sex determination

[Robert Butcher]

Joe,
thanks for the clear message to Andrea.
 I knew I should have waited longer before replying, and trusted that 
someone in the know would give the correct, and nice and clear, 
info!!

The difference you highlighted is i think important. That is 
although the timing of expression /action of the initial regulatory 
(and possible master) and downstream switch genes (such as Dsx /Tra 
etc in most insects, Sxl being so far restricted i believe to 
drosophila), and consequently downstream male/ female determining 
genes of the cascade indeed is very variable in different somatic 
tissues in drosophila (and probably so in leps), it is restricted by 
the material available, that is the master gene(s) on the sex 
chromosome. Thus simplisttically it could be viewed that in one way 
sex determination is principally decided, although not strictly 
committed sensu, at fertilisation, given that failed sex chromosome 
segregation is not that common, possibly fatal in cases where dosage 
compensation fails, and in gametic tissue may even lead to sterility. 

In contrast,However, your point emphasising that cellular sex 
commitment is independent  and variable in its onset, and for some 
tissue progenitors can be as late as metamorphosis, has importance 
beyond explaining the observation of rare intersexes or (sometimes) 
common gynandromorph development, since in addition to alteration 
by abiotic and biotic factors leading to the above results,  it is 
open to manipulation by "selfish" genetic elements or parasites. For 
example, Wolbachia (a diverse group intracellular parasitic bacteria) 
mediated feminisation of genetically determined males (i.e. ZZ, in a 
female heterogametic system) is well established in certain isopods, 
but has fairly recently been reported in one Lepidoptera (i think 
this still awaits confirmation). 
Anyway, thanks again
 Rob
Robert Butcher,
Evolutionary and Ecological Entomology Unit,
Department of Biological Sciences,
Dundee University,
Dundee, DD1 4HN,
Tayside, Scotland,
UK.
Work Phone:- 01382-344291 (Office), 01382-344756 (Lab).
Fax:- 01382-344864
e-mail:- r.d.j.butcher at dundee.ac.uk