Abstract
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 gtf3ab 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.
Highlights
A great variety of anthropogenic chemicals present estrogen- or androgen-like properties displaying biological activities similar to those of endogenous hormones. Upon bioaccumulation such chemicals interfere with the normal hormonal function altering, for example, the normal control of sexual differentiation, gametogenesis and reproduction
Teleost genomes show a partially duplicated genome after the teleost specific genome duplication event and this further complicates the identification of potential sex linked genes [8]. Both sex determination and differentiation are environmentally driven in fish
In zebrafish gtf3aa would code for a protein of367 aa and and estimated molecular weight of 42,6 kDa, this values being i 318 aa and 37.15 kDa for the case of gf3ab. This is very close to the two protein sequences obtained through alternative promoter usage of the unique gtf3a gene present in the X. laevis genome [29, 30]
Summary
A great variety of anthropogenic chemicals present estrogen- or androgen-like properties displaying biological activities similar to those of endogenous hormones Upon bioaccumulation such chemicals interfere with the normal hormonal function altering, for example, the normal control of sexual differentiation, gametogenesis and reproduction. Teleost genomes show a partially duplicated genome after the teleost specific genome duplication event and this further complicates the identification of potential sex linked genes [8]. Both sex determination and differentiation are environmentally driven in fish. Exposure to such compounds during critical periods of development in zebrafish (Danio rerio) has been reported to impair gonadal development [11, 12], alter sex phenotypes [12, 13] and to feminise and/or masculinise individuals [1, 13, 14]
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