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Fish oogenesis is characterised by a massive growth of oocytes each reproductive season. This growth requires the stockpiling of certain molecules, such as ribosomal RNAs to assist the rapid ribosomal assembly and protein synthesis required to allow developmental processes in the newly formed embryo. Massive 5S rRNA expression in oocytes, facilitated by transcription factor 3A (Gtf3a), serves as marker of intersex condition in fish exposed to xenoestrogens. Our present work on Gtf3a gene evolution has been analysed  in silico  in teleost genomes and functionally in the case of the zebrafish  Danio rerio . Synteny-analysis of fish genomes has allowed the identification of two  gtf3a  paralog genes, probably emerged from the teleost specific genome duplication event. Functional analyses demonstrated that  gtf3ab  has evolved as a gene specially transcribed in oocytes as observed in  Danio rerio , and also in  Oreochromis niloticus . Instead,  gtf3aa  was observed to be ubiquitously expressed. In addition, in zebrafish embryos  gtf3aa  transcription began with the activation of the zygotic genome (~8 hpf), while  gtf3ab  transcription began only at the onset of oogenesis. Under exposure to 100 ng/L 17β-estradiol, fully feminised 61 dpf zebrafish showed transcription of ovarian  gtf3ab , while masculinised (100 ng/L 17α-methyltestosterone treated) zebrafish only transcribed  gtf3aa . Sex related transcription of g tf3ab  coincided with that of  cyp19a1a  being opposite to that of  amh  and  dmrt1 . Such sex dimorphic pattern of  gtf3ab  transcription was not observed earlier in larvae that had not yet shown any signs of gonad formation after 26 days of oestradiol exposure. Thus,  gtf3ab  transcription is a consequence of oocyte differentiation and not a direct result of estrogen exposure, and could constitute a useful marker of gonad feminisation and intersex condition.

plos one

Duplication and subfunctionalisation of the general transcription factor IIIA (gtf3a) gene in teleost genomes, with ovarian specific transcription of gtf3ab