Transcription levels of GnRH and serotonin signaling genes in the tissues and gametogenic stages of Mediterranean mussel (Mytilus galloprovincialis, Lamarck, 1819).

Supervisors: Maren Ortiz-Zarragoitia, Oihane Diaz de Cerio (PiE-UPV/EHU).
Mussels are among the most ubiquitous bivalves with great economic and ecological significance. Despite this, the fundamental aspects of their reproductive endocrinology are not fully elucidated, which can be primarily attributed to their primitive organ system and undefined signaling pathways. Hence, their decentralized nervous system is assumed to fulfill the primary regulatory functions controlling reproduction through the action of well-conserved signaling molecules. Among these, the presumed roles of the vertebrate-like gonadotropin-releasing hormones (GnRH) and the serotonin (5-HT) neurotransmitter systems have already been investigated in other bivalves but not in Mytilus galloprovincialis or mussels in general. Thus, this study aimed to characterize the genes involved in these signaling systems and evaluate their involvement in the reproduction of M. galloprovincialis populations in Arriluze and Plentzia, Spain. Cloning of the GnRH receptor (gnrhr), tryptophan hydroxylase (tph, serotonin synthesis-limiting enzyme) and serotonin receptor (5-htr) was successfully performed using the M. galloprovincialis ganglia. Transcriptomic analysis (qPCR) of sequenced reproductive genes reveals higher expression levels in the three ganglia (i.e. cerebral, visceral and pedal ganglia) compared to the peripheral tissues, which confirms that the nervous system performs the key neuroendocrinological regulations in mussels. Transcription levels of the reproductive genes showed a consistent upregulation during the onset of oogenesis and spermatogenesis followed by a constant downregulation until reaching sexual maturity, which suggests a direct involvement of these genes on the initiation of gametogenic development. The gradual elevation in the transcription of the serotonin-signaling genes (tph, 5-htr) during the spawning stage coincides with the widely reported spawn-inducing activity of serotonin in bivalves. The continued elevation in the transcription of these genes at the post-spawning stage, including gnrhr, suggests that the mussel population in Plentzia is preparing for another cycle of reproduction and that these genes are indeed involved in the onset of gametogenesis. An evaluation on the validity of 18s rRNA as a reference gene on the same experimental treatments revealed that it has significant differential transcription in the tissues, gametogenic stages and sex in M. galloprovincialis, implying its inadequacy as an internal control in this study. However, its significant upregulation in mature ovaries may serve as a potential marker for vitellogenesis in this species.