Quest for Better Asparagus Solves Mystery of Gender Origins

A honeybee collects pollen from a male garden asparagus plant. (Piet Beurskens)

(CN) – A long-standing theory of the origin and early evolution of sex chromosomes has been confirmed after scientists sequenced the genome of asparagus.

The findings, published Thursday in the journal Nature Communications, will aid asparagus breeding efforts and resolves questions surrounding the original formation and development of male sex chromosomes.

Most flowering plants are hermaphrodites. Asparagus plants, on the other hand, are generally either male (XY) or female (XX), though “supermales” (YY) can also be produced. Growers favor all-male (XY) plants because they live longer and do not self-breed. To produce all-male XY seeds, breeders cross XX females with YY supermales.

Until now, however, such breeding efforts have required time-consuming test crosses to differentiate between XY males and YY supermales.

“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 senior author Jim Leebens-Mack, a plant biology professor at the University of Georgia.

The identification of the genes that determine sex will also allow breeders to more efficiently develop and produce 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,” Leebens-Mack said.

“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.”

The diversity of sexual systems in plants has sparked questions since the days of Charles Darwin, while Danish geneticist Mogens Westergaard proposed a two-gene model for the origin of sex chromosomes in the early 20th century. However, the theory was impossible to test using human and mammal sex chromosomes as the divergence of the X and Y chromosomes occurred millions of years ago.

But the origin of separate sexes and sex chromosomes is more recent for flowering plants like asparagus, which enabled the team to test Westergaard’s two-gene model while also aiding crop breeding programs.

As predicted by Westergaard, and others, the connection between a gene necessary for male function and a gene that stunts the development of female organs on a small part of the Y chromosome did in fact spark the evolution of asparagus sex chromosomes.

“Over the last hundred years, evolutionary biologists have hypothesized several ways that a regular pair of chromosomes can evolve into an X and Y pair that determine sex,” said lead author Alex Harkess, a former doctoral student at Leebens-Mack’s lab.

“Our work confirms one of these hypotheses, showing that a sex chromosome pair can evolve by mutations in just two genes – one that influences pollen (male) development, and one that influences pistil (female) development.”

 

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