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Scientists discover papaya have sex chromosomes

By Cat Holmes
University of Georgia

Research has uncovered sex chromosomes, rare in plants, in papayas, according to a study that appears in the January 22 issue of Nature.

The discovery was a serendipitous one and "may shed new light on how sex chromosomes evolve," said Andrew Paterson, a University of Georgia plant geneticist and collaborator on the project.

The newly revealed papaya sex chromosomes appear to have formed in the last few million years. This is relatively recent when compared to human and animal sex chromosomes which are thought to be several hundred million years old.

"To me this is a nice example of why it's important to preserve and explore biodiversity," Paterson said. "Here in this relatively minor crop, someone found an outcome that may considerably advance our understanding of the evolution of sex chromosomes."

Papaya production

The original goal of the project, led by Ray Ming of the Hawaii Agricultural Research Center, was to find genetic markers in papaya to help farmers determine the sex of papayas earlier in their development. That's because determining the sex of papayas is directly related to efficient commercial fruit production.

Papayas, for those who don't know, come in three sexes: male, female and hermaphrodite. Hermaphrodites produce the sweetest fruit and are the most productive, so they're the ones farmers want to flourish. Females produce good fruit, but aren't as productive and males are uncommon and undesirable.

Unfortunately for papaya farmers, there is no way to determine a plant's sex from its seed. In a typical planting, about two- thirds of the plants are hermaphroditic and one-third are female. Once the plants have flowered, workers go through the fields and laboriously remove the females.

Since papayas typically flower between 6 and 12 months of age, farmers had to wait a good while before they could determine the sex. In addition to the labor cost of removing large female plants, competition from the female plants reduces the size and growth rate of the hermaphrodites, costing the farmers even more.

The research team found the genetic markers they were looking for quickly.

In fact, they found more than they were looking for.

Reviewing recombination

The scientists discovered that the area of the papaya chromosome where sex determination occurs has "few genes but lots of junk," Paterson said. Junk DNA is basically DNA that doesn't serve a purpose. It often accumulates in areas where recombination is suppressed, Patterson said.

Recombination is what allows the genes from the parents to be joined in different combinations to produce a seemingly infinite variety in offspring.

Most genes recombine but those on sex chromosomes, for the most part, do not.

"In the course of evolution, when a group of genes expresses sex, it seems to be so important that nature finds a way to protect those genes from recombination," Paterson explained. "The accumulation of junk [genes] seems to be a by- product of that preservation."

While about 90 percent of the human sex chromosome suppresses recombination, only about 10 percent of the papaya sex chromosome suppresses recombination. This suggests that the papaya sex chromosome is at a much earlier stage of evolution.

When the scientists investigated more thoroughly, they discovered that papaya sex chromosome have virtually all of the features that the human sex chromosomes have.

"Who knows what other kinds of messages are hidden out there in crops and other plants we haven't thought to look at yet?" Paterson said.

(Cat Holmes was a science writer with the University of Georgia College of Agricultural and Environmental Sciences.)

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