Papaya
Sex-Chromosome Study Provides New Glimpse of Evolution
By Marcia Wood
January 23, 2004 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.
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