By Cat Holmes
University of Georgia
Research has uncovered sex chromosomes, rare in plants, in
papayas, according to a study that appears in the January 22
issue of Nature.
The discovery was a serendipitous one and “may shed new light
on how sex chromosomes evolve,” said Andrew Paterson, a
University of Georgia plant geneticist and collaborator on the
project.
The newly revealed papaya sex chromosomes appear to have formed
in the last few million years. This is relatively recent when
compared to human and animal sex chromosomes which are thought
to be several hundred million years old.
“To me this is a nice example of why it’s important to preserve
and explore biodiversity,” Paterson said. “Here in this
relatively minor crop, someone found an outcome that may
considerably advance our understanding of the evolution of sex
chromosomes.”
Papaya production
The original goal of the project, led by Ray Ming of the
Hawaii Agricultural Research Center, was to find genetic
markers in papaya to help farmers determine the sex of papayas
earlier in their development. That’s because determining the
sex of papayas is directly related to efficient commercial
fruit production.
Papayas, for those who don’t know, come in three sexes: male,
female and hermaphrodite. Hermaphrodites produce the sweetest
fruit and are the most productive, so they’re the ones farmers
want to flourish. Females produce good fruit, but aren’t as
productive and males are uncommon and undesirable.
Unfortunately for papaya farmers, there is no way to determine
a plant’s sex from its seed. In a typical planting, about two-
thirds of the plants are hermaphroditic and one-third are
female. Once the plants have flowered, workers go through the
fields and laboriously remove the females.
Since papayas typically flower between 6 and 12 months of age,
farmers had to wait a good while before they could determine
the sex. In addition to the labor cost of removing large female
plants, competition from the female plants reduces the size and
growth rate of the hermaphrodites, costing the farmers even
more.
The research team found the genetic markers they were looking
for quickly.
In fact, they found more than they were looking for.
Reviewing recombination
The scientists discovered that the area of the papaya
chromosome where sex determination occurs has “few genes but
lots of junk,” Paterson said. Junk DNA is basically DNA that
doesn’t serve a purpose. It often accumulates in areas where
recombination is suppressed, Patterson said.
Recombination is what allows the genes from the parents to be
joined in different combinations to produce a seemingly
infinite variety in offspring.
Most genes recombine but those on sex chromosomes, for the most
part, do not.
“In the course of evolution, when a group of genes expresses
sex, it seems to be so important that nature finds a way to
protect those genes from recombination,” Paterson
explained. “The accumulation of junk [genes] seems to be a by-
product of that preservation.”
While about 90 percent of the human sex chromosome suppresses
recombination, only about 10 percent of the papaya sex
chromosome suppresses recombination. This suggests that the
papaya sex chromosome is at a much earlier stage of
evolution.
When the scientists investigated more thoroughly, they
discovered that papaya sex chromosome have virtually all of the
features that the human sex chromosomes have.
“Who knows what other kinds of messages are hidden out there in
crops and other plants we haven’t thought to look at yet?”
Paterson said.
