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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