性染色体的进化历史

Two new studies offer insight into sex chromosome evolution by focusing on papaya（木瓜） , 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 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（常染色体）, 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 research with Ming while she was a doctoral student in his lab. "Usually people will focus on the ancient sex chromosomes because they are the most relevant to us," she said. "So this is the first direct and complete look at a more recently evolved sex chromosome system."

Analyzing the X chromosome is vital to understanding the evolution of sex, said Ming, an affiliate of the Institute for Genomic Biology at Illinois. The new findings in papaya suggest that the human X chromosome, too, has undergone numerous changes since it first distinguished itself from the autosomes, Ming said. Such changes are not detectable because the ancestral autosomes are no longer available for comparison, he said.

Because the papaya sex chromosomes are young and can be compared to closely related autosomes in a sister species, they offer a view of the early events of both X and Y chromosome evolution, Ming said.

Studying papaya sex chromosomes is a complicated task, however. The papaya has male, female and hermaphrodite sexual types, with two kinds of Y chromosomes (the male Y and the slightly modified, hermaphrodite Yh). Papaya plants may produce combinations of male and female (from the XY system) or hermaphrodite and female (from the XYh system) plants.

This complexity causes problems for papaya growers, Ming said. Hermaphrodites are the most productive of the papaya sexual types and yield the best fruit, but the offspring of hermaphrodites are not all hermaphrodites. To aid growers, Ming and his colleagues aim to develop a "true-breeding" hermaphrodite（雌雄同体的） papaya variety that consistently produces hermaphrodite offspring.