London, Apr 24 : Research institutions in the U.S. and China have produced a first draft of the papaya genome, detailing more than 90 percent of the plant's gene coding sequence, which may shed new light on the evolution of flowering plants.
As the research was based on a genetically modified plant, the newly sequenced papaya genome ends up giving the most exhaustive account of the genetic changes till date that makes it resistant to the papaya ringspot virus (PRSV).
Papaya is known to be one of the most nutritious fruits known and has now become the fifth angiosperm (flowering plant), after Arabidopsis (a well-studied member of the mustard family that includes species such as cabbage and radish), rice, poplar and grape, for which detailed genome information is available.
"One of the implications of this study is, on a larger scale, to understand the genome evolution of angiosperms," Nature quoted Ray Ming, a University of Illinois professor of plant biology and co-lead author on the study, as saying.
Ming revealed that recent findings suggested that the papaya genome ventured on a different evolutionary path after its divergence from that of Arabidopsis about 72 million years ago.
In fact, it has also been found that the number of functional genes in papaya is much lower than any other flowering plant for which genome sequence is available. Also, there is a significant difference in its allotment of genes for key enzymes as compared to its counterparts.
Besides, it is rich in genes for enzymes involved in cell-wall expansion and starch production unlike Arabidopsis. Papaya is also ahead in number of genes for volatile compounds, the odours that attract pollinators and animals that eat the fruit and disperse its seeds.
Papaya production worldwide is affected by PRSV, which hinders its ability to photosynthesize, resulting in stunted plants often producing deformed and inedible fruit.
In the early 1990s, scientists at Cornell and the University of Hawaii developed a transgenic papaya that was resistant to PRSV by using a technique developed in 1986 that involved randomly inserting a viral coat protein gene into a plant to give the plant immunity to the virus.
In the new study it was revealed that the transgenic insertions occurred in only three places in the papaya genome, without any disruption in any nuclear gene.
The findings of this study appeared as the cover article in the journal Nature.