Effects of Salinity and pH of Seawater on the Reproduction of the Sea Urchin Paracentrotus lividus

Application of Raman spectroscopy to the evaluation of F-actin changes in sea urchin eggs at fertilization

The actin filaments on the surface of echinoderm oocytes and eggs readily undergo massive reorganization during meiotic maturation and fertilization. In sea urchin eggs, the actin cytoskeletal response to the fertilizing sperm is fast enough to accompany Ca2+ signals and to guide sperm's entry into the egg. Although recent work using live cell imaging technology confirmed changes in the actin polymerization status in fertilized eggs, as was previously shown using light and electron microscopy, it failed to provide experimental evidence of F-actin depolymerization a few seconds after insemination, which is concurrent with the sperm-induced Ca2+ release. In the present study, we applied Raman microspectroscopy to tackle this issue by examining the spectral profiles of the egg's subplasmalemmal regions before and after treating the eggs with actin drugs or fertilizing sperm. At both early (15 s) and late (15 min) time points after fertilization, specific peak shifts in the Raman spectra revealed change in the actin structure, and Raman imaging detected the cytoskeletal changes corresponding to the F-actin reorganization visualized with LifeAct-GFP in confocal microscopy. Our observation suggests that the application of Raman spectroscopy, which does not require microinjection of fluorescent probes and exogenous gene expression, may serve as an alternative or even advantageous method in disclosing rapid subtle changes in the subplasmalemmal actin cytoskeleton that are difficult to resolve.

The Therapeutic and Diagnostic Potential of Phospholipase C Zeta, Oocyte Activation, and Calcium in Treating Human Infertility

Oocyte activation, a fundamental event during mammalian fertilisation, is initiated by concerted intracellular patterns of calcium (Ca2+) release, termed Ca2+ oscillations, predominantly driven by testis-specific phospholipase C zeta (PLCζ). Ca2+ exerts a pivotal role in not just regulating oocyte activation and driving fertilisation, but also in influencing the quality of embryogenesis. In humans, a failure of Ca2+ release, or defects in related mechanisms, have been reported to result in infertility. Furthermore, mutations in the PLCζ gene and abnormalities in sperm PLCζ protein and RNA, have been strongly associated with forms of male infertility where oocyte activation is deficient. Concurrently, specific patterns and profiles of PLCζ in human sperm have been linked to parameters of semen quality, suggesting the potential for PLCζ as a powerful target for both therapeutics and diagnostics of human fertility. However, further to PLCζ and given the strong role played by Ca2+ in fertilisation, targets down- and up-stream of this process may also present a significantly similar level of promise. Herein, we systematically summarise recent advancements and controversies in the field to update expanding clinical associations between Ca2+-release, PLCζ, oocyte activation and human fertility. We discuss how such associations may potentially underlie defective embryogenesis and recurrent implantation failure following fertility treatments, alongside potential diagnostic and therapeutic avenues presented by oocyte activation for the diagnosis and treatment of human infertility.

The Effect of Acidic and Alkaline Seawater on the F-Actin-Dependent Ca2+ Signals Following Insemination of Immature Starfish Oocytes and Mature Eggs

In starfish, the addition of the hormone 1-methyladenine (1-MA) to immature oocytes (germinal vesicle, GV-stage) arrested at the prophase of the first meiotic division induces meiosis resumption (maturation), which makes the mature eggs able to respond to the sperm with a normal fertilization response. The optimal fertilizability achieved during the maturation process results from the exquisite structural reorganization of the actin cytoskeleton in the cortex and cytoplasm induced by the maturing hormone. In this report, we have investigated the influence of acidic and alkaline seawater on the structure of the cortical F-actin network of immature oocytes of the starfish (Astropecten aranciacus) and its dynamic changes upon insemination. The results have shown that the altered seawater pH strongly affected the sperm-induced Ca²⁺ response and the polyspermy rate. When immature starfish oocytes were stimulated with 1-MA in acidic or alkaline seawater, the maturation process displayed a strong dependency on pH in terms of the dynamic structural changes of the cortical F-actin. The resulting alteration of the actin cytoskeleton, in turn, affected the pattern of Ca²⁺ signals at fertilization and sperm penetration.

