Abstract
The production of Venerupis decussata relies on wild seed collection, which has been recently compromised due to recruitment failure and severe mortalities. To address this issue and provide an alternative source of seed, artificial spawning and larval rearing programs were developed. However, hatchery-based seed production is a relatively new industry and it is still underdeveloped. A major hurdle in the European clam seed production is the control of spawning and reproduction, which is further hindered by the impossibility of obtaining fertile gametes by gonadal “stripping”, as meiosis re-initiation is constrained to a maturation process along the genital ducts. In the present study, oocytes were collected from 15 females and microarray analyses was performed to investigate gene expression profiles characterizing released and stripped ovarian oocytes. A total of 198 differentially expressed transcripts between stripped and spawned oocytes were detected. Functional analysis carried out on these transcripts highlighted the importance of a few biological processes, which are most probably implicated in the control of oocyte competence. Significant differences were observed for transcripts encoding proteins involved in meiosis progression (e.g. dual specificity phosphatase CDC25), WNT signalling (e.g. frizzled class receptor 8, wingless-type MMTV integration site family member 4), steroid synthesis (e.g. progestin and adipoQ receptor family member 3, cytochrome P450-C17), mRNA processing (e.g. zinc finger protein XlCOF28), calcium regulation (e.g. regucalcin, calmodulin) and ceramide metabolism (ceramidase B, sphingomyelinase). This study provides new information on transcriptional profiles putatively associated with ovarian egg infertility, and suggests potential mechanisms regulating early oocyte development in clams. Genes which were differentially expressed between stripped and spawned oocytes might have a pivotal role during maturation process in the gonadal duct and could be interesting targets for further functional studies aiming to make ovarian oocytes fertilizable.
Highlights
The grooved carpet shell Venerupis decussata is a native European bivalve species and, its global aquaculture production is still relatively low in Europe (4.137 tons in 2011) [1], it has a high economic value
stripped oocytes (STR), low hatching rate (LHR) and medium hatching rate (MHR) eggs were clearly separated along the Principal Component 1 (x axis), which explained 29% of the variation
The expression profiles were separated along the Principal Component 2 (y axis, 11% of the variation), but in this case STR and LHR oocytes did not show a marked divergence in expression patterns, while the separation of MHR eggs was remarkable
Summary
The grooved carpet shell Venerupis decussata is a native European bivalve species and, its global aquaculture production is still relatively low in Europe (4.137 tons in 2011) [1], it has a high economic value. Spawning success in the European clam is not predictable, with frequent failures to induce gametes emission This cannot be overcome by stripping, a practice for collecting oocytes before egg emission, widely used in some bivalve species (e.g. in cupped oysters), where stripped eggs can be fertilized. While full-grown oocytes of all bivalves are blocked in ovaries at prophase I stage, some crucial differences are observed in spawned eggs. In bivalves such as Spisula or Barnea spawned oocytes are arrested at prophase I and fertilization occurs at this stage leading to meiosis re-initiation [3,4,5]. The molecular determinants of this crucial difference are still unknown
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