Towards a molecular understanding of mosquito host detection
Mosquitoes obtain blood-meals from vertebrate hosts to enable egg production. Blood is not the only source of nourishment for mosquitoes. Both male and female mosquitoes seek nectar sources from plant hosts. Since males male do not bite, nectar is their principle energy source. Mosquitoes gain access to these nutrients by sensing volatile chemicals in their environment. Olfactory cues such as body odor, carbon dioxide, and nectar volatiles are sufficient to enable mosquito attraction. Understanding how mosquitoes sense their hosts is a focus of the DeGennaro lab. Using a molecular genetic analysis of mosquito behavior, we seek to identify the odors and olfactory receptors that are necessary for mosquito host detection.
Dr. DeGennaro with Ir8a mutant mosquitoes.
Current Projects
Identifying the mosquito receptors for human odor
Mosquitoes must bite to transmit pathogens. This observation stresses the importance of understanding how mosquitoes find their human hosts. The specific genes and neural circuits mosquitoes use when host-seeking are largely unknown, but it is clear from my work on the Orco and Ir8a co-receptors that key olfactory pathways play an essential role. Aedes orco is required for DEET repellency, human host preference, and nectar-seeking. Surprisingly, removing Orco function only reduced mosquito host-seeking in the absence of CO2. This immediately suggested that mosquito host-seeking involved the integration of multiple olfactory pathways. To test this, we explored the role of Ionotropic Receptors (IRs) in mosquito host detection. We made mutations in the Ir8a olfactory co-receptor to understand the role of this olfactory pathway in human host detection. We found that Aedes Ir8a mutants cannot sense acidic volatiles found in human odor or behaviorally respond to lactic acid, a component of human sweat that has been identified as a key kairomone. Attraction to humans and human odor is significantly reduced in Ir8a mutants. Our epistatic analysis showed that CO2 sensitization and host odor detection by other olfactory receptor pathways is not sufficient to rescue this host-seeking defect, highlighting the importance of acid volatile detection in mosquito attraction to humans. The predictions of our epistatic analysis have been supported by the overlapping expression of Ir25a and orco as well as the lack of significant co-expression of other olfactory receptors in Ir8a neurons. We are currently working on characterizing odor-tuned IRs that mosquitoes use to sense acidic volatiles.
CO2-independent detection of acidic volatiles by the Ir8a olfactory pathway. In contrast to Ir8a, our evidence suggests there may be significant interactions between orco and Ir25a that require CO2 sensing by Gr3 to promote mosquito host-seeking.
Species-specific mosquito preferences for the bacteria and odor of individual humans
Comprehensive analysis of mosquito host detection also requires an understanding of the components of human odor and how they are produced by the host skin microbiome. We have recently completed a DARPA funded project to design a next generation insect repellent. Our goal was to understand and exploit the differences in mosquito attraction to individual humans to help build a microbiome-based repellent. To accomplish our goal, we tested 119 human subjects for their attractiveness to Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus mosquitoes. During each subjects visit we also assayed their skin microbiome composition, odor profiles, and chemosensory skin contact cues. Our analysis has revealed key human odors and bacteria that are preferred or avoided by different mosquito species. As we work through this large data set, we are finding new species and subspecies of skin bacteria that are candidates to include in our in vitro skin bacterial community models.
Dr. John Castillo demonstrating the use of the uniport olfactometer to understand mosquito attraction.
Vector ecology of Aedes aegypti in South Florida
Our lab is located in a region where Aedes aegypti is prevalent and local transmission of the dengue and Zika viruses has occurred. We are developing projects to understand the vector ecology of mosquito host interactions in south Florida. Our work is supported by the National Institutes of Health and the Centers for Disease Control as part of the statewide "Southeastern Regional Center of Excellence in Vector-Borne Diseases: The Gateway Program". We are conducting a large-scale analysis of mosquito populations using adult mosquito trapping and oviposition-based surveillance methods to understand the connection between the landscape and Aedes aegypti and Aedes albopictus populations in Miami, Houston, and Charleston. This GIS-based study seeks to build models to predict population dynamics to guide vector control interventions. This work is being conducted in collaboration with Dr. Melissa Nolan at the University of South Carolina and Dr. Sarah Gunter at the Baylor College of Medicine.
Roberto Palacios counting Aedes aegypti eggs
Manipulating where mosquitoes lay their eggs
We have identified a novel aggregation behavior in female mosquitoes. When equal oviposition site choices are offered, Aedes gravid females are more likely to choose one site over another. This aggregation occurs regardless of egg deposition or the ability of females to contact water. Aggregation decreases as the number of females increases which suggests that both attractive and repellent cues could govern this behavior. To test this, we developed an avoidance assay by pre-placing multiple females in one of two oviposition chambers. Gravid wild-type females choose to aggregate in sites without pre-placed females while orco, but not Ir8a mutant females, show increased avoidance, indicating that attractive olfactory cues influence aggregation. In contrast to orco, Ir8a and wild-type females, Gr3 mutants did not avoid populated oviposition sites, and carbon dioxide causes females to avoid empty breeding sites. Our work has shown that chemosensory-dependent communication between female mosquitoes can orient their choices when selecting an oviposition site.
Understanding mosquito behavior in the laboratory is essential, but application of this knowledge requires testing in field conditions. In collaboration with Dr. Marcus Stensmyr at Lund University, we identified geosmin as a potent attractant for ovipositing female mosquitoes. We are currently conducting applied research to understand how geosmin-rich beet root peel can be used to increase the efficacy of current oviposition and adult gravid traps that usually contain hay or oak leaf water with our partners in the Southeastern CoE for Vector-borne Disease.
Genetic analysis of mosquito nectar-seeking
Identifying the receptors that enable mosquito nectar-seeking behavior is one of our interests. We know that the olfactory receptor co-receptor, ORCO is necessary for both male and female mosquitoes to respond to nectar volatiles. The ORs that provide the odor-ligand specificity are unknown. In this project, we seek to identify receptors using a combination of loss-of-function behavioral studies and cell-based assays. In addition, the first meal a female mosquito ingests consists of nectar not blood. We are interested in learning more about the molecular basis of this switch from nectar-seeking to human host-seeking behavior. This research could set the stage for a novel approach to lock female mosquitoes in a behavioral state where they never become interested in human hosts.
Aedes albopictus male caught in the act of nectar-seeking.
