Papayas may contain the secrets of the evolution of sex chromosomes, according to new research. "This is the first time we've documented an X-Y system at the beginning stage, at the start of evolution towards a conventional X-Y system," says molecular biologist Ray Ming.
Papaya (Carica papaya) is unusual among plants in having three sexes--male, female, and hermaphrodite, each with distinctive flowers. Their odd system of sex determination has intrigued scientists for decades and is of economic importance to Hawaiian papaya farmers because their customers prefer hermaphrodite fruit. Now, molecular techniques have revealed what makes a male.
Using three genetic mapping approaches, Ming and colleagues at the Hawaii Agriculture Research Center in Aiea identified a small block of DNA that codes for maleness. This region accounts for about 10% of one chromosome and is only found in males and hermaphrodites. Unlike the Y chromosome in animals and a handful of other plants, it appears to be the same size as its X-like counterpart. But like the Y chromosome in animals, the male region appears to have lost some DNA that codes for proteins; that loss is generally thought to be a step along the way to the Y chromosome diverging from the X.
Scientists believe this happens through suppression of recombination, a process in which chromosomes in a pair crossover to produce novel combinations of the parents' genetic material. Ming's team found no recombination in the male region of 2190 papaya plants, further suggesting that this it is an incipient sex chromosome.
Sex chromosomes are generally thought to have evolved from ordinary "autosomal" chromosomes. The revelation that the male- specific region of the papaya is small and had a lower proportion of defunct genes than the human Y chromosome--about 38%, compared to 95%--suggests that it's just started to diverge from an autosomal chromosome. "The papaya [male-specific] region might resemble the ancestor of the human Y chromosome as it existed about 240 million to 320 million years ago," the researchers report in the 22 January issue of Nature.
However, not everyone is convinced. Evolutionary geneticist Dmitry Filatov, of the University of Birmingham, U.K., cautions that while "this paper seems to be quite convincing that there is a sex chromosome ... the evolutionary side of the story is not that clear." Filatov says he would like to see papayas compared to another species in which X and Y haven't diverged at all.