Plasticity of fertilization rates under varying temperature in the broadcast spawning mussel, Mytilus galloprovincialis

Environmental microplastics compromise reproduction of the marine invertebrate Mytilus galloprovincialis: A holistic approach

The extensive presence of microplastics (MPs) in marine ecosystems constitutes a major threat to aquatic environments. The gametes of the marine invertebrate Mytilus galloprovincialis, which is essential for coastal ecosystems, are released directly into the water, potentially exposing them to environmental microplastics (EMPs). This study examined the effects of exposing M. galloprovincialis gametes to 50 or 100 µg/L EMP for 1 h on fertilization rates, larval quality, and the molecular mechanisms underlying the induction of apoptosis and shell growth. Our findings show that increased EMP concentrations correlate with reduced fertilization success and higher rates of larval malformations, indicating negative impacts on embryonic development. Additionally, DNA degradation in larvae is related to the EMP concentration. The apoptosis-associated proteins Bax, P53, and Cas-3 are upregulated, whereas Bcl-2 and DNA-ligase are downregulated with increasing EMP concentrations. Prothymosin-ɑ (PTMA), which is crucial for cell proliferation, also decreases with increasing EMP concentrations, contributing to impaired cell proliferation and growth imbalances. Reduced HRG gene expression is correlated with decreased shell growth and larval malformations. This study underscores the detrimental impact of EMPs on bivalve gametes, which impacts fertilization success and larval quality and highlights the potential risks to species survival and marine ecosystem stability.

Heat stress mediates toxicity of rutile titanium dioxide nanoparticles on fertilisation capacity in the broadcast spawning mussel Mytilus galloprovincialis

Titanium dioxide nanoparticle (nTiO2) pollution of marine environments is rapidly increasing with potentially deleterious effects on wildlife. Yet, the impacts of nTiO2 on reproduction remain poorly understood. This is especially the case for broadcast spawners, who are likely to be more severely impacted by environmental disturbances because their gametes are directly exposed to the environment during fertilisation. In addition, it is unclear whether rising water temperatures will further exacerbate the impact of nTiO2 toxicity. Here, in a series of fertilisation trials, we systematically examine the main and interactive effects of nTiO2 exposure and seawater temperature on fertilisation success in the Mediterranean mussel Mytilus galloprovincialis. Specifically, our fertilisation trials explored whether nTiO2 exposure influences fertilisation rates when (i) eggs alone are exposed, (ii) both sperm and eggs are exposed simultaneously, and (iii) whether increases in seawater temperature interact with nTiO2 exposure to influence fertilisation rates. We also ask whether changes in nTiO2 concentrations influence key sperm motility traits using computer-assisted sperm analysis (CASA). In fertilisation trials for treatment groups (i) and (ii), we found no main effects of nTiO2 at environmentally relevant concentrations of 5, 10 and 50 μg L-1 on fertilisation capacity relative to the control. Consistent with these findings, we found no effect of nTiO2 exposure on sperm motility. However, in treatment group (iii), when fertilisation trials were conducted at higher temperatures (+6 °C), exposure of gametes from both sexes to 10 μg L-1 nTiO2 led to a reduction in fertilisation rates that was significantly greater than when gametes were exposed to elevated temperature alone. These interacting effects of nTiO2 exposure and seawater temperature demonstrate the toxic potential of nTiO2 for fertilisation processes in a system that is likely to be impacted heavily by predicted future increases in sea surface temperatures.

Polystyrene Nanoplastics in Aquatic Microenvironments Affect Sperm Metabolism and Fertilization of Mytilus galloprovincialis (Lamark, 1819)

The continuous and unregulated discharge of wastes and pollutants into the aquatic environment has required constant monitoring of the risks incurred by aquatic ecosystems. Alarmism arises from plastic pollution as larger artifacts release nanoscale fragments that can contact free-living stages such as gametes, embryos, and larvae. Specifically, the interaction between spermatozoa, released in water in externally fertilizing species, and the surrounding microenvironment is essential for successful fertilization. Activation and kinematics of movement, proper maintenance of ionic balance, and chemotactism are processes highly sensitive to even minimal perturbations caused by pollutants such as polystyrene nanoplastics. Spermatozoa of Mytilus galloprovincialis (M. galloprovincialis), an excellent ecotoxicological model, undergo structural (plasma membrane ruptures, DNA damage) and metabolic (reduced motility, fertilizing capacity) damage upon exposure to 50 nm amino-modified polystyrene nanoplastics (nPS-NH2). Nanoplastics of larger diameter (100 nm) did not affect sperm parameters. The findings highlighted the negative impact that plastic pollution, related to nanoparticle diameter and concentration, could have on sperm quality and reproductive potential of organisms, altering the equilibrium of aquatic ecosystems.

