This Wasp’s Larvae Sometimes Grow Hundreds of Soldier Clones—But Why?
Imagine, for a moment, having 3,000 twin sisters. You all live inside a caterpillar, but it’s home, and your numbers serve as a veritable army against any trespassers.
Such is the life of the parasitoid wasp Copidosoma floridanum, one of the insect world’s most fascinating examples of polyembryony, in which multiple embryos form from a single egg. C. floridanum females lay eggs inside the eggs of certain moth species; when the moth larva hatches, the wasp egg gives rise to as many as 3,000 clone embryos.
But wait, there’s more.
C. floridanum also exhibits polyphenism, in which different types of wasp larvae develop from the same genetic material—best known for its role in the caste system (queen, worker, soldier, etc.) appearing in social insects such as ants and bees. C. floridanum produces two different types of larvae: reproducers and soldiers. The reproducers emerge during the moth larva’s last instar, consume it, and pupate into adult wasps. The soldiers, however, emerge earlier but never molt, and they die when the host dies. Instead, they spend their time defending their reproductive siblings by attacking other parasitoids present in the same host larva. The script isn’t always the same, though, as C. floridanum can shift the number of soldier larvae it produces, and it is also known that male eggs tend to produce fewer and less-aggressive soldier larvae.
Researchers, of course, want to know why all of this happens. In a study published in July in the Annals of the Entomological Society of America, a team led by Margaret Smith, Ph.D., assistant professor at the University of North Georgia (and previously a postdoctoral researcher at the University of Georgia, where a portion of the research was conducted), explore the conditions that spur increased soldier development in Copidosoma floridanum as well as some of the underlying biological mechanisms driving it. “We decided to do both experiments in this paper because we knew part of the story but wanted to fill in a few important missing pieces,” says Smith.
Previous research showed that the presence of competitors was linked with increased soldier production in C. floridanum. Smith and colleagues tested whether other external factors were also related. (Shifts in caste development in other insect groups are commonly exhibited as responses to environmental conditions.) They found, however, that heat shock and bacterial infection had no influence on soldier production. Only multiparasitism, in the form the presence of competitor parasitoid Microplitis demolitor, led to increased soldier production in their experiment.
What about M. demolitor causes C. floridanum to shift toward more soldier production? The researchers tested that, too, and found that the M. demolitor egg chorion is the trigger.
As C. floridanum embryos proliferate, the differentiator between reproductive and soldier larvae is the presence or absence of a “primary germ cell” during embryo division. Embryos without the primary germ cells become soldier larvae. Smith and colleagues found that, in response to multiparasitism, female broods of C. floridanum rapidly shift (within 16 hours) to produce more embryos without primary germ cells. The same shift did not occur, however, in male broods. All of this adds up to a deeper understanding of how polyembryony and polyphenism have evolved in C. floridanum. “I think that these findings are important because they advance our understanding of caste formation in this species,” Smith says. “C. floridanum is different than many social insects in that caste is determined by the presence of germline stem cells and not environmentally. Understanding this species more helps us better understand the variation in evolution of caste differentiation and plasticity in general.”
Left unanswered, however, is exactly how C. floridanum engineers the caste shift, something Smith says he hopes to answer in the future.
“I’d really like to understand more about the cellular basis of caste formation,” says Smith. “Is gene expression altered so that primary germ cells adopt a non-germline fate? Do broods physically move the primary germ cell so that they fail to end up in daughter embryos during proliferation?”
Annals of the Entomological Society of America