CHAMPAIGN, Ill. Two new studies offer insight into sex chromosome evolution by focusing on papaya, a multimillion dollar crop plant with a sexual problem (as far as growers are concerned) and a complicated past. The findings are described in two papers in the Proceedings of the National Academy of Sciences.
The research reveals that the papaya sex chromosomes have undergone dramatic changes in their short evolutionary histories (they are about 7 million years old; by comparison, human sex chromosomes began their evolution more than 167 million years ago). One of the two studies compares the papaya X chromosome with that of a closely related non-sex chromosome (called an autosome) in a sister species. The other looks at differences between the X and Y chromosomes.
The studies show that the papaya sex chromosomes are increasing in size mostly through the accumulation of repetitive sequences while also reorganizing themselves and losing some genes carried over from their days as autosomes. Some of the lost genes are gone without a trace, while other remnants of genes that are no longer functional called "pseudogenes" are still present. The papaya Y chromosome also has independently gained some genes from the autosomes, the researchers report.
Gene loss in the Y chromosome is well documented in ancient Y chromosomes, but gene loss in the X chromosome, particularly at this early stage, is unexpected, as is the expansion of the X chromosome, said University of Illinois plant biology professor Ray Ming, who led both studies.
"The pace of gaining repetitive sequences and losing genes is faster in the Y than in the X chromosome, however," he said.
"This is the first look at an early stage of sex chromosome evolution," said Andrea Gschwend, who conducted the r
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University of Illinois at Urbana-Champaign