Papaya is a multimillion-dollar crop with a complicated sex life. Now plant geneticist Ray Ming at the University of Illinois at Urbana-Champaign and his colleagues find that hermaphrodite papayas, the version most useful to farmers, most likely arose due to the ancient Maya roughly 4,000 years ago.
Papaya, the first fruit tree to get its genome sequenced, is a key crop across the world, yielding $130 million annually in Hawaii alone. The hermaphrodites produce the papayas that are sold commercially — the males do not produce fruit, and the females produce round fruit that occupy more space, making them less cost-effective to ship. “The same size box packs either 12 hermaphrodite fruit or 10 female fruit,” says Ming, who co-led the international team that produced the first draft of the papaya genome in 2008.
However, growing hermaphrodite papaya is costly and inefficient because a significant fraction of their seeds are female, which are of no use to growers. Farmers cannot tell the sex of a seed until it has grown and flowered, meaning they have to plant five or more seeds together to maximize their chances of getting at least one hermaphrodite. Hence, farmers end up wasting fertilizer and water on useless female papaya trees. And the typical crowding of papayas on farms results in poor root systems and small canopies that delay fruit production.
Scientists want to identify the mutation that caused this hermaphroditism in order to develop papaya trees that produce only hermaphrodite seeds, cutting costs for farmers while improving the health of the papayas and the environment. To that end, Ming and his colleagues analyzed the hermaphrodite papaya sex chromosome known as Yh, which is a mutant form of the male papaya Y chromosome.
The researchers sequenced and compared the hermaphrodite-specific and male-specific regions of the Yh and Y chromosomes, respectively, in 12 cultivated and 24 wild male papaya plants. They found less than half of 1 percent difference between the hermaphrodite and male sequences, which suggests they diverged unexpectedly recently, only about 4,000 years ago. In addition, the Y chromosomes most closely related to the Yh chromosomes all came from the northwest Pacific coast of Costa Rica.
These findings, reported online in the March 11 issue of Genome Research, suggest hermaphrodite papaya emerged at roughly the same time and place as the rise of the ancient Maya. “This is the only documented case that domestication of a crop plant contributed to the evolution of a sex chromosomes,” Ming says.
These findings should help narrow down which mutations led to hermaphrodite papaya – both male and hermaphrodite papaya can change sex under certain conditions. Ming adds that his group’s discovery of the ancestral male population that the hermaphrodites evolved from should help point to the gene that controls sex determination in male flowers. This, he says, could help researchers understand the molecular basis of sex reversal, and eventually eliminate natural sex reversal in the fruit.
Geneticist Michael Freeling, at the University of California, Berkeley, says that the work means that the mystery of papaya hermaphrodity is now understood to the point that a “genetically satisfying, gene-regulatory explanation is within reach.”