|Epigenetic Mechanism Links Temperature and Gonadal Sex in Fish|
|SciMed - Genetics & Genome|
|TS-Si News Service|
|Tuesday, 03 January 2012 03:00|
Barcelona, Spain. Scientists have found the epigenetic mechanism that links temperature and gonadal sex in fish, a step toward examining whether a similar mechanism exists in other vertebrates.
The study built on previous knowledge that environmental temperature has measureable effects on sex determination.
The research team observed that increased temperature increases DNA methylation of the gonadal aromatase promoter in females, a factor in gene silencing. There are species, such as the Atlantic silverside fish, that show temperature-dependent sex determination. And there are other species for which temperature variations can override the genentic instructions within its DNA.
The study was performed by the Spanish National Research Council (CSIC), in collaboration with researchers from the Centre for Genomic Regulation (CRG).
The team, lead by Francesc Piferrer, a CSIC professor at the Institute of Marine Sciences (ICM) in Barcelona in Barcelona, exploited the properties of aromatose, an enzyme that transforms androgens into estrogens, which are essential for the development of ovaries in all non-mammalian vertebrates.
If there is no aromatase there are no estrogens, and without estrogens the development of ovaries is not possible.
The findings appear in the journal PLos Genetics.Previous work with sex determination of the European sea bass, dependent on a combination of genetic and environmental factors, had shown that starting with a normal sex ratio population for the species – equal proportions of male and females – it was possible to obtain an all-male group just through an increase in water temperature during a critical period of early development. The most intriguing observation was that effects of temperature were maximum at a moment when gonads were not differentiated nor had they even started to form. Why was this happening, what makes temperature override the genetic component and so early was, until now, a long-standing puzzle.
In the experiment, scientists exposed two groups of European seabass larvae at different temperatures, normal and high temperature, during their first weeks of life. Results show that high temperature increases the DNA methylation of the gonadal aromatase promoter (cyp19a), which, in turn drives its silencing as its transcriptional activation is inhibited.
In the group exposed to high temperature there were genetic females that were only partially affected and yet developed as females. However, there were other genetic females with the highest level of DNA methylation that therefore developed as males because aromatase was fully inhibited.
This is the first time that an epigenetic mechanism linking an environmental factor to a cellular mechanism related to the sexual determination has been described in any animal. Previously, only a similar mechanism had been described in some plants.
As researcher Francesc Piferrer points out, ‘animals are affected very soon, before differences between females and males become visible in histological samples, which happens on the 150th day of life, and even before the gonads start to form, which happens on the 35th day of life’.
This work explains why a few degrees of temperature rise masculinize these animals, something relevant in a context of global change. It also explains why many fishes raised on farms are males, since farmers raise larvae in warmer waters in order to accelerate their growth. Piferrer adds that ‘sex determination by temperature is very common in reptiles.’.
FundingLaia Navarro-Martín and Noelia Díaz were supported by predoctoral scholarships and Jordi Viñas and Laia Ribas by postdoctoral grants from the Spanish Ministry of Science and Innovation (MCINN).
Research was funded by the MCINN projects “Sexgene”, “Aquagenomics”, and “Epigen-Aqua” to Francesc Piferrer and by a MICINN project and AGAUR grant to Luciano Di Croce.
CitationDNA methylation of the gonadal aromatase (cyp19a) promoter is involved in temperature-dependent sex ratio shifts in the European sea bass. Laia Navarro-Martín, Jordi Viñas, Laia Ribas, Noelia Díaz, Arantxa Gutiérrez, Luciano Di Croce, Francesc Piferrer. PLos Genetics 2011; 7(12): e1002447. doi:10.1371/journal.pgen.1002447
Sex ratio shifts in response to temperature are common in fish and reptiles. However, the mechanism linking temperature during early development and sex ratios has remained elusive. We show in the European sea bass (sb), a fish in which temperature effects on sex ratios are maximal before the gonads form, that juvenile males have double the DNA methylation levels of females in the promoter of gonadal aromatase (cyp19a), the enzyme that converts androgens into estrogens. Exposure to high temperature increased the cyp19a promoter methylation levels of females, indicating that induced-masculinization involves DNA methylation-mediated control of aromatase gene expression, with an observed inverse relationship between methylation levels and expression. Although different CpGs within the sb cyp19a promoter exhibited different sensitivity to temperature, we show that the increased methylation of the sb cyp19a promoter, which occurs in the gonads but not in the brain, is not a generalized effect of temperature. Importantly, these effects were also observed in sexually undifferentiated fish and were not altered by estrogen treatment. Thus, methylation of the sb cyp19a promoter is the cause of the lower expression of cyp19a in temperature-masculinized fish. In vitro, induced methylation of the sb cyp19a promoter suppressed the ability of SF-1 and Foxl2 to stimulate transcription. Finally, a CpG differentially methylated by temperature and adjacent to a Sox transcription factor binding site is conserved across species. Thus, DNA methylation of the aromatase promoter may be an essential component of the long-sought-after mechanism connecting environmental temperature and sex ratios in vertebrate species with temperature-dependent sex determination.
Temperature changes during early embryonic and/or larval stages are able to modify sex ratios in fish and reptiles. However, the underlying mechanism by which temperature is able to modify the molecular pathways that developing gonads follow to become ovaries or testes is still unknown. One of the most interesting questions raised from previous studies with our model species, the European sea bass, was how temperature could affect the developmental fate of the gonads at a time when they were not even formed in the most rudimentary manner. This was the telltale sign of an epigenetic mechanism. In this study, DNA methylation levels of the aromatase promoter were analyzed in European sea bass exposed to different temperatures during early developmental stages. Aromatase is the enzyme that converts androgens (male hormones) into estrogens (female hormones), which are essential for ovarian development in all non-mammalian vertebrates. We show that increased temperature during a critical period in early development is able to increase DNA methylation of the aromatase promoter, preventing aromatase gene expression. We conclude that gonadal aromatase promoter methylation is most likely part of the long-sought-after mechanism connecting temperature and environmental sex determination in vertebrates.
|Last Updated on Monday, 02 January 2012 22:17|