In many species, the possession of X and Y chromosomes determines whether an individual develops into a male or female. In humans, for example, individuals who inherit their father's Y chromosome become male (XY), and individuals who inherit their father's X chromosome become female (XX).
This system of sex determination has evolved independently multiple times and a striking feature of its evolution is that Y chromosomes have degenerated genetically, losing many genes over time. What is not well understood, however, is what happens to the Y chromosome during the earliest stages of this evolution, or the time scales over which degeneration occurs.
U of T Department of Ecology and Evolutionary Biology (EEB) professor Spencer Barrett, co-investigator of a study, said in humans, the Y chromosome has undergone extensive gene loss over its roughly 200-million-year evolutionary history, and now retains only about three per cent of its ancestral genes. We know very little about the early stages of the process, however, because it happened so long ago.
He said that the most well-studied Y chromosomes, including those in humans and other animal species, began degenerating hundreds of millions of years ago. Not so with plants.
Barret said that the emergence of separate sexes in plants is a relatively recent evolutionary innovation, making them ideal for this study, asserting that only about six per cent of flowering plants have males and females. The remainder are hermaphrodites.
The scientists used a plant species with separate sexes whose X and Y chromosomes probably first evolved around 15 million years ago at the most, making them relatively young compared to those in animals.
The study has been published in the journal Proceedings of the National Academy of Sciences.