Researchers peek at the early evolution of sex chromosomes

Two new studies offer insight into sex chromosome evolution by focusing on papaya, a multi-million 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 was done by Ray Ming, a professor of plant biology, and Andrea Gschwend, when she was a doctoral student in his lab. | Photo by L. Brian Stauffer

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 reorganising 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 pseudogenes give researchers an opportunity to see evolution in action; they are evidence that the chromosomes are in the process of losing them.)

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.