A team of university and agriculture industry researchers has set out to develop a bisexual variety of papaya that could improve farming one of Hawaii’s biggest crops.
An international team with representatives from the University of Hawaii and Hawaii Agriculture Research Center published new research on papaya sex genes in Monday’s issue of the Proceedings of the National Academy of Sciences of the United States of America.
The research is featured on the cover of the weekly publication, and advances the study of early-stage sex chromosome evolution in other plants and animals, including humans. But a major advance in farming papaya also could come out of the work.
Commercial papaya farmers in Hawaii typically plant three to seven seedlings to ensure they get at least one plant that will produce fruit on its own.
This inefficient practice is necessary because papaya plants occur in three sexual forms: male, female and hermaphrodite, or bisexual — though only bisexual plants produce good fruit.
Male plants can’t produce fruit and don’t occur in commercial papaya varieties grown in Hawaii.
Female plants need to be pollinated by a male or bisexual plant but don’t produce commercial-grade fruit.
So bisexual plants are what farmers want, but any given seed has only a 50 percent chance of being bisexual for the popular Rainbow variety. The odds are 66 percent for the more disease-susceptible Solo variety.
Ken Kamiya, owner of Oahu papaya farm Kamiya Gold Inc., said not having to weed out female plants would be a boon to the industry.
"If you eliminate females you save the farmer money," he said. "It’s a costly deal."
There were 172 commercial papaya farms covering 2,000 acres in Hawaii last year, according to the National Agricultural Statistics Service. Papaya was the state’s seventh-biggest crop by value at $9.7 million. But the industry has suffered in recent years from pests, disease, low prices, vandalism and high operating costs, according to the agency.
Hawaii papaya production peaked in 1984 at 306 farms that produced 80.5 million pounds of fruit. Last year’s crop totaled 28.6 million pounds, which was down from 30.1 million pounds in 2010 and the previous decade’s high of 55 million pounds in 2001.
A change Japan made in January to allow imports of Hawaii’s genetically modified Rainbow variety has opened new opportunities for local papaya farmers. Now the prospect of a new genetically modified variety stands to help the industry further.
Genetically modified papaya was commercialized in 1998 with the Rainbow variety engineered to resist the ringspot virus that threatened to wipe out Hawaii papaya farming. Such fruits have gained large consumer acceptance but also have posed trouble for some farmers who have had trees chopped down by vandals suspected of being motivated by anti-GMO sentiment.
The latest research focused on sex chromosome evolution in papaya, and involved sequencing a hermaphrodite-specific region of the plant’s sex chromosome.
The authors said papaya can serve as a window into understanding early stages of evolution in sex chromosomes that’s not possible in more evolved plants and animals, including humans, because the sex chromosomes in papaya arose only about 1.9 million years ago, or "just yesterday in evolutionary terms."
Researchers from Hawaii Agriculture Research Center, or HARC, first discovered that papaya had relatively young sex chromosomes, and published their findings in the British journal Nature in 2004.
The new research was conducted by a team led by Ray Ming, a former HARC employee now with the University of Illinois at Urbana-Champaign. Others on the team were from HARC, UH, Texas A&M University, the University of Wisconsin at Madison, the University of Georgia, Youngstown State University in Ohio and the University of Edinburgh in Scotland.
Local contributors to the research were Ching Man Wai, Eric J. Tong, Ratnesh Singh, Ming-Li Wang, Maqsudul Alam and Paul H. Moore.
The continuing research on developing a hermaphrodite papaya variety is being pursued by HARC, the University of Illinois and Texas A&M.
