SINGAPORE - Their day job is cancer research. But a bunch of scientists here has a pricklier passion.
The self-confessed durian connoisseurs, who get their fix in Chinatown weekly during durian season, began to ponder the complexities of its odour and the mysterious allure of the king of fruit, even as they enjoyed eating it.
So in their free time, they attempted to unravel the durian's DNA. Three years later, they have a complete genetic map of the fruit - a world first - and some answers to their questions.
"I was naturally curious about the durian genome - what gene causes its pungent smell? How did its spiny husk arise?" said Professor Teh Bin Tean, deputy director for research at the National Cancer Centre Singapore (NCCS).
Their research was done on a particular durian variety, Musang King (Mao Shan Wang in Chinese), one of the most popular commercial varieties.
The scientists found that a durian has some 46,000 genes - double that of humans. Plants are known to have more complex genomes than animals, with multiple gene repeats.
And one type of gene, in particular, is responsible for its notorious smell - methionine gamma lyases (MGL), which regulates odour compounds called volatile sulphur compounds (VSC) - the same chemicals which give the pungent smell to rotten eggs.
Unlike other plants which typically have just two MGL copies, durians have four, which means their production of VSC is "turbocharged", and explains why they are so pungent, said study co-leader Patrick Tan, who is from Duke-NUS Medical School.
6 interesting durian facts
- The durian and the cacao plant, which produces the beans that create chocolate, shared a common ancestor 65 million years ago. The durian's closest relative is the cotton plant.
- It has about 46,000 genes - double that of humans, who have 23,000.
- A class of genes called methionine gamma lyases (MGL) is behind its pungent smell. They regulate volatile sulphur compounds, which can smell like rotten eggs. Plants typically have two MGL copies but durians have four, which could explain why it produces more of the smelly compounds.
- The genes involved in sulphur and flavour pathways in Musang King (Mao Shan Wang in Chinese) are more active than in the other durian cultivars such as D24, which may explain why it has the strongest taste and smell.
- A durian's pungent smell could be important in the wild in attracting animals, like elephants and orang utans, to eat and disperse its seeds to other regions.
- There are about 30 known durian species in South-east Asia, of which 11 are edible. Some of them are endangered.
The team also traced the lineage of the durian back some 65 million years to a common ancestor with the cacao plant, whose beans are used to make chocolate. Its closest relative is the cotton plant.
A paper on their study will be published online in the prestigious journal Nature Genetics on Monday (Oct 9) night.
The research was carried out using advanced sequencing technology that is ideal for studying plant genomes, and which became mainstream only a few years ago.
But while technology had a role to play in kick-starting the research project, it was the love of the fruit that ultimately made it possible, the team said.
The $500,000 project was funded by "fellow durian lovers", who wished to be anonymous.
It was, at times, a rough ride.
The scientists had many bumpy drives to durian plantations in Johor Baru where they begged farmers to let them buy unripe durians for their research, said Mr Cedric Ng, senior research associate at NCCS. Unripe fruit were needed to study how the level of MGL changed as the fruit developed.
"It is taboo to pluck off unripe fruits because they think their tree will die after," he said.
"But it has been an exhilarating journey."
The team had to extract the DNA from the durian plant multiple times to be sequenced.
The durian genome data has been donated to the National Parks Board. By unravelling the durian genome, the group has taken the first step towards identifying genes involved in disease resistance, flavour, drought tolerance, and other traits.
The team hopes that its effort will pave the way for researchers to create new durian species that are drought-resistant, or low-sugar varieties suitable for diabetics, for instance.
There are 30 other durian species in nature, of which 11 are edible. Some are endangered.
Said Dr Nigel Taylor, group director of the Singapore Botanic Gardens: "Understanding the genome of a species can enable you to understand how to conserve it, and also its relatives."