Species-Specific Gamete Interaction during Sea Urchin Fertilization: Roles of the Egg Jelly and Vitelline Layer

In sea urchins, the sequence of the cellular and molecular events characterizing the fertilization process has been intensively studied. We have learned that to activate the egg, the fertilizing sperm must undergo morphological modifications (the acrosome reaction, AR) upon reaching the outer gelatinous layer enveloping the egg (egg jelly), which triggers the polymerization of F-actin on the sperm head to form the acrosomal process. The AR exposes bindin, an adhesive sperm protein essential for the species-specific interaction with the cognate receptor on the egg vitelline layer. To investigate the specific roles of the egg jelly and vitelline layer at fertilization of sea urchin eggs, Paracentrotus lividus eggs were incubated in acidic seawater, which removes the egg jelly, i.e., experimental conditions that should prevent the occurrence of the AR, and inseminated in the same medium. At variance with the prevailing view, our results have shown that these dejellied P. lividus eggs can still interact with sperm in acidic seawater, albeit with altered fertilization responses. In particular, the eggs deprived of the vitelline layer reacted with multiple sperm but with altered Ca2+ signals. The results have provided experimental evidence that the plasma membrane, and not the vitelline layer, is where the specific recognition between gametes occurs. The vitelline layer works in unfertilized eggs to prevent polyspermy.

Profiling Analysis of Filter Feeder Polypedilum (Chironomidae) Gut Contents Using eDNA Metabarcoding Following Contrasting Habitat Types-Weir and Stream

We analyzed the dietary composition of Polypedilum larvae among two contrasting habitats (river and weir). Our approach was (i) to apply eDNA-based sampling to reveal the gut content of the chironomid larvae, (ii) the diversity of gut contents in the two aquatic habitats, and (iii) assessment of habitat sediment condition with the food sources in the gut. The most abundant food was Chlorophyta in the gut of the river (20%) and weir (39%) chironomids. The average ratio of fungi, protozoa, and zooplankton in river chironomids gut was 5.9%, 7.2%, and 3.8%, while it was found decreased to 1.2%, 2.5%, and 0.1% in weir chironomids. Aerobic fungi in river midge guts were 3.6% and 10.34% in SC and IS, while they were in the range of 0.34-2.58% in weir midges. The hierarchical clustering analysis showed a relationship of environmental factors with food contents. Abiotic factors (e.g., pH) in the river and weir habitats correlated the clustered pattern with phytoplankton and minor groups of fungi. This study could help understand the food source diversity in the chironomid and habitat environmental conditions by using eDNA metabarcoding as an effective tool to determine dietary composition.

Transgenerational effects and phenotypic plasticity in sperm and larvae of the sea urchin Paracentrotus lividus under ocean acidification

In marine organisms, differing degree of sensitivity to ocean acidification (OA) is expected for each life stage, and disturbance at one stage can carry over into the following stage or following generation. In this study we investigated phenotypic changes of sperm and larvae of the sea urchin Paracentrotus lividus in response to different pH conditions (8.0, 7.7, 7.4) experienced by the parents during gametogenesis. In sperm from two-months exposed males, sperm motility, velocity, ATP content, ATP consumption and respiration rate were evaluated at three pH values of the activating medium (8.0, 7.7 and 7.4). Moreover, larvae from each parental group were reared at pH 8.0 and 7.7 for 20 days and larval mortality and growth were then assessed. Sperm motility and respiration rate were not affected either by exposure of males to low pH or by the post-activation pH. Sperm velocity did not differ among post-activation pH values in all sperm groups, but it decreased slower in sperm developed under acidified conditions, suggesting the presence of positive carryover effect on sperm longevity. This positive carryover effect of exposure of males to low pH values was highlighted also for the sperm ATP content, which was higher in these groups of sperm. ATP consumption rate was affected by post-activation pH with higher values at pH 8.0 in sperm from males maintained at control condition and pH 7.7 while the energy consumption appeared to be differently modulated at different experimental conditions. A negative carry over effect of OA was observed on survival of larvae from parents acclimated at pH 7.4 and additive negative effects of both parental and larval exposure to low pH can be suggested. In all groups of larvae, decreased somatic growth was observed at low rearing pH, thus larvae from parents maintained at low pH did not show an increased capability to cope with OA.

