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Of chicken and men

Research on chicken chromosomes reassures men that the Y chromosome is here to stay.

Egg-laying chickens and human males don’t seem to have much in common. But they inherited their sex in a similar way. In humans, maleness is determined by the Y chromosome. All eggs carry an X chromosome, while sperm carry X or Y. When a sperm with a Y chromosome fertilizes an egg, congratulations! It’s a boy. Chickens and other birds do the same thing in reverse: all sperm carry a Z chromosome while the egg has either a Z or a W. The ZZ combination is a male rooster and ZW lays eggs.

The Y and W chromosomes are unique. Half the population doesn’t have one, so they carry only nonessential genes. They are alone in cells, without a similar partner chromosome, so they can’t undergo recombination. This process of swapping DNA is crucial for creating the genetic variety that is the basis of evolution and for eliminating harmful mutations from the gene pool.
“A popular hypothesis is that sex-specific chromosomes like Y and W will go extinct because they accumulate bad mutations that can’t be removed by recombination,” says Sofia Berlin, associate professor, Department of Plant Biology and Forest Genetics, Uppsala BioCenter, Swedish Agricultural University. This is supported by the shrinking size of the Y chromosome over millions of years of evolution, fueling speculation that it will eventually disappear altogether.

This idea is refuted by a 2012 PNAS study by Berlin and colleagues at the University of Oxford and University College London. They investigated the evolution of sex-specific chromosomes like W and Y by measuring the expression of genes on W. They looked at five different chicken breeds, two that had been selected for high egg production, two selected for the male traits of plumage or cockfighting, and a wild ancestor of chickens. They found that chickens selected for egg laying expressed W genes more highly than the ancestral chicken or birds selected for male traits.

The researchers concluded that W genes had adapted to selective pressure for female fertility – more eggs, please – by increasing gene expression. This shows that the W chromosome is responsive and evolutionarily healthy, not dying out. Genes on W can’t recombine, but they aren’t static. Says Berlin, “We found that the W chromosome is not accumulating bad things. It’s accumulating good things for female birds. It’s undergoing positive selection, not degradation. Maybe Y could accumulate good things too, who knows?”

The research had an added benefit. The W chromosome is highly repetitive, making sequencing difficult, so the initial chicken genome sequence had poor coverage of W. Berlin says the study found previously unknown genes on W. Discovering their functions will be the basis for new research projects.

Berlin was involved in the W project because it related to her PhD work, and she still collaborates with the study leader, University of Oxford’s Judith E. Mank. However, Berlin’s main work at the Uppsala BioCenter is plant genetics. She applies molecular tools and works with plant breeders on the willow, a common bioenergy source in Sweden, optimizing varieties for fast growth and pathogen resistance. Some of Berlin’s work is in familiar territory, though. She also studies sex determination in plants. g

Article: Moghadama HK, Pointera MA, Wright AE, Berlin S, Mank, JE. W chromosome expression responds to femalespecific selection. Published in PNAS, May 22, 2012 vol. 109 no. 21 8207-8211.