Ocean acidification alters sperm responses to egg-derived chemicals in a broadcast spawning mussel

The continued emissions of anthropogenic carbon dioxide are causing progressive ocean acidification (OA). While deleterious effects of OA on biological systems are well documented in the growth of calcifying organisms, lesser studied impacts of OA include potential effects on gamete interactions that determine fertilization, which are likely to influence the many marine species that spawn gametes externally. Here, we explore the effects of OA on the signalling mechanisms that enable sperm to track egg-derived chemicals (sperm chemotaxis). We focus on the mussel Mytilus galloprovincialis, where sperm chemotaxis enables eggs to bias fertilization in favour of genetically compatible males. Using an experimental design based on the North Carolina II factorial breeding design, we test whether the experimental manipulation of seawater pH (comparing ambient conditions to predicted end-of-century scenarios) alters patterns of differential sperm chemotaxis. While we find no evidence that male-female gametic compatibility is impacted by OA, we do find that individual males exhibit consistent variation in how their sperm perform in lowered pH levels. This finding of individual variability in the capacity of ejaculates to respond to chemoattractants under acidified conditions suggests that climate change will exert considerable pressure on male genotypes that can withstand an increasingly hostile fertilization environment.

Post-ejaculation thermal stress causes changes to the RNA profile of sperm in an external fertilizer

Sperm cells experience considerable post-ejaculation environmental variation. However, little is known about whether this affects their molecular composition, probably owing to the assumption that sperm are transcriptionally quiescent. Nevertheless, recent evidence shows sperm have distinct RNA profiles that affect fertilization and embryo viability. Moreover, RNAs are expected to be highly sensitive to extracellular changes. One such group of RNAs are heat shock protein (hsp) transcripts, which function in stress responses and are enriched in sperm. Here, we exploit the experimental tractability of the mussel Mytilus galloprovincialis by exposing paired samples of ejaculated sperm to ambient (19°C) and increased (25°C) temperatures, then measure (i) sperm motility phenotypes, and (ii) messenger RNA (mRNA) levels of two target genes (hsp70 and hsp90) and several putative reference genes. We find no phenotypic changes in motility, but reduced mRNA levels for hsp90 and the putative reference gene gapdh at 25°C. This could reflect either decay of specific RNAs, or changes in translation and degradation rates of transcripts to maintain sperm function under stress. These findings represent, to our knowledge, the first evidence for changes in sperm RNA profiles owing to post-ejaculation environments, and suggest that sperm may be more vulnerable to stress from rising temperatures than currently thought.

RNA-seq coupled to proteomic analysis reveals high sperm proteome variation between two closely related marine mussel species

Speciation mechanisms in marine organisms have attracted great interest because of the apparent lack of substantial barriers to genetic exchange in marine ecosystems. Marine mussels of the Mytilus edulis species complex provide a good model to study mechanisms underlying species formation. They hybridise extensively at many localities and both pre- and postzygotic isolating mechanisms may be operating. Mussels have external fertilisation and sperm cells should show specific adaptations for survival and successful fertilisation. Sperm thus represent key targets in investigations of the molecular mechanisms underlying reproductive isolation. We undertook a deep transcriptome sequencing (RNA-seq) of mature male gonads and a 2DE/MS-based proteome analysis of sperm from Mytilus edulis and M. galloprovincialis raised in a common environment. We provide evidence of extensive expression differences between the two mussel species, and general agreement between the transcriptomic and proteomic results in the direction of expression differences between species. Differential expression is marked for mitochondrial genes and for those involved in spermatogenesis, sperm motility, sperm-egg interactions, the acrosome reaction, sperm capacitation, ATP reserves and ROS production. Proteins and their corresponding genes might thus be good targets in further genomic analysis of reproductive barriers between these closely related species. SIGNIFICANCE: Model systems for the study of fertilization include marine invertebrates with external fertilisation, such as abalones, sea urchins and mussels, because of the ease with which large quantities of gametes released into seawater can be collected after induced spawning. Unlike abalones and sea urchins, hybridisation has been reported between mussels of different Mytilus spp., which thus makes them very appealing for the study of reproductive isolation at both pre- and postzygotic levels. There is a lack of empirical proteomic studies on sperm samples comparing different Mytilus species, which could help to advance this study. A comparative analysis of sperm proteomes across different taxa may provide important insights into the fundamental molecular processes and mechanisms involved in reproductive isolation. It might also contribute to a better understanding of sperm function and of the adaptive evolution of sperm proteins in different taxa. There is now growing evidence from genomics studies that multiple protein complexes and many individual proteins might have important functions in sperm biology and the fertilisation process. From an applied perspective, the identification of sperm-specific proteins could also contribute to the improved understanding of fertility problems and as targets for fertility control.