LAUSR

Tsetse flies transmit trypanosomes—parasites that cause devastating diseases in humans and livestock—across much of sub-Saharan Africa. Chemical communication through volatile pheromones is common among insects; however, it remains unknown if and how such chemical communication occurs in tsetse flies. We identified methyl palmitoleate (MPO), methyl oleate, and methyl palmitate as compounds that are produced by the tsetse fly Glossina morsitans and elicit strong behavioral responses. MPO evoked a behavioral response in male—but not virgin female—G. morsitans. G. morsitans males mounted females of another species, Glossina fuscipes, when they were treated with MPO. We further identified a subpopulation of olfactory neurons in G. morsitans that increase their firing rate in response to MPO and showed that infecting flies with African trypanosomes alters the flies’ chemical profile and mating behavior. The identification of volatile attractants in tsetse flies may be useful for reducing disease spread. Description The scent of tsetse Volatile sex attractants can be used in traps to monitor and control insect spread. Tsetse flies spread disease among humans and livestock in sub-Saharan Africa, and methods for controlling them are lacking. Ebrahim et al. identified fatty acid methyl esters as the volatile sex attractants in the tsetse fly Glossina morsitans, and showed that mating behavior was promoted for male files in response to these compounds (see the Perspective by Syed). Mating behavior in this species was also modulated by infection with parasites that cause African trypanosomiasis. These results suggest that sex attractants might be used in traps for controlling tsetse spread in sub-Saharan Africa. —MM Methyl palmitoleate is a pheromone of the tsetse fly Glossina morsitans. INTRODUCTION Tsetse flies are vectors of trypanosomes that cause human and animal diseases in sub-Saharan Africa. Historically these flies and the trypanosomes they transmit have had extremely detrimental effects on the health and development of this region. Tsetse continues to be a major cause of rural poverty in this area because of the impact of trypanosomiasis on people and domestic farm animals. Moreover, the geographic range of tsetse is expected to extend to new regions as a result of climate change, thus placing more people and domestic animals at risk. Despite more than a century of tsetse research, volatile sex pheromones have not been identified. Because pheromones have been successfully used in the control of a wide variety of other insects, their identification in tsetse could be useful in controlling these flies and trypanosomiasis. RATIONALE To identify volatile pheromones of tsetse, we used gas chromatography-mass spectrometry (GC-MS). We studied the behavioral responses of tsetse flies to pheromones and to other flies perfumed with them. To elucidate the cellular effects of pheromones we analyzed the electrophysiological responses they elicited from olfactory neurons in the antenna. We analyzed both males and females. Although we focused on Glossina morsitans, we also used the related species Glossina fuscipes, which accounts for the greatest number of cases of human African trypanosomiasis. We also investigated the effects of trypanosome infection on the chemical profiles and sexual behavior of flies. RESULTS When introduced into a simple mating paradigm, pairs of tsetse began copulating within seconds. A single male placed in a chamber with a tsetse-sized decoy landed on it and stayed attached for minutes if the decoy was dosed with female extract, but not if dosed with male extract. Using GC-MS we identified methyl palmitoleate (MPO), methyl oleate (MO), and methyl palmitate (MP) as volatile compounds produced by G. morsitans that elicited strong behavioral responses. All three compounds have a 16-carbon unbranched backbone. We focused on MPO, which elicited a strong response from males—but not virgin females—in the decoy assay. This response depended on the antenna. MPO also elicited an attractive response from males in a T-maze paradigm. Male G. morsitans mounted G. fuscipes females perfumed with MPO, but not control females. We characterized olfactory neurons of the G. morsitans and G. fuscipes antennae. We identified a class of neurons on the G. morsitans antenna that responds to MPO. These responses are sexually dimorphic, and this class of neurons also responded to certain odorants that attract tsetse in the field. MPO had little if any effect on G. fuscipes in our physiological or behavioral testing. The pheromone profiles of the two species are different. Notably, infection of mated G. morsitans with trypanosomes resulted in the appearance of 21 small volatile compounds in their chemical profiles. Infection also reduced the sexual receptivity of females. CONCLUSION We have identified a volatile sex attractant of the tsetse fly. MPO acts as an aphrodisiac, elicits sex-specific behavioral effects, and evokes sexually dimorphic physiological responses. It also elicits species-specific behavioral and physiological effects. Our work invites exploration of whether MPO may be useful in tsetse control. We also found that infection with trypanosomes affects both the chemical profile and sexual behavior of tsetse. Volatile chemicals of tsetse flies. Methyl palmitoleate (MPO), produced by the tsetse fly Glossina morsitans, elicits physiological responses from olfactory neurons. MPO also elicits behavioral responses, acting as an attractant and an arrestant, and as an aphrodisiac when used to perfume females of another species. Trypanosome infection alters the tsetse chemical profile. When placed with two females, one uninfected and one infected, males mated with the uninfected female. CREDIT: TSE TSE FLY ILLUSTRATION: ESTEFANIA ALONSO GÓMEZ, CC BY-SA 4.0