sex determination

[Robert Butcher]

Hi Andrea,
In the absence (so far) of any replies, here is a stab at your 
question, but please treat with caution. I am no expert in this 
field. Hopefully any mistakes here will draw a correct answer out 
from those who know.

 The background:- 
Most, but not all, existant Lepidoptera have a sex chromosome based 
sex determination system where sex determining genes are segregated 
onto different chromosomes and so sex determination is determined by 
their segregation during oogenesis, akin to say the familiar human 
system (males XY, females XX), except that it is the female that is 
heterogametic. That is WZ is females, ZZ as males. 
This is however, not universal across all Lepidoptera, and several 
familiesy have a system more akin to the ancestral Trichoptera.
Of course, there may well be interplay between maternally derived 
factors (mRNA in the egg) as well as the parental sex chromosome 
determined genes in the fertilised egg (Zygote).
However, sex is determined in a cell indipendent (speciific) manner, 
not in the egg initially prior to division (see below).

 So to your questions, 
(1) Sex of what will become the somatic (body) tissue will be 
determined once these early zygote (parentally derived) genes are 
transcribed and translated, that is early after fertilisation and 
certainly long before first instar hatching. I would guess that 
gametic sex determination (the sperm or egg producing tissue), which 
is not likely to have an identical molecular genetic pathway but be 
based upon the same initial regulatory genes, would be similiar, that 
is at pole cell commitment which would be shortly after the 
fertilised egg starts to divide and form the multicellular embryo, 
before a first instar larvae even begins to develop.

(2)  Or is it possible to change the sex by 
> e.g. temperature after the egg is laid like in some fish?

Essentially No. Sex is therefore not determined outright by 
temperature (environmental sex determination), as occurs in some 
reptiles, nor by environmental cues such as sex ratio as occurs in 
certain fish.
Temperature, and other factors, may affect the apparent sexual 
phenotpye of some individuals, and this is the cause of many of the 
observed gynandromorphs, and the few intersexes, observed in 
Lepidoptera (individuals with characteristics of both 
males and females)
For example, as a WZ fertilised egg divides to form the cellular 
mass that will become the embryo, anything that can cause a failed 
segregtion (partitioning)of the W chromosome, such as temperature 
shock during mitosis (the seperation of the duplicated chromosomes 
into two nucleii that will result in two daughter cells) will result 
in one cell, and all its daughters down the line being Z (as long as 
this is not fatal), and so develop as male, whilst the other ceklls 
are till WZ and so feamle, because sex is determined at the cell 
level (see above).This mosiac pattern leads to gynadromorphs, or 
interesexes when it is not developmentally fatal.

Hope this anserwed your questions and that this simplified version 
helps explain the above answers.
If you need more info, i could look it up for you, or alternatively 
try contacting, for example, F. Marec or W. Tautz at the below 
address, as this is really their field (i think)
All the best
Rob

Frantisek Marec is at:- marec at entu.cas.cz
Walther Traut is at Institut fur Biologie, Medizinische Universitat 
zu Lubeck, Ratzeburger Allee 160, Lubeck, D23538, Germany, Europe


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