Some chickens appear to be male on one side of the body and female on the other, and researchers writing in Nature this week say they know why.
It was previously thought that sex chromosomes in birds control whether a testis or ovary forms, with sexual traits then being determined by hormones.
The authors of the new study, however, identified differences between male and female cells that control the development of sexual traits. The scientists have named the phenomenon, cell autonomous sex identity (CASI).
The findings may also be relevant to why males and females differ in behavior and in susceptibility to disease.
They could also lead to improvements in poultry production - identification of some of the molecular differences between male and female cells should lead to better tests for sexing embryos prior to hatch. It might even be possible to devise ways of obtaining the growth characteristics of male birds in females, with improvements in feed efficiency and productivity that could contribute to future food security.
"This research has completely overturned what we previously thought about how sexual characteristics were determined in birds. We now believe that the major factors determining sexual development are built into male and female cells and derive from basic differences in how sex chromosome genes are expressed. Our study opens a new avenue for our understanding of sexual development in birds, said
Dr Michael Clinton, who led the study.
"It also means we must now reassess how this developmental process occurs in other organisms. There is already some evidence that organs such as the heart and brain are intrinsically different in males and females and birds may provide a model for understanding the molecular basis for these gender differences."
Citation: et al., 'Somatic sex identity is cell autonomous in the chicken', Nature, March 2010, 464, 237-242; doi:10.1038/nature08852
