I thought that the X and Y chromosomes determined gender in animals, but I see no mention of them in your article. Does this mean that on a genetic basis, males and females in these organisms are identical?

Neil H. Murphy
Walnut Creek, Calif.

In a sense, yes, says Philip Hastings of the Scripps Oceanographic Institute in La Jolla, Calif. Sex-changing fish don’t rely on chromosomes to determine sex, so there’s no special DNA difference between males and females. However, other fish species, such as swordtails and guppies, do depend on chromosomes to determine sex .—S. Milius