Re: Gender genetics and expression in sex-changing animals

Jason, the chromosomal structure and sexual development of fish are 
quite different from mammals such as humans.  Fish often depend on a much 
more complicated system of sexual determination than simple sex 
chromosomes, a system which is usually autosomal in nature.  This alone 
would suggest that sex determination and sexual differentiation are more 
labile in fish. Here, these processes depend more on quantitative 
interactions among genes spread throughout the genome, rather than on 
qualitative determination conferred by genetic components on heteromorphic 
sex chromosomes. Some organisms (as in the case of some turtles and 
alligators) depend totally on environmental cues, such as temperature, for 
determining sex.   

As in many vertebrates, it appears that the default sex in fish is female.
However, protogynous fish (the best studied example being members of the 
wrasse family) change their sex in response to environmental cues, or as a 
consequence of a regular developmental plan.  Some species start as males 
when they are small and switch to being female when they are large enough 
to "afford" the metabolic cost of producing "expensive" eggs versus "cheap" 
sperm.  Some go further and switch to male again when they are VERY large, 
since then they can defend territories which increase their access to 
females. Some fish undergo only a single sex change, and either sex may be 
the first in the sequence.

The developmental processes by which gonads switch from testes to ovaries 
and back again are quite interesting, but complicated.  Nevertheless, 
considerable histological and endocrinological work has been done to track 
these processes.  If you want to find the best starting point to understand 
the steps by which such changes occur, try the following references:

Bull, J. J. (1983) Evolution of Sex Determining Mechanisms (Benjamin 
Cummings, Menlo Park, CA)

Francis, RC and GW Barlow (1993) Social control of primary sex 
differentiation in the Midas cichlid.  PNAS 90:10673-10675