Crude Papaya Chromosome Tied to Human Sex Marker

Researchers looking for a simple way to determine the sex of papayas have discovered that the plant has a primitive Y chromosome that scientists believe is similar to the human Y chromosome before hundreds of millions of years of evolution. The Y chromosome confers maleness on its bearer.

The discovery was expected to be very valuable to papaya growers, but should also shed light on how the Y chromosome developed and achieved its unusually small size.

Papayas come in three sexes: male, female and hermaphrodite. Hermaphrodites produce the sweetest fruit and are the most productive; females produce good fruit, but are less productive; and males are uncommon and undesirable.

Unfortunately, there has been no way to determine a papaya’s sex from its seed. Now, when the plants flower at six to 12 months, workers go through fields and remove all but the hermaphrodites.

Biologist Ray Ming of the Hawaiian Agricultural Research Center reported in the Jan. 22 issue of Nature that they quickly found genetic markers that would allow seed selection. They also found a distinct Y chromosome, which is very unusual in plants.

The area of the chromosome where sex determination occurs has “few genes, but lots of junk,” said Andrew Patterson of the University of Georgia, a member of the team.

The junk DNA, which basically serves no purpose, accumulates in chromosomes when recombination -- the exchange of genetic information during sexual reproduction -- is suppressed. In the papaya Y chromosome, about 10% of the chromosome suppresses recombination, compared with 90% of the human Y chromosome. That indicates the papaya chromosome is at a much earlier stage of evolution, Ming said.