sex determination in caterpillars
ROBERT BUTCHER
R.D.J.BUTCHER at dundee.ac.uk
Tue Jul 7 10:55:38 EDT 1998
Hi,
> I am not a butterfly or moth expert
Nor am I so please bear in mind that i cannot vouch for either the
accuracy of these replies, but as a starting point:-
.
I am guessing that caterpillars are sexually dimorphic.
Well, yes they are, in that there are male and female forms but not
haemaphrodite. There are parthenogenetic species (only females) in
Lepidoptera, but the majority are sexual:
>
> A. How can one determine the sex of a caterpillar
The gonads develop fairly early on and can be visible from the
second to third instar larval stage. However, it isnt always that
easy. If the species has a thin non-pigmented cuticle you should have
an easy time of seeing the gonads and sexing.
If the species has a darkly pigmented skin and / or a dense mass of
bristles emminating, then life is harder. this scenario is not
uncommon, and so for many species sexing at the larvael stage is
either not possible. Of course you could dissect it, but
then that is fatal. Sex is therefore often (usually?) determined in
the imago (butterfly / moth) form. Even in the imago, in many species
no clear sexual dimorphism such as a difference in body or antennal
size or wing pattern exists and so again sexing is using the
genitalia morphology, although it is at least partially external in
the imago!, so no invasive dissection (e.g. Ephestia genus of
Pyralidae). In contrast some species have dramatically different
morphology between males and females!!
>
> B. How or when is the sex of a caterpillar determined (for example, is it
> environmental sex determination?)
> No, environmentakl sex determination is not known in lepidoptera.
certainly the secondary (imago) sex ratio is influenced by a wide
range of factors (diet, density, temperature, diapause etc) but these
are affecting differential mortality (one sex dies more frequently
than the other) rather than the sex determination system. However it
has been claimed that feminisation (conversion of genetic males into
functional females) by an intracellular parasite (Wolbachia) occurs
in one lepidoptera species, but this awaits confirmation (Wolbachia
feminisation is common in isopods).
Somatic and gametic sex determination in lepidoptera is not known, to
my knowledge, at the molecular level at all. Simplistically it is sex
chromosome determined. That is the master sex determining genes are
segregated on chromosomes that are inherited in a sex-linked manner,
like in mammals (us). Except, it is the female that is heterogametic.
So, simplistically, in mammals sperm contain either an X or a Y
chromosome, eggs always a X. male determining genes are on the Y
chromosome and "dominant" over female determining genes, and
therefore XX=female, XY = male.(this is not the case in the
simplistically similar Diptera or Coleoptera, where autsome:X ratios
are involved). However in lepidoptera this is reversed. Sperm only
contain W, and eggs either W or Z, with WW = male, WZ = female
(Warning i may have got W and Z the wrong way round here, but the
principal is still true). Well downstream of the sex determining
genetic cascade somatic sex determination is mediated via variouse
steroid like hormones (including JH III), analagous in some aspects
to human sex steroids, but the molecular linkage of the master sex
genes (and maternally derived mRNA in the ova) is, to my knowledge,
still to be elucidated, as is the diferences between somatic and
gametic sex determination.
When?. Pass .After fertilisation and early on in the ova development
prior to late blastulation and well before a first instar caterpillar
hatches both gametic and somatic sex determination will be"
complete". The time scale as to when cellular sex commitmment on the
appropriate cells that will become gonad imaginal disc cells is
probably known, but sorry, not by me.
> C. Why don't caterpillars mate??
Physiologically they cant. they do not have external genitalia, in
early instars spermatogenesis and oogenesis will not have commenced
so there are no gamets to exchange. their isprobably no neurological
preparation for mating (recognition of sex pheromones, courtship etc
all essential for restricting cross species mating as well as in mate
selection on fitness grounds etc etc) etc etc
Evolutionary, the imago stage is " designed" for gamete exchange,
the larval stage for aquiring the resources neccessary for this (and
dispersal of the progeny to new resources etc)
> D. Can caterpillars change their sex?
To my knowledge no evidence exists of sex change in caterpillars (no
evidence of environmental sex determination). However, this does not
exclude the possibility of such, or of gynandromorph development per
se, but rather if it occurred it would be rare i guess.
>
> Finally, an unrelated question, can a caterpillar die of old age?
In strict terms, No. Senescence will not have evolved in
caterpillars, and im not aware of any data to support it either. In
contrast the larval form is under selection to survive to imago and
mate.Of course not all do, but that isnt senescence ("old age") but
just mortality. Senescence will have evolved on the reproductive
form, the imago (butterfly / moth)in terms of resource allocation to
adult longeivity versus gamete production etc etc.
However, caterpillars will certainly die in a time dependent manner
under certain abnormal (?) conditions, e.g. extended diapause when
you prevent them eclosing, inability to pupate when you prevent
pupation and maintain as final instar larvaec Its just this is not
really "old age" (= senescene), any more than any other non-optimal
condition is such as predation or parasitism, desease and
infection etc, but rather is a form of starvation. larvae are
evolved to make it to the imago stage, reproduce and thereafter "die
of old age". Well not sure if that helps at all, hope some of it
will, and i hope you get some more informative replies as well
Cheers
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
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