Fitness costs of alarm cues in sea urchins: Fertilization, hatching, and larval size

Sea urchins sense alarm cues extracted from crushed conspecifics and perform anti-predation behaviors in exposure to alarm cues. This indicates of the fitness benefits of alarm cues in sea urchins. The present study investigated whether fitness costs of alarm cues exist in fertilization, hatchability, deformity, and larval size of the sea urchin Mesocentrotus nudus. In the present study, we found that fertilization and hatching rates were significantly lower in the group with alarm cues than those in the group without alarm cues, indicating that fitness costs of alarm cues exist in sea urchins. However, there was no significant difference in deformity rate, larval length, stomach length, and stomach width of M. nudus with and without alarm cues. The group with alarm cues showed significantly shorter larval width than the group without alarm cues. This indicates that smaller larvae of sea urchins more probably survive in the environment with alarm cues. The present study reveals that fitness costs of alarm cues exist in sea urchins and sheds light on the ecological roles of alarm cues in kelp bed ecosystems.

Regulation of the Actin Cytoskeleton-Linked Ca2+ Signaling by Intracellular pH in Fertilized Eggs of Sea Urchin

In sea urchin, the immediate contact of the acrosome-reacted sperm with the egg surface triggers a series of structural and ionic changes in the egg cortex. Within one minute after sperm fuses with the egg plasma membrane, the cell membrane potential changes with the concurrent increases in intracellular Ca²⁺ levels. The consequent exocytosis of the cortical granules induces separation of the vitelline layer from the egg plasma membrane. While these cortical changes are presumed to prevent the fusion of additional sperm, the subsequent late phase (between 1 and 4 min after fertilization) is characterized by reorganization of the egg cortex and microvilli (elongation) and by the metabolic shift to activate de novo protein and DNA syntheses. The latter biosynthetic events are crucial for embryonic development. Previous studies suggested that the early phase of fertilization was not a prerequisite for these changes in the second phase since the increase in the intracellular pH induced by the exposure of unfertilized sea urchin eggs to ammonia seawater could start metabolic egg activation in the absence of the cortical granule exocytosis. In the present study, we have demonstrated that the incubation of unfertilized eggs in ammonia seawater induced considerable elongations of microvilli (containing actin filaments) as a consequence of the intracellular pH increase, which increased the egg’s receptivity to sperm and made the eggs polyspermic at fertilization despite the elevation of the fertilization envelope (FE). These eggs also displayed compromised Ca²⁺ signals at fertilization, as the amplitude of the cortical flash was significantly reduced and the elevated intracellular Ca²⁺ level declined much faster. These results have also highlighted the importance of the increased internal pH in regulating Ca²⁺ signaling and the microvillar actin cytoskeleton during the late phase of the fertilization process.

Application of the Paracentrotus lividus sea-urchin embryo-larval bioassay to the marine pollution biomonitoring program in the Tunisian coast

The pollution of the marine environment by treated and untreated effluents has increased due to human activities. Monitoring the marine ecosystem is nowadays a global concern. In this work, we evaluated the effect of contaminated and uncontaminated seawater, from different Tunisian coastal areas, on the fertilization, gastrulation, and embryo-larval development events of sea urchins (Paracentrotus lividus). The station of Salakta (SA) is considered as a control station, while the stations of Hamdoun Wadi (HW), Port of Monastir (PM), Karaia Monastir (KM), Teboulba (TE), and Khniss Lagoon (KL) are considered to be contaminated stations. The analysis of seawater physicochemical characteristics showed that levels of the total suspended matter (TSM), chemical oxygen demand (COD), biochemical oxygen demand (BOD), total organic carbon (TOC), and nitrate (NO^3-) were lower in the seawater of the reference site Salakta (SA) when compared to those of the contaminated seawater sites. In addition, a very strong variation in the levels of trace metals in seawaters sampled in the studied sites was noted. In fact, the highest concentrations of Pb and Cu were observed in Hamdoun Wadi (HW), port of Monastir (PM), and Karaia Monastir (KM), while the highest concentration of Zn was noted in the Teboulba lagoon (TE) and Khniss (LK). Alterations in physicochemical characteristics as well as elevated trace metal levels in the studied seawater samples were correlated with reduced fertility rate, gastrulation rate, and the frequency of normal sea urchin larvae. The total absence of normal sea urchin pluteus larvae in the sea waters of heavily polluted sites proves the great sensitivity of the larval frequency to mixed pollution. This work recommends the utility of urchin fertilization and gastrulation rates and normal pluteus larval frequencies as useful bioassays to monitor the exposure of marine ecosystems to mixed pollution.

