© 2018. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2018) 221, jeb157131. doi:10.1242/jeb.157131 REVIEW Olfaction, experience and neural mechanisms underlying mosquito host preference Gabriella H. Wolff* and