The thermal environment of sperm affects offspring success: a test of the anticipatory paternal effects hypothesis in the blue mussel

Silent cells? Potential for context-dependent gene expression in mature sperm

Sperm are traditionally viewed as transcriptionally and translationally silent cells. However, observations that components of the cellular machinery of gene expression are maintained in ejaculated sperm are increasingly cited as challenges to this fundamental assumption. Here, we critically evaluate these arguments and present three lines of evidence from both model and non-model systems that collectively raise the question of whether ejaculated sperm may be capable of active gene expression. First, and critical for arguments surrounding the possibility of differential gene expression, we review recent evidence that spermatozoa may retain the capacity to transcribe and translate their genomes. Second, we highlight how sperm cells can exhibit differential transcript quantities across different post-ejaculation environments. Third, we ask whether the accumulating evidence of remarkable phenotypic plasticity in post-ejaculatory sperm phenotypes could be mechanistically underpinned by changes in sperm gene expression. While these lines of evidence are indirect and do not definitively show transcription of sperm genomes, we highlight how emerging technologies may enable us to test this hypothesis explicitly. Our review advocates for progress in this field and highlights several important evolutionary, ecological and practical implications that will probably transcend disciplines to the clinical and applied reproductive sectors.

Heatwave-Induced Paternal Effects Have Limited Adaptive Benefits in Offspring

As the threat of climate change and associated heatwaves grows, we need to understand how natural populations will respond. Inter-generational non-genetic inheritance may play a key role in rapid adaptation, but whether such mechanisms are truly adaptive and sufficient to protect wild populations is unclear. The contribution of paternal effects in particular is not fully understood, even though the male reproductive system may be highly sensitive to heatwaves. We used the zebrafish Danio rerio to investigate the effects of heatwaves on male fertility and assess potential adaptive benefits to their offspring in a number of large-scale heatwave experiments. Heatwave conditions had negative effects on male fertility by reducing gamete quality and fertilisation success, and we found indications of an adaptive effect on hatching in offspring produced by heatwave-exposed males. Our findings highlight the importance of including male and female fertility when determining species ability to cope with extreme conditions and suggest that parental effects provide limited adaptive benefits.

Pre-fertilization gamete thermal environment influences reproductive success, unmasking opposing sex-specific responses in Atlantic salmon (Salmo salar)

The environment gametes perform in just before fertilization is increasingly recognized to affect offspring fitness, yet the contributions of male and female gametes and their adaptive significance remain largely unexplored. Here, we investigated gametic thermal plasticity and its effects on hatching success and embryo performance in Atlantic salmon (Salmo salar). Eggs and sperm were incubated overnight at 2°C or 8°C, temperatures within the optimal thermal range of this species. Crosses between warm- and cold-incubated gametes were compared using a full-factorial design, with half of each clutch reared in cold temperatures and the other in warm temperatures. This allowed disentangling single-sex interaction effects when pre-fertilization temperature of gametes mismatched embryonic conditions. Pre-fertilization temperature influenced hatch timing and synchrony, and matching sperm and embryo temperatures resulted in earlier hatching. Warm incubation benefited eggs but harmed sperm, reducing the hatching success and, overall, gametic thermal plasticity did not enhance offspring fitness, indicating vulnerability to thermal changes. We highlight the sensitivity of male gametes to higher temperatures, and that gamete acclimation may not effectively buffer against deleterious effects of thermal fluctuations. From an applied angle, we propose the differential storage of male and female gametes as a tool to enhance sustainability within the hatcheries.

Paternal environment effects are driven by female reproductive fluid but not sperm age in an external fertilizer

Sperm ageing after ejaculation can generate paternal environment effects that impact offspring fitness. In many species, female reproductive fluids (FRFs), i.e. ancillary fluids released by eggs or within the female reproductive tract, may protect sperm from ageing and can additionally interact with sperm to influence offspring viability. This raises the intriguing prospect that FRFs may alleviate paternal effects associated with sperm ageing. Here, we test this novel hypothesis using the broadcast spawning mussel, Mytilus galloprovincialis. We show that incubating sperm in FRF prior to fertilization increases offspring viability, and that these effects occur independently of sperm age. Our results provide novel evidence that FRFs allow females to selectively bias fertilization toward higher quality sperm within an ejaculate, which in turn yields more viable offspring. We consider this FRF-mediated paternal effect in the context of female physiological control over fertilization and the transgenerational effects of female-regulated haploid selection.

Marine heatwave temperatures enhance larval performance but are meditated by paternal thermal history and inter-individual differences in the purple sea urchin, Strongylocentrotus purpuratus

Marine heatwave (MHW) events, characterized by periods of anomalous temperatures, are an increasingly prevalent threat to coastal marine ecosystems. Given the seasonal phenology of MHWs, the full extent of their biological consequences may depend on how these thermal stress events align with an organism's reproductive cycle. In organisms with more complex life cycles (e.g., many marine invertebrate species) the alignment of adult and larval environments may be an important factor determining offspring success, setting the stage for MHW events to influence reproduction and development in situ. Here, the influence of MHW-like temperatures on the early development of the California purple sea urchin, Strongylocentrotus purpuratus, were explored within the context of paternal thermal history. Based on temperature data collected during MHW events seen in Southern California from 2014-2020, adult urchins were acclimated to either MHW or non-MHW temperatures for 28 days before their sperm was used to produce embryos that were subsequently raised under varying thermal conditions. Once offspring reached an early larval stage, the impact of paternal and offspring environments were assessed on two aspects of offspring performance: larval size and thermal tolerance. Exposure to elevated temperatures during early development resulted in larger, more thermally tolerant larvae, with further influences of paternal identity and thermal history, respectively. The alignment of paternal and offspring exposure to MHW temperatures had additional positive benefits on larval thermal tolerance, but this tolerance significantly decreased when their thermal experience mismatched. As the highest recorded temperatures within past MHW events have occurred during the gametogenesis of many kelp forest benthic marine invertebrate species, such as the purple sea urchin, such parental mediated impacts may represent important drivers of future recruitment and population composition for these species.

The Physiological and Evolutionary Ecology of Sperm Thermal Performance

Ongoing anthropogenic climate change has increased attention on the ecological and evolutionary consequences of thermal variation. Most research in this field has focused on the physiology and behavior of diploid whole organisms. The thermal performance of haploid gamete stages directly tied to reproductive success has received comparatively little attention, especially in the context of the evolutionary ecology of wild (i.e., not domesticated) organisms. Here, we review evidence for the effects of temperature on sperm phenotypes, emphasizing data from wild organisms whenever possible. We find that temperature effects on sperm are pervasive, and that above normal temperatures in particular are detrimental. That said, there is evidence that sperm traits can evolve adaptively in response to temperature change, and that adaptive phenotypic plasticity in sperm traits is also possible. We place results in the context of thermal performance curves, and encourage this framework to be used as a guide for experimental design to maximize ecological relevance as well as the comparability of results across studies. We also highlight gaps in our understanding of sperm thermal performance that require attention to more fully understand thermal adaptation and the consequences of global change.