Serotonergic mechanisms mediating spawning and oocyte maturation in the zebra mussel,Dreissena polymorpha

Toward invasive mussel genetic biocontrol: Approaches, challenges, and perspectives

Invasive freshwater mussels, such as the zebra (Dreissena polymorpha), quagga (Dreissena rostriformis bugensis), and golden (Limnoperna fortunei) mussel have spread outside their native ranges throughout many regions of the North American, South American, and European continents in recent decades, damaging infrastructure and the environment. This review describes ongoing efforts by multiple groups to develop genetic biocontrol methods for invasive mussels. First, we provide an overview of genetic biocontrol strategies that have been applied in other invasive or pest species. Next, we summarize physical and chemical methods that are currently in use for invasive mussel control. We then describe the multidisciplinary approaches our groups are employing to develop genetic biocontrol tools for invasive mussels. Finally, we discuss the challenges and limitations of applying genetic biocontrol tools to invasive mussels. Collectively, we aim to openly share information and combine expertise to develop practical tools to enable the management of invasive freshwater mussels.

Compositional variability of Mg/Ca, Sr/Ca, and Na/Ca in the deep-sea bivalve Acesta excavata (Fabricius, 1779)

Acesta excavata (Fabricius, 1779) is a slow growing bivalve from the Limidae family and is often found associated with cold-water coral reefs along the European continental margin. Here we present the compositional variability of frequently used proxy elemental ratios (Mg/Ca, Sr/Ca, Na/Ca) measured by laser-ablation mass spectrometry (LA-ICP-MS) and compare it to in-situ recorded instrumental seawater parameters such as temperature and salinity. Shell Mg/Ca measured in the fibrous calcitic shell section was overall not correlated with seawater temperature or salinity; however, some samples show significant correlations with temperature with a sensitivity that was found to be unusually high in comparison to other marine organisms. Mg/Ca and Sr/Ca measured in the fibrous calcitic shell section display significant negative correlations with the linear extension rate of the shell, which indicates strong vital effects in these bivalves. Multiple linear regression analysis indicates that up to 79% of elemental variability is explicable with temperature and salinity as independent predictor values. Yet, the overall results clearly show that the application of Element/Ca (E/Ca) ratios in these bivalves to reconstruct past changes in temperature and salinity is likely to be complicated due to strong vital effects and the effects of organic material embedded in the shell. Therefore, we suggest to apply additional techniques, such as clumped isotopes, in order to exactly determine and quantify the underlying vital effects and possibly account for these. We found differences in the chemical composition between the two calcitic shell layers that are possibly explainable through differences of the crystal morphology. Sr/Ca ratios also appear to be partly controlled by the amount of magnesium, because the small magnesium ions bend the crystal lattice which increases the space for strontium incorporation. Oxidative cleaning with H₂O₂ did not significantly change the Mg/Ca and Sr/Ca composition of the shell. Na/Ca ratios decreased after the oxidative cleaning, which is most likely a leaching effect and not caused by the removal of organic matter.

Characterizing transcriptome in female scallop Chlamys farreri provides new insights into the molecular mechanisms of reproductive regulation during ovarian development and spawn

Bivalve mollusks are descendants of an early-Cambrian lineage and have successfully evolved unique strategies for reproduction. Nonetheless, the molecular mechanisms underlying reproductive regulation in mollusks remain to be elucidated. In this study, transcriptomes of ovary at four reproductive stages in female Chlamys farreri were characterized by RNA-Seq. Regarding signaling pathways, ECM-receptor interaction pathway, mTOR signaling pathway, Fanconi anemia pathway, FoxO signaling pathway, Wnt signaling pathway and Hedgehog signaling pathway were enriched during ovarian development processes. In addition, pathways related to energy metabolism such as Nitrogen metabolism and Arachidonic acid metabolism were enriched at spawn stage. Interestingly, Neuroactive ligand-receptor interaction was significantly enriched involved in ovarian development and spawn, and indicated the potential functions of nervous system on reproductive regulation in C. farreri. What's more, this study identified and characterized fourteen genes involved in "sex hormones synthesis and regulation", "ovarian development and spawn" and "maternal immunity" during the four reproductive stages in C. farreri. We determined that CYP17 uniquely affected gamete release by influencing the physiological balance among the steroid hormones and showed that receptors of the 5-HT and GABA neurotransmitters were tightly associated with ovarian maturation. Furthermore, to the best of our knowledge, this is the first study to report the maternal effect gene Zar1 in bivalve mollusks, likewise the maternal immunity genes displayed coordinated and cooperative expression during reproductive periods, which strengthened the environmental adaptation mechanisms of bivalves. Taken together, this study provides the first dynamic transcriptomic analysis of C. farreri at four key reproductive stages, which will assist in revealing the molecular mechanisms underlying bivalves on reproductive regulation in ovarian development and spawn.