Effects of Dithiothreitol on Fertilization and Early Development in Sea Urchin

The vitelline layer (VL) of a sea urchin egg is an intricate meshwork of glycoproteins that intimately ensheathes the plasma membrane. During fertilization, the VL plays important roles. Firstly, the receptors for sperm reside on the VL. Secondly, following cortical granule exocytosis, the VL is elevated and transformed into the fertilization envelope (FE), owing to the assembly and crosslinking of the extruded materials. As these two crucial stages involve the VL, its alteration was expected to affect the fertilization process. In the present study, we addressed this question by mildly treating the eggs with a reducing agent, dithiothreitol (DTT). A brief pretreatment with DTT resulted in partial disruption of the VL, as judged by electron microscopy and by a novel fluorescent polyamine probe that selectively labelled the VL. The DTT-pretreated eggs did not elevate the FE but were mostly monospermic at fertilization. These eggs also manifested certain anomalies at fertilization: (i) compromised Ca2+ signaling, (ii) blocked translocation of cortical actin filaments, and (iii) impaired cleavage. Some of these phenotypic changes were reversed by restoring the DTT-exposed eggs in normal seawater prior to fertilization. Our findings suggest that the FE is not the decisive factor preventing polyspermy and that the integrity of the VL is nonetheless crucial to the egg's fertilization response.

Fertilization and development of Arbacia lixula eggs are affected by osmolality conditions

The sea urchin Arbacia lixula coexist with Paracentrotus lividus in the Mediterranean, but the two sea urchin species are quite different from each other. Concerning the female gamete, A. lixula eggs are much darker than those of P. lividus due to the characteristic pigmentation. Upon insemination, the fertilization envelope formed by A. lixula eggs is remarkably thinner than that of P. livius eggs, which implies that the cortical organization of the eggs in the two species may be quite different. In this communication, we examined the phenotypic plasticity of A. lixula eggs in the changing osmolality. The plasma membrane, cortical actin cytoskeleton and vesicles are extensively altered in the eggs exposed to 40% seawater for 15 min. When fertilized, the Ca²⁺ response in these eggs was significantly compromised and the sperm often failed to enter the eggs. Remarkably, the pattern of the Ca²⁺ response was restored when these eggs were transferring back to the natural seawater before fertilization, while the actin cytoskeleton partially reverted to the original state. Nonetheless, these eggs restored in seawater failed to regain the innate sperm receptivity that allows only one sperm to enter in natural seawater. Thus, the ability to guide monospermic fertilization is lost by water entry into the eggs, and the eggs incorporated either multiple or no sperm. On the other hand, eggs briefly exposed to hypertonic seawater exhibited no evident morphological anomaly. Nonetheless, the monospermic eggs that experienced a brief exposure (15 min) to hypertonic seawater prior to fertilization in natural seawater displayed a subtly altered sperm-induced Ca²⁺ response and morpho-functional anomaly around the pluteus stage. Our results suggest that A. lixula eggs attain only a limited extent of cytological plasticity, and that the osmolality shock affects the physical nature of the egg surface which in turn affects the developmental programming.

Major challenges in cryopreservation of sea urchin eggs

Cryopreservation of gametes, embryos and larvae of marine invertebrates has been investigated in many studies throughout the years. There are many favorable studies on sperm cryopreservation but oocytes are still under research as no successful results have been sustainably obtained for this type of cells. The preservation of both maternal and paternal gametes separately would provide a reliable source of genetic material for their application to conservation, aquaculture and fundamental research. Unfortunately to date, it has not been possible to cryopreserve eggs from marine organisms. The aim of this review is to go over the factors that have been historically considered as obstacles for oocyte cryopreservation in aquatic organisms and discern those that may specifically apply to eggs of the sea urchin Paracentrotus lividus.