Abstract
The estimation of maturity and sex of fish stocks in European waters is a requirement of the EU Data Collection Framework as part of the policy to improve fisheries management. On the other hand, research on fish biology is increasingly focused in molecular approaches, researchers needing correct identification of fish sex and reproductive stage without necessarily having in house the histological know-how necessary for the task. Taking advantage of the differential gene transcription occurring during fish sex differentiation and gametogenesis, the utility of 5S ribosomal RNA (5S rRNA) and General transcription factor IIIA (gtf3a) in the molecular identification of sex and gametogenic stage was tested in different economically-relevant fish species from the Bay of Biscay. Gonads of 9 fish species (, Atlantic, Atlantic-chub and horse mackerel, blue whiting, bogue, European anchovy, hake and pilchard and megrim), collected from local commercial fishing vessels were histologically sexed and 5S and 18S rRNA concentrations were quantified by capillary electrophoresis to calculate a 5S/18S rRNA index. Degenerate primers permitted cloning and sequencing of gtf3a fragments in 7 of the studied species. 5S rRNA and gtf3a transcript levels, together with 5S/18S rRNA index, distinguished clearly ovaries from testis in all of the studied species. The values were always higher in females than in males. 5S/18S rRNA index values in females were always highest when fish were captured in early phases of ovary development whilst, in later vitellogenic stages, the values decreased significantly. In megrim and European anchovy, where gonads in different oogenesis stages were obtained, the 5S/18S rRNA index identified clearly gametogenic stage. This approach, to the sexing and the quantitative non-subjective identification of the maturity stage of female fish, could have multiple applications in the study of fish stock dynamics, fish reproduction and fecundity and fish biology in general.
Highlights
There are more than 30000 fish species inhabiting a wide range of aquatic habitats worldwide [1], some of them being very important in the global economy and human diet
The electropherograms obtained from gonads of commercially-relevant fish species showed different RNA patterns when comparing total RNA extracted from adult testes and ovaries
No signs of RNA degradation, clearly observable in Agilent RNA chips run in the 2100 Bioanalyzer, were observed in any of the samples, In the case of ovaries, the relative 5S ribosomal RNA (5S rRNA) signal was always higher than in the testes
Summary
There are more than 30000 fish species inhabiting a wide range of aquatic habitats worldwide [1], some of them being very important in the global economy and human diet. The impossibility of fish populations to breed on time is resulting in a significant loss of potential yield. In this scenario, it is imperative to study the reproduction biology of fish stocks, understand the population dynamics and the different changes that can occur in their life history [3]. As reproduction largely determines productivity and the resilience of populations, it is crucial to estimate the quality and quantity of gametes. These are indicators of the reproductive capacity of commercial fish populations towards scientifically-based fisheries management [3, 4]
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