Molecular cloning and functional expression of the Penaeus monodon 5-HT receptor

Serotonin (5-HT) mediates a number of diverse physiological functions in crustaceans by interacting with various 5-HT receptor subtypes. A putative 5-HT receptor cloned from the ovary of the black tiger prawn (Penaeus monodon) consisted of 2291 nucleotides, encoding a putative 5-HT(1Pem) receptor protein of 591 amino acids. Transient expression of 5-HT(1Pem) in HEK293 cells demonstrated a saturable [3H]-5-HT binding with a Kd of 10.43+/-1.13 nM and Bmax of 1.53+/-0.06 pmol/mg. The putative 5-HT(1Pem) receptor is expressed in all tissues examined and is constitutively expressed in the ovary during ovarian maturation and spent phase. Polyclonal antibodies against the third intracellular loop (i3 loop) of the 5-HT receptor showed that the 5-HT(1Pem) receptor protein was expressed in the trabeculae of ovarian stages 1 and 2 but on the cortical rod and surrounding the oocyte membrane of stages 3 and 4, suggesting that receptor localization plays a critical role in regulating ovarian maturation and spawning in penaeus shrimp.

Sensory innervation of the ovotestis in the snail Helix aspersa

Because oviposition in the land snail Helix aspersa is a metabolically expensive process coupled to a high fixed cost, one expects oviposition to occur only when the clutch size surpasses a minimum value at which the reproductive benefit exceeds the cost. We propose that neural innervation of the gonad allows H. aspersa to monitor oocyte production and ensure an adequate supply of gametes prior to ovulation. The ovotestis is innervated by a branch of the intestinal nerve in which the majority of axon fibres measure <0.2 μm in diameter. We found a strong positive correlation between the number of mature oocytes in the ovotestis and the frequency of spontaneous afferent spikes in the nerve branch. Tactile stimulation of the ovotestis resulted in a 20-fold increase in afferent spikes and an efferent reflex directed towards the ovotestis and the pericardium. Afferent activity also increased 10-fold after an experimentally induced increase in the volume of the ovotestis. These results suggest that the growing oocytes expand the walls of the acini and trigger action potentials in the mechanosensitive nerve terminals that lie within the acinar walls. We hypothesize that the resulting tonic signal is permissive for ovulation. In addition, a phasic sensory signal may occur during ovulation to trigger CNS motor output related to oviposition.

Putative serotonin reuptake inhibitor-induced spawning and parturition in freshwater bivalves is inhibited by mammalian 5-HT2 receptor antagonists

Selective serotonin reuptake inhibitors (SSRIs) can mimic the physiological actions of serotonin, and in bivalve molluscs they induce zebra mussel spawning and fingernail clam parturition. We have elucidated further the pharmacology of SSRI-induced spawning and part-urition by blocking these reproductive processes with two mammalian 5-HT(2) receptor antagonists, cyproheptadine and mianserin. These two antagonists were potent inhibitors of both spawning and parturition induced by the SSRIs fluvoxamine, fluoxetine, and zimelidine. In zebra mussels, both cyproheptadine and mianserin significantly blocked spawning induced by fluvoxamine and by zimelidine. In the fingernail clams Sphaerium spp., both cyproheptadine and mianserin blocked fluvoxamine-induced parturition. A possible mechanism of action for SSRI-induced spawning and parturition in bivalves is suggested.

Monoamines in the albumen gland, plasma, and central nervous system of the snail Biomphalaria glabrata during egg-laying

The potential role of selected biogenic monoamines and related compounds in the reproductive physiology of the freshwater snail Biomphalaria glabrata was investigated. Extracts of the albumen gland (AG), plasma, and central nervous system (CNS) were subjected to high pressure liquid chromatography with electrochemical detection (HPLC-ED), and under the extraction and separation conditions employed the following amines were detected: tyrosine, dihydroxyphenylalanine (DOPA), dopamine, and tryptophan in the AG; DOPA, tyrosine, and tryptophan in the plasma; DOPA, tyrosine, dopamine and 5-hydroxytryptamine in the CNS. These compounds were then quantified in individual samples taken from snails known to be in a particular stage of the egg-laying process. AG dopamine levels were highest in snails in the first stage of the reproductive process, when the AG is secreting perivitelline fluid around each fertilized ovum. Significant decreases in AG protein content during the later stages of the egg-laying process were also evident. Plasma tyrosine and DOPA levels were lowest in snails that contained a fully packaged egg mass, while no changes in monoamine content were observed in the CNS. These data provide insights into the role(s) that monoamines, especially catecholamine-related compounds, may play in B. glabrata reproductive physiology.

