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Study reveals complexities governing sex of birds

A gene controls the sex of gonads in birds but does not influence their bodies’ sexual development, highlighting an evolutionary divide from mammals.

Sex reserved male head
Sexual development in birds is determined by individual cells throughout the body.

A gene that determines whether birds develop testes or ovaries has been identified by scientists, in a discovery that highlights a key difference between the biology of birds and mammals.

The gene, known as DMRT1, controls the development of birds’ gonads but does not determine other sexual characteristics such as physical appearance, scientists found.

The development illustrates that, in contrast with mammals, sexual development in birds is determined by individual cells throughout the body, and not by gonadal hormones.

Findings from the study, carried out by the Roslin Institute, the Francis Crick Institute and the National Avian Research Facility, could help to address a welfare problem in the egg industry, in which male chicks are routinely culled.

Sex-reversal

Researchers used genome editing in male chick embryos to remove one copy of the gene, which previous studies had linked to sex determination.

The chicks developed ovaries instead of testes. However, the birds retained the physical characteristics of males – such as size and male feather patterns – and they did not lay eggs.

The hormone oestrogen has a key role in determining whether birds develop ovaries or testes, and helps control activity of the DMRT1 gene, the study also showed.

Outcomes from the research could inform the development of methods to identify chick sex early in development, so and prevent male eggs incubating to hatch.

In addition they may inform further research to understand how to produce sex-reversed chickens with functioning ovaries.

The research is published in Proceedings of the National Academy of Sciences.

In mammals, sex chromosomes determine the type of sexual organ formed and then sex hormones largely define the secondary sexual attributes, such as body size and shape. In birds, it seems that individual cells give rise to sexual characteristics and this is not significantly influenced by hormones.

Dr Jason Ioannidisformerly of the Roslin Institute, who led the analysis

These finding highlight the  evolutionary divide existing between mammals and birds.

Dr Mike Clintonthe Roslin Institute, who directed the research

The unique method of sex determination in birds highlights the difficult problem of reducing the excess numbers of male chicks in poultry breeding.

Dr Mike McGrewThe Roslin Institute, who co-directed the research

** The Roslin Institute receives strategic investment funding from the Biotechnology and Biological Sciences Research Council and it is part of the University of Edinburgh’s Royal (Dick) School of Veterinary Studies. **

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