How sex chromosomes diverged within a species

New research from an international team of breeders, genome scientists, and plant biologists at UGA sheds light on longstanding questions about the origin and early evolution of sex chromosomes, and at the same time serves as a foundation for asparagus breeding efforts:

While most flowering plants are hermaphrodites, garden asparagus plants are typically either male (XY) or female (XX), although YY "supermales" can be produced in the greenhouse. Growers prefer all-male plants, as they live longer and do not self-seed. Breeders produce all-male XY seed by crossing an XX female, with a YY supermale. Until now the differences between asparagus X and Y chromosomes were not understood and breeders were not able to distinguish XY males from YY supermales without time-consuming test crosses.

"One of the things that we were able to do pretty early in our collaboration was to identify genetic markers that allowed breeders to efficiently distinguish XY males from YY males and then use those YY males to produce all-male seed," said Jim Leebens-Mack, professor of plant biology and senior author on the study.

Understanding the genetic variation in plants that allows for XY and YY males was advanced by identification of the genes that determine sex, which paves the way for more efficient development and production of valuable hybrid asparagus plants.

"In addition to more rapid identification of sex genotypes, our collaborators are now able to manipulate the asparagus Y chromosome to convert males to females or hermaphrodites. In the near future, breeders will be able to cross whatever lines they want, without having to look within a particular line for the female that has one set of characteristics, and in another line for a male with complementary traits," Leebens-Mack said.

Questions about the great diversity of sexual systems in plants go back to Charles Darwin, and a two-gene model for the origin of sex chromosomes was coined by Danish geneticist Mogens Westergaard in the early 20th century. But the theory was impossible to test through analyses of humans and mammal sex chromosomes, where divergence of the X and Y chromosomes happened tens of millions ago.

Great new work, broadening some of the most fundamental insights in science and bringing them into today for use by scientists and industry. The march of knowledge continues. Congratulations to Leebens-Mack, Harkess and the entire team on this important new article published in Nature.

Image: photo of a honeybee collecting pollen from a male garden asparagus plant, by Piet Beurskens of Limgroup