HONG KONG, April 24 (Reuters) - A toxic and smelly Polynesian fruit nicknamed the "vomit fruit" drives away most predators, but scientists in Japan may have discovered how it can serve as food and nest for a species of fruit fly.

The researchers hope the discovery can help in the fight against pests.

"We can also begin to understand how to manipulate insects' behaviour by changing their preference for particular substances," they said in an article published in the Public Library of Science journal PLoS Biology.

The researchers said they had identified two genes in the Drosophila sechellia fruit fly that make the fruit so appealing to the insect.

No other type of fruit fly would go near the fruit of the Tahitian Noni shrub and those that land on it die. But the D. sechellia feeds on it and lays eggs on the fruit, ensuring a bountiful meal for its young.

Such symbiotic relationships draw the attention of researchers, who hope to manipulate sensory preferences of pests like mosquitos, which are drawn to human blood and sweat and responsible for spreading disease among them.

Using genetic analysis, the Japanese scientists found the D. sechellia possessed two olfactory genes - Obp57d and Obp57e - which were different from corresponding genes in other fruit flies.

"We found, for the first time, the genes that determine the insects' preference (for) their host plants," Takashi Matsuo, assistant professor of biological sciences at the Tokyo Metropolitan University, wrote in an email to Reuters.

To prove that these were the genes that made the offending fruit so attractive to the D. sechellia, Matsuo's team bred a closely related species of fruit fly and replaced their olfactory genes with those of the D. sechellia.

"(They) adopted the behaviour of the donor fly ... replacing the Obp57d and Obp57e genes changed the fly's response to the host toxins," the scientists wrote in the article found online: http://dx.doi.org/10.1371/journal.pbio.0050118.

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