sex determination

Joseph G. Kunkel joe at bio.umass.edu
Wed Apr 28 09:19:07 EDT 1999


Rob,

Thanks for your underlining the critical points I made.  The more we
repeat it the more interesting facts come out.

Robert Butcher wrote:

> ... Thus simplistically it could be viewed that in one way
> sex determination is principally decided, although not strictly
> committed sensu, at fertilisation, ...

Yes, this is true I believe for insects.  For fish and other yolky-egg
laying vertebrates a tissue can be chromosomally male and yet behave
like a female.  Its chromosomal sex is not relevant.  Such a tissue is
the liver.  A male vertebrate liver (frog, chicken, fish, lizard) will
respond to estrogen by producing the egg yolk protein, vitellogenin, in
the same way that a female can.  Male humans can develop breasts under
the pathologic or artificial influence of female hormones.  Insect
tissues are much less apt to react this way.  I did some research on
cockroach capability to respond to the gonadotrophic hormone that showed
that both male and female larvae, close to metamorphosis, could both
respond weakly to gonadotrophin by producing vitellogenin but when they
metamorphosed to the adult form the male lost all ability to respond
while the female was enhanced orders of magnitude in its response.  The
mechanisms behind these changes may, sadly, never be known due to the
overpowering predominance of Drosophila as an experimental organism.  It
is so much more difficult to study a "non-genetic" organism like a
cockroach than the ultimate genetic organism, Drosophila.  However, as
you point out, Drosophila, and presumably most of Diptera, do many
things in their unique way.  While segmentation genes seem to be
universal, Diptera are one of the few yolky egged animals that do not
use vitellogenin as an egg storage protein.  That is true despite the
homology of vitellogenin genes between most insects, frogs, lizards,
birds and nematodes!

   It will take some very dedicated lepidopterist to understand the
mechanisms that Andrea has questioned in Lepidoptera.

                         -or-
> ... 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 ....

 - a unique puzzle that presents itself to a biologist may gain us
insight via serendipity, which is always a welcome surprise.  Perhaps
the timing of the Wolbachia feminization can be used as a tool tol learn
about the timing of Lepidopteran tissue sex!  That is just one reason
why we need continued work in all groups and species since; if no one is
looking, we may miss our opportunity to learn from the valuable clues
with which nature is teasing us.

Cheers,
Joe
-- 
______________________________
Joe Kunkel, Professor
Biology Department, University of Massachusetts at Amherst
joe at bio.umass.edu  http://www.bio.umass.edu/biology/kunkel/


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