WESLACO – Two scientific papers on the study of the sex chromosomes of papaya have been published as cover stories in the most recent issue of Proceedings of the National Academy of Science of the United States of America, according to one of the contributing scientists.
Dr. Qingyi Yu, far right, is shown here with her team members. They include, from left, Samuel Pina Jr., Jiangbo Zhou, Christine Bonczyk, Ninfa Ramos and Ratnesh Singh. (Photo courtesy of Dr. Qingyi Yu)
Dr. Qingyi Yu, a Texas AgriLife Research plant molecular biologist at Weslaco and a member of the scientific team doing the research, said their work is yielding new information about the evolution of plant sex chromosomes.
Yu said she and her colleagues are studying the sex chromosomes of papaya to help growers become more efficient in the production of the fruit, estimated to grow on about one million acres worldwide.
“Our team sequenced the hermaphrodite-specific region of the Y chromosome, HSY, and its X counterpart,” she said. “By comparing HSY and its X counterpart, the team revealed two large inversions occurred in the Y chromosome, which likely caused cessation of recombination between HSY and its X counterpart and resulted in additional genome rearrangements.”
In the other paper, the team compared the X chromosome of papaya with its homologous autosome from its close relative, Vasconcellea monoica, Yu said.
“V. monoica has no sex chromosomes and is the only monoecious species in the Caricaceae family,” she said. “It served as an outgroup to trace the evolutionary history of the papaya X chromosome.”
It had been thought the X chromosome preserved the gene content of its ancestral autosome, Yu said.
“It is surprising that the papaya X chromosome has expanded due to accumulation of transposable elements,” she said. “The results changed our previous views on sex chromosome evolution.”
Because there is no true hermaphrodite breeding line available, which is preferrable because it self-pollinates, papaya farmers have to plant seedlings at random and thin out all but the hermaphrodites after plants flower, she said.
“That’s a very inefficient way to plant papayas,” Yu said. “It also means that until they are thinned out, plants compete with each other, which delays flowering and growth. The goal is to eventually clone the genes that control sex types in papayas and develop true hermaphrodite papaya varieties.”
Papayas have relatively young sex chromosomes in an evolutionary sense and offer a rare glimpse into how sex chromosomes evolved at an early stage, she said.
Yu’s work is funded by a grant from the National Science Foundation plant genome program.
The studies were led by Dr. Ray Ming, a plant biology professor at the University of Illinois, in collaboration with Texas A&M University, Hawaii Agriculture Research Center, the University of Wisconsin, the University of Georgia, the University of Hawaii, the University of Edinburgh, and Youngstown State University.
Details of the studies can be found at http://www.pnas.org/content/early/2012/07/31/1121096109
and
http://www.pnas.org/content/early/2012/08/01/1207833109 .
