Biology in the News Explained

Cool bugs #10 – Fruit fly parasites

Leptopilina is a genus of parasitic wasps present mainly in Europe, that attack various fruit fly species in the genus Drosophila. Females lay an egg in a fly larva, and the wasp larva develops inside while the fly larva continues to grow, and to begin to pupate. During the pupal stage, the wasp larva begins to feed and develop on the fly’s tissues. It pupates within the fly’s pupal case, from which it emerges as an adult wasp – only one wasp can emerge from a single host in these species.

Leptopilina species have been popular model organisms in the study of behavioral ecology; in particular, they have often been used to test foraging theory models. This is because they are very easy to work with in the laboratory. They can be observed through a dissecting scope as they search for Drosophila host larvae in a petri dish. In the case of parasitic wasps, “foraging” actually refers specifically to females searching for hosts in which to lay their eggs, even though classic “foraging” models refer more generally to any animal searching for food. It is reasoned that evolutionarily, there will be stronger selection for optimal foraging strategies if you are a female depositing eggs, because the consequence of your “foraging” decisions directly affects your reproductive success. In the case of food foraging, reproductive success is affected indirectly through the animal obtaining sufficient energy to reproduce.

Work with Leptopilina species has led to advances in foraging models. For example, through the late 1970′s at least, it was generally believed that when a female parasitic wasp laid an egg in a host that had already been parasitized, either by another female or herself, she was engaging in suboptimal behavior: she was wasting her eggs because only one wasp could emerge from the host. Thus when this behavior was observed, it was argued by some that the wasps might not be able to tell whether or not a host was parasitized. But in the 1980s and 1990s, researchers such as Marcel Visser demonstrated mathematically that “superparasitizing” a host (laying more than one egg in a host) could be adaptive behavior under specific conditions. This is because although the first egg laid in a host is usually the one to survive, there is a small probability (that drops off exponentially with time) that the second one laid will “win” and emerge as the adult. It appears that Leptopilina can detect when a host has already been attacked, but still makes appropriate foraging decisions. For example, if there is a lot of competition with other female wasps within a patch of larvae, and larvae are hard to find, a wasp might be better off laying a few eggs in previously parasitized hosts than not laying any at all.

L. heterotoma female parasitizing Drosophila larvae.

L. heterotoma parasitizing Drosophila larvae.

L. heterotoma is one of the useful members of the genus that attack Drosophila melanogaster larvae. D. melanogaster is the common fruit fly that can be collected almost anywhere in the world; it has very happily gone with humans wherever they have gone, and it is easy to breed them, and thus L. heterotoma, in the laboratory. To observe L. heterotoma ovipositing, all you have to do is mix some yeast with warm water, spread it on some filter paper in a petri dish, and add Drosophila larvae. Because Drosophila are always found in rotting fruit or vegetables, their parastioids are cued in to odors associated with fermentation – where there are yeast doing their work, there are probably Drosophila larvae – so providing the yeast tells a female Leptopilina that this is a good spot to search for larvae. When she enters the yeast patch and is close to larvae, she detects a different odor, specific to the fly larvae. This causes her to unfurl a long ovipositor that is usually stored in a protected spot along her abdomen. She places the tip of the ovipositor in the yeast, and starts jabbing at random, until she hits a larva. She then makes the decision whether or not to lay an egg. There is indeed a chemical marker left by previous females that allows her to detect whether or not a larva is already parasitized. An observer watching her can tell if she has accepted or rejected the host: she either jerks the ovipositor out after a few seconds, or leaves it in much longer.

The behavior of the foraging wasp can be manipulated in several ways, such as letting her deplete her eggs before the test, housing her with other females so that she perceives competition, giving her previous foraging experience in the presence of particular odors, etc.

Another wasp in this genus, L. clavipes, has been the subject of much research because some strains are infected with a bacterial species, Wolbachia, that cause them to produce only female offspring. This has dramatic effects on the population genetics of L. boulardi, because normally in wasps, unfertilized eggs become males, and fertilized eggs become females. Infected L. boulardi strains are all-female, and lay female eggs without fertilization.

L. boulardi can harbor a virus that manipulates their foraging behavior, causing them to superparasitize more often than would be expected given foraging models. The benefit to the virus appears to be that it can be transmitted from one wasp larva to another within a Drosophila host. This illustrates that even when we think we are studying a single organism and using it to test and refine evolutionary models, we have to keep in mind that all animals (including humans) harbor many symbiotic organisms, and our assumption that bacteria and viruses that do not cause disease also have no other effects on them may often be wrong.

(Photo used by permission.)

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