Birds of a feather don't always flock together




Sex, Ys, and Platypuses
By Jocelyn Selim
April 25, 2005 | Biology & Medicine



For mammals, gender is usually a simple affair. From mice to elephants,
a single pair of chromosomes control sex-inheriting double Xs makes
females, while inheriting a Y makes males. But for the platypus, the
story is a bit more complicated.



Unable to locate a single Y chromosome in the platypus, researchers had
long considered its gender-determining genes a mystery. By using
fluorescent labeling to track platypus chromosomes during cell
division, Frank Gruetzner, a molecular biologist at the Australian
National University, solved the puzzle. Instead of a single pair of sex
chromosomes, the platypus has five-a record for vertebrates.
"It's not as confusing as it might be," Gruetzner says. "The
sex chromosomes link up in a chain, so a male platypus is always
XYXYXYXYXY."



Even more intriguing, one of the platypus's Y chromosomes shares
genes with the ZZ/ZW sex chromosomes found in birds, which are thought
to have evolved separately. "We're not certain exactly how the two
are related, because in birds the system is inverted," says
Gruetzner. "Males are the ones with two identical sex chromosomes."
The presence of both bird and mammal chromosomes in the platypus
implies that sex determination evolved just once among a common
ancestor of both groups, and then diverged into distinct systems.



The platypus has always looked a little ambiguous. Its signature
duckbill and egg-laying are birdlike, but its fur and milk are
mammalian. "Now," says Gruetzner, "we know this ambiguity goes
all the way down to its genome."


Frank Gruetzner and his colleagues want to sequence the platypus
genome. Read their proposal at
www.genome.gov/Pages/Research/Sequencing/SeqProposals/PlatypusSEQ.pdf#search='Frank%20Gruetzner'


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