Multiple triggers of oocyte maturation in nemertean worms: the roles of calcium and serotonin

To analyze the process of oocyte maturation in nemertean worms, oocytes with a large nucleus (=germinal vesicle, or GV) were removed from gravid ovaries of Cerebratulus lacteus and Micrura alaskensis. Following transfer to natural seawater (NSW), fully grown oocytes spontaneously matured as indicated by their completion of germinal vesicle breakdown (GVBD), whereas GVBD was reversibly blocked if the oocytes were initially placed in calcium-free seawater (CaFSW). Similarly, calcium ionophore treatments triggered GVBD in calcium-containing artificial seawater (ASW) but not in CaFSW, suggesting that external calcium influx may facilitate maturation. However, compared to the overall levels of maturation elicited by ASW, significantly higher percentages of GVBD were achieved with NSW or with ASW that had been conditioned with marine sediment. Moreover, calcium channel blockers decreased GVBD rates in ASW but not in NSW, which is consistent with the view that substances other than external calcium ions can trigger maturation. Accordingly, oocytes underwent equally high levels of GVBD when treated with serotonin (=5-hydroxytryptamine, or 5-HT) in ASW or CaFSW. The 5-HT-induced maturation was blocked by inhibitors of 5-HT receptors but continued to occur in the presence of calcium channel blockers or the calcium chelator BAPTA. In addition, oocytes microinjected with fluorescent calcium indicators underwent GVBD in response to 5-HT without displaying marked calcium transients during confocal imaging runs. Collectively, such findings suggest that nemertean oocytes can mature via multiple pathways that may include external calcium influx or a 5-HT-induced signaling cascade that lacks prominent calcium fluctuations. J. Exp. Zool. 287:243-261, 2000.

Serotonergic responses of the siphons and adjacent mantle tissue of the zebra mussel, Dreissena polymorpha

Bivalve siphons have important functional roles in nutritional physiology, defense, and reproductive mechanisms, yet little is known about their neuromuscular control. In the present study, tension measurements and video observations of siphons and adjacent mantle tissue were used to investigate responses to serotonin (5-HT). 5-HT caused relaxation at 10(-6) M and 10(-5) M, contraction or biphasic responses at 10(-4) M and 10(-3) M, and siphon opening at 10(-4) M and 10(-3) M. Responses were slow and lasted 5-10 min after much shorter (20 s) applications of 5-HT. The relaxation phase was enhanced in high potassium medium. Contractile responses could be mimicked by alpha-1-methyl-5-HT and 2-methyl-5-HT but not by 8-OH-DPAT. The responses were not affected by methiothepin, TFMPP, 1-(1-naphthyl)piperazine, metergoline, NAN-190, mianserin, cyproheptadine, and ketanserin. The pharmacology of the 5-HT receptor(s) mediating these siphon/mantle responses is, therefore, different from previously described 5-HT receptors involved in spawning. The siphon/mantle contains previously undescribed longitudinal muscle fibers in the mantle and circular muscle fibers around the siphons. Serotonergic innervation of the siphon margins and mantle tissue was observed by immunohistology. The presence of 5-HT in the siphon/mantle tissue and the responsiveness of these preparations to 5-HT suggest that 5-HT may be a physiological regulator of mantle and siphon movements in the zebra mussel.

Cloning and sequence analysis of two cDNAs encoding cyclin A and cyclin B in the zebra mussel Dreissena polymorpha

Cyclins are key components in the progression of both mitotic and meiotic cell cycle control. Full-length cDNA clones encoding cyclin A and cyclin B were isolated from a zebra mussel testis cDNA library. The clones contained open reading frames of 419 and 434 amino acids, had similarity to cyclins A and B from other species, but also some unique features in their sequences. Cyclin A and B mRNA was expressed in testis, ovary, gill, mantle, muscle, and eggs, as shown by specific polymerase chain reaction.

Tricyclic antidepressants suppress spawning and fertilization in the zebra mussel, Dreissena polymorpha

Tricyclic antidepressants (TCAs), which are psychotropic drugs that work in vertebrates by interfering with serotonergic mechanisms, were tested for their effects on the serotonin-elicited spawning behavior of the zebra mussel, Dreissena polymorpha. Exposure of mussels to 10(-4) M imipramine or desipramine for 2 hr prior to serotonin treatment inhibited spawning in male, but not female, zebra mussels (p < 0.05). Clomipramine (10(-4) M) inhibited spawning of both sexes (p < 0.01). Inhibition of spawning was more effective with 2 hr preexposure time than with shorter times (p < 0.0001). Oocytes released in the presence of TCAs had a normal appearance, with no germinal vesicle present; however, fertilization and embryonic development were adversely affected in oocytes released into TCA concentrations as low as 10(-6) M. Oocytes fertilized after TCA treatment rarely developed normally. This is the first report of an inhibitory effect of TCAs on spawning, fertilization, and early embryonic development in any animal. The concentrations that affect embryonic development in zebra mussels are in the same range as therapeutic plasma concentrations in humans.