What makes a good egg ? Molecular mechanisms defining egg developmental competence in teleost fish
Résumé
Egg quality (i.e. the egg’s ability to be fertilized and subsequently develop into a normal embryo) is highly variable in the wild or under aquaculture conditions. Yet, the egg components – and associated molecular processes – responsible for its developmental competence remain poorly understood. In teleost fish, in which a high fecundity can be observed in comparison to other vertebrate models, it is possible to sample individual egg clutches in which both developmental success assessment and analytical studies can be performed in parallel. Several types of approaches have been conducted to draw the molecular portrait of a developmentally competent fish egg by studying its composition in terms of maternal mRNAs, miRNAs, and proteins. Correlative studies have shown a link between the abundance of specific mRNAs and/or proteins in the eggs and the overall developmental success of the corresponding egg clutches. In rainbow trout (Oncorhynchus mykiss), a correlation between developmental success and the abundance of nucleoplasmin (npm2) mRNA in the egg was previously established. In zebrafish (Danio rerio), a model species with transparent eggs and rapid development, a knockdown (KD) approach showed that maternally-inherited npm2 mRNA was crucial to allow developmental success beyond zygotic genome activation (ZGA). Similar approaches at the proteome and miRNA repertoire levels have yielded interesting results that are currently being further analyzed. Taking advantage of the wide diversity of fish models (over 30000 species), future studies will be designed to identify key molecular mechanisms that could be shared by evolutionary distant species.
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