French and American researchers have discovered a new function of a gene for sexual switching in melons that can provide a great deal of information about evolution of sexual systems in plants. While French scientists led by Abdel Bendahmane of the National Institute for Agricultural Research (INRA), isolated the melon sex determination gene and determined its function, New York University biologists Jonathan Flowers and Michael Purugganan conducted the evolutionary analysis of the study.
The sexual system in melons, called andromonoecy, carries both male and bisexual flowers and appears to have evolved recently, and the scientists in the study aimed at understanding what determines the recent formation of melons' new sexual system.
"If we can understand how different sexual systems in plants have evolved, we can then begin to understand how sex in general evolves," explained Purugganan.
Focussing their study on the role of hormone ethylene, which is known to help fruit ripen, the French scientists determined that an enzyme involved in making this gaseous hormone also plays a role in the evolution of the sexual switch of female flowers to hermaphrodites. This finding links hormone levels to sex determination in flowers.
They also wanted to find out if the change in ethylene levels, and therefore the resulting sexual system, was the result of evolutionary selection. Thus, they looked at the ethylene enzyme gene, called CmACS-7, which had the mutation that causes the sex change in melons.
When the researchers examined the molecular diversity in this gene, compared it with other genes in the melon genome, and used mathematical modeling, it was concluded that the level of molecular variation at the sex determining ethylene enzyme gene was not something that occurred by chance.
Instead, the pattern was consistent with evolutionary selection favouring the sex switch mutation in melons.
"Humans and other mammals generally have only two sexes - males and females. But other species, including plants, can evolve bewildering arrays of sexual combinations," observed Purugganan.
He suggested that this study provides us with new insights into the molecular basis for sex determination, enabling us to understand the advantages of different sexual systems.
The study appears in the latest issue of the journal Science.