What Are Gonads For (Among Else)?

This post from January 21, 2005, is about insects, parasitoids and the mental approach to science:

This really cool science post (Speaking of sex differences reminds me of a seminar I attended a few years ago, about a parasitoid wasp that injects a single egg (together with some toxins and a DNA virus) into a (somewhat larger) egg of its moth host.

The speaker spent his 50 minutes describing his painfully difficult and inconclusive molecular experiments, trying to figure out where the DNA (from the injected viruses) inserts itself into the host genome and how does that insertion affect the host. The effects of parasitism were some small changes in size and head-to-body proportions, complete loss of gonads (in both sexes), and a developmental change: instead of going through four instars before pupation, the parasitized host went through five instars and then, after a very long period in the fifth instar, died during the process of pupation.

Everyone in the room, including the speaker, made an assumption that DNA virus somehow disrupts development, ensuring sufficient time for the parasitoid to hatch inside the host and eat its way out of it. That is why the guy tried so hard to find the insertion place. Of course, the inevitable question arose during the Q&A session: why are the hosts castrated? Neither the speaker, nor anyone in the audience had a faintest idea. It seemed quite superfluous: all of the hosts die before adulthood, so there is no sense in preventing them from breeding after they are dead anyway.

I thought of it completely differently. My understanding (a testable hypothesis, I guess) was that viral DNA inserts in such a way that it induces castration. Perhaps it inserts smack in the middle of one of the genes responsible for the embryonic development of the gonad. But then, the lack of gonads and their hormones results in botched-up development. After all, a number of hormones, including juvenile hormone, ecdisone and some gonadal steroids are involved in control of metamorphosis (both from one larval instar to the next and from larva to pupa). While others saw castration as a mysterious side-effect, I saw it as the main mechanism by which the parasitoid manipulates the developmental timing of the host.

And if I am right, all that expensive and inconclusive molecular tinkering was unneccessary. Remembering the photo of the larva from the slide-show, I remember the gonads being huge and just below the surface (cuticle). Surgical castration should be technically relatively easy. Removing the gonads would then, if I am right, result in the appearance of the fifth instar and death in pupation in non-parasitized larvae. Injections of gonadal extracts, or candidate hormones, into the castrated hosts at approriate times would recover the normal pattern of development. Actually , such work could help discover what the "appropriate times" are, thus helping us understand insect development better.

So, (as I wrote before on the difference between US and Russian science), don't automatically switch on your PCR machine. Think first!