Hawaiian papayas.  Click the image for additional information about it.

Papaya Sex-Chromosome Study Provides New Glimpse of Evolution

What do people and papayas have in common?

Both have specialized chromosomes carrying genes that determine the gender of their offspring. These so-called "sex chromosomes" are markedly different from ordinary chromosomes.

In the current issue of the journal Nature, scientists from industry, university and the Agricultural Research Service provide the first direct evidence that papaya sex chromosomes are evolving from other chromosomes--ordinary ones in the plants' genetic makeup. The same process is thought to have occurred in the human genome during the millions of years of human sex chromosome evolution.

In humans, sex-chromosome studies may explain inheritance of gender-linked conditions such as sickle-cell anemia or hemophilia. In papaya, studies of sex chromosomes may help scientists understand inheritance of traits responsible for the size, shape and quality of this popular tropical fruit. Apparently, only papaya trees that inherit a specific combination of genes on their sex chromosomes produce fruit that has the elongated shape that consumers of Hawaii-grown papaya prefer. Today, just half of all commercial papaya seedlings grown in Hawaii inherit this prized combination of genes.

ARS research plant physiologist Paul H. Moore at Aiea, Hawaii, and ARS research horticulturist Francis T.P. Zee at Hilo participated in the papaya study. Both are with the ARS U.S. Pacific Basin Agricultural Research Center. Plant molecular geneticist Ray Ming of the grower-sponsored Hawaii Agriculture Research Center, Aiea, led the investigation.

The scientists analyzed genetic material from more than 2,000 fresh papayas. They found that a chromosome with a small region of genes for male traits--comprising only about 10 percent of the chromosome's length--determines sex in papaya. This papaya chromosome resembles a primitive version of the human Y chromosome--perhaps as it existed 240 to 320 million years ago. The papaya research provides a unique opportunity to examine the beginnings of sex chromosome evolution.

ARS is the U.S. Department of Agriculture's chief scientific research agency.