Researchers have identified a system of three genes that are responsible for hybrid sterility in rice, or the inability of many hybrid rice species to pass their genes on to the next generation. Their findings suggest one way that hybrid sterility is maintained across rice species, and they might also lead to the genetic improvement of rice as a food stock.
The hybrid rice (middle) created by crossing indica and japonica subspecies is sterile but vigorous. | Photo courtesy of Qifa Zhang
When two different species mate, like a horse and a donkey, their hybrid offspring—a mule in this example—is born reproductively sterile. But this phenomenon, known as hybrid sterility, isn’t limited to the animal kingdom. Plenty of plants produce viable hybrids that are more robust than their parental strains, but they are often genetic dead-ends as well.
Jiangyi Yang and colleagues from Huazhong Agricultural University in Wuhan, China, along with other researchers from the Chinese University of Hong Kong, have now investigated the sterility that occurs in hybrids of the indica and japonica subspecies of the cultivated rice, Oryza sativa.
Their results appear in the 14 September issue of the journal Science.
The researchers homed in on a specific region of the rice chromosome, called S5, which they had associated with hybrid sterility during an earlier study. They found three tightly linked genes—ORF3, ORF4, and ORF5—that control fertility in indica-japonica rice hybrids.
Among indica and japonica subspecies of cultivated rice, the ORF5 gene and its partner ORF4 work together to kill female gametes, or eggs, while the ORF3 gene actively tries to save those gametes. Specifically, the researchers suggest that ORF5 produces a molecule that is sensed by ORF4 and leads to an increase in stress on a cell’s endoplasmic reticulum, a structure inside the cell that helps to fold and transport proteins. That stress eventually activates ORF3, they say, which works to stabilize and protect the endoplasmic reticulum.
This kind of killer-protector system underlies the hybrid sterility between these two species of cultivated rice, according to the new study. Yang and the other researchers suggest that non-lethal combinations of ORF4 and ORF5 may allow indica-japonica hybrids to pass their genes on to subsequent generations—and that overcoming hybrid sterility in rice could lead to more robust crops and higher yields in the future.
13 September 2012