A new study in which the ICM-CSIC has participated reveals, for the first time, how to accurately predict the sex of an animal without sex chromosomes or a master sex-determining gene from a model that integrates all genetic information.
A new study by the Marine Biodiversity Exploitaiton and Conservation (MARBEC) in which the Institut de Ciències del Mar (ICM-CSIC) in Barcelona has participated has revealed the mechanisms that determine sex in animals without sex chromosomes, as is the case of some fish, where the genetic basis of sex is a continuous character that goes from one extreme, with one sex, to the other extreme, with the other sex. However, in these cases there is a threshold which, in addition, is influenced by the environment, and causes, at the individual level, sex to be resolved in a binary way, giving rise to variable proportions of males and females.
Finding out the genetic basis of sex determination in an individual of a species without sex chromosomes, and the precise contribution of genetic and environmental influences in these species, had been impossible until now. This is the first time that a sex determination system has been shown to be influenced by both genetic and environmental variation, both of which are continuous.
The influence of genes and environment
According to the work, recently published in the journal Proceedings of the National Academy of Sciences of the Unites States of America (PNAS), sex determination in some fish such as European sea bass is polygenic, as the species does not have sex chromosomes, but is determined by the combined effects of many genes on different chromosomes. It is also influenced by temperature, as many of its genes or the proteins they encode are susceptible to this environmental variable.
This could explain, for example, the increasingly common cases of sex reversal in fish, i.e. when the phenotypic (gonadal) sex of individuals does not correspond to their genetic sex. .
"Understanding how external factors can reverse genetic information, affecting an essential trait such as the sex of the gonads, is of great importance in the current scenario of global warming," says Francesc Piferrer, ICM-CSIC researcher and author of the study.
The role of energy and epigenetics
The study also reveals that energy and epigenetics also play a key role in determining sexual fate. Specifically, the work shows that energy pathways, including fat and sugar regulation, are involved in sex determination and, in the case of European sea bass, explain why females have higher energy levels and a higher growth rate than males.
Finally, it shows that early exposure to high temperature up-regulates two key sex-determining genes, i.e. it causes them to be more highly expressed in individuals with a poorly defined tendency to be female, which explains temperature-induced masculinisation.
"Our results provide a significant advance in our understanding of the epigenetic mechanisms underlying temperature-induced masculinisation in fish," explains ICM-CSIC researcher Núria Sánchez, who carried out the epigenetic analyses.
The European bass, a model species
The study has used European sea bass as a model to describe in-depth this polygenic sex determination system and its interaction with temperature. This is currently the best characterised species with polygenic sexual determinism, and the results may be useful for other species with similar systems, some of which are of commercial interest.
In this case, by examining the entire genome, including all its modifications and the proteins it encodes, it has been possible to demonstrate that the genetic sex tendency, defined as the genetic propensity to become male or female, accurately estimates the future sex of the individual.
"Thanks to this research, we have been able to demonstrate, for the first time, the existence of a clearly continuous sex determination system in an animal," says MARBEC researcher and lead author of the study Benjamin Geffroy. All in all, the results of this study will contribute to a better understanding of the possible impacts of climate change on species without sex chromosomes, as is the case in many fish and reptiles.