derived glucocorticoids, leading to altered offspring phenotypes. Although these maternal stress effects are often considered negative, recent studies suggest this maternal stress signal may prepare offspring for a similarly stressful environment (environmental match). We applied the environmental match hypothesis to examine whether a prenatal stress signal can dampen the effects of elevated water temperatures on body size, condition, and survival during early development in Chinook salmon Oncorhynchus tshawytscha from Lake Ontario, Canada. We exposed fertilized eggs to prenatal exogenous egg cortisol (1,000 ng/ml cortisol or 0 ng/ml control) and then reared these dosed groups at temperatures indicative of current (+0°C) and future (+3°C) temperature conditions. Offspring reared in elevated temperatures were smaller and had a lower survival at the hatchling developmental stage. Overall, we found that our exogenous cortisol dose did not dampen effects of elevated rearing temperatures (environmental match) on body size or early survival. Instead, our eyed stage survival indicates that our prenatal cortisol dose may be detrimental, as cortisol-dosed offspring raised in elevated temperatures had lower survival than cortisol-dosed and control reared in current temperatures. Our results suggest that a maternal stress signal may not be able to ameliorate the effects of thermal stress during early development. However, we highlight the importance of interpreting the fitness impacts of maternal stress within an environmentally relevant context.

Swimming kinematics and temperature effects on spermatozoa from wild and captive shortnose sturgeon (Acipenser brevirostrum)

Computer-assisted semen analysis (CASA) and cluster analysis were used to compare spermatozoa swimming kinematics and milt quality between wild and captive shortnose sturgeon (Acipenser brevirostrum). Milt samples from 27 shortnose sturgeon were collected in May 2016 and June 2017. Of these, 19 were wild caught in the Saint John River, New Brunswick, Canada, and eight were from a captive population at the Mactaquac Biodiversity facility. The following kinematic variables were measured immediately following sperm activation (˜5 s), at 30, 60, and 180 s post-activation; average path velocity (VAP); straight-line velocity (VSL); curvilinear velocity (VCL); amplitude of lateral head displacement (ALH); beat cross frequency (BCF); straightness (STR); linearity (LIN); wobble (WOB); percent motility (MOT). Analyses were conducted at 7, 10, and 14 °C to determine potential effects of temperature on kinematics. Principal components analysis (PCA) of original kinematic variables yielded two main components, a speed/wobble component along with a movement pattern component. Hierarchical cluster analysis (HCPC) indicated there were distinct subpopulations, with composition of clusters the result of fish source (wild-caught or captive). Wild-caught fish had greater sperm densities (P = 0.0064) and sperm swimming speeds compared to captive fish (P < 0.05). Temperature had a significant effect only on captive spermatozoa, and this result was not consistent between time periods. There was no effect of hormonal manipulation on spermatozoa motility kinematics. Results indicate there are significant differences in measures of milt quality between wild and captive shortnose sturgeon, indicating an effect of rearing condition on reproductive potential, which may affect fertilization success.

No support for cryptic choice by ovarian fluid in an external fertilizer

Whether the ovarian fluid (OF) represents a selective environment influencing cryptic female choice was tested using an external fertilizer experiencing intense sperm competition and large effects of OF on sperm swimming behavior-the Arctic charr (Salvelinus alpinus). We physically separated the OF from the eggs of reproductively active females and reintroduced either their own OF or fluid from another female to the eggs. The eggs were then fertilized in vitro in a replicated split-brood design with sperm from two males under synchronized sperm competition trials, while also measuring sperm velocity of the individual males in the individual OFs. We found large effects of males, but no effect of females (i.e., eggs) on paternity, determined from microsatellites. More important, we found no effect of OF treatments on the relative paternity of the two competing males in each pair. This experimental setup does not provide support for the hypothesis that OF plays an important role as medium for cryptic female choice in charr. Power analyses revealed that our sample size is large enough to detect medium-sized changes in relative paternity (medium-sized effect sizes), but not large enough to detect small changes in relative paternity. More studies are needed before a conclusion can be drawn about OF's potential influence on paternity under sperm competition-even in charr.