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Structural differences in Nasonia wasps linked to evolution, behavior and disease

What can a tiny wasp with a rather gruesome parasitic life cycle teach us about evolution, behavior and human developmental diseases? In a new paper, researchers led by István Mikó and Holly Hoag at the University of New Hampshire (UNH) suggest that it may be a lot.

The research team took a thorough three-dimensional look at four species of Nasonia wasps. The paper, on the cover of the Journal of Insect Science, describes subtle structural differences in the lower head regions between species that have important effects on courtship and mating behaviors. The findings yield insight into a much larger scope of development and disease research.

"Insects have very simple development pathways and tissues," says Mikó. "They allow us to learn how genes contribute to basic differences in structures and behaviors in ways that are not possible in humans."

Revealing the intricacies of the genetic interactions that lead to defects and differences in Nasonia therefore provides a key foundation for understanding them in other species, including humans. For example, Nasonia is an important model organism for craniofacial abnormalities, which are the most common human birth defects.

Courtship between parasites

Nasonia wasps lay their eggs in flies. To be more precise, females seek out the pupae—analogous to butterfly cocoons—of larval flies transforming into adults. After drilling a small hole through the tough outer layer, they inject venom and lay 20–40 small eggs. The venom stops development in the host flies, making them a better food source for the larval wasps once hatched.

The growing larvae devour their hosts from within, then progress through their own pupal stage. The adult wasps eventually leave the host, but some species mate even before they emerge. The parasitism is so effective that Nasonia are employed as an important biocontrol for pest flies.

Parasitic behavior aside, Nasonia are of particular interest to biologists. Their different species are quite closely related, providing an excellent model system for studying the evolution of species and the biological mechanisms driving specific behaviors.

Mikó, the collections manager of the UNH Collection of Insects and Other Arthropods, and Hoag, a 2021 UNH graduate who worked with Mikó for her senior capstone project, focused on the mating behavior of Nasonia males in conjunction with the structural features of their lower heads. Previous studies showed that differences in the lower head region of males can be used to distinguish between Nasonia species. Could those differences also be connected to variations in courtship behavior?

In general, male Nasonia are observed to initiate courtship with a sequence of "head-nods" and wing vibrations. There have also been indications that the male produces an aphrodisiac pheromone, though its composition and origin remain unknown. The female, in turn, chooses whether to signal receptivity and mate or to reject the male, ending the courtship process. The specifics of nodding intensity and duration and where courtship takes place, whether still within or outside of the host fly, depend on the Nasonia species.

The researchers used advanced three-dimensional microscopy technologies to examine the head regions in four Nasonia species—vitripennis, longicornis, giraulti and oneida—to gain further insight.

Head shapes and aphrodisiacs

The research team found that the gena, the "cheek" area between the compound eye and mouth region, showed important differences between the males of different species. Also of note, they identified and characterized three glands in the head region that had not been previously described. Two of them showed no notable differences between species or sexes, but the other one, named the genomandibular gland, was either absent or so small that it was not measurable in females.

Accurately measuring such small structures usually requires expensive software packages to analyze the image data. Hoag used her ingenuity to create an effective, lower-cost option.

"I saw that dentists were using a free, open-source program to measure tooth volume," says Hoag. "I was able to apply their workflow and use it for measuring the structures in Nasonia."

Based on their findings, the researchers hypothesize that the genomandibular gland is in fact the source of the aphrodisiac pheromone. Further, they propose that the differences in courtship behaviors, lower head structures and gland size are connected and are associated with whether each species mates inside or outside of the fly host, or both.

Further investigation of the specific differences between species of courtship behavior and pheromone production will yield increased knowledge of the relationship between evolution and development in the wasps as well as other species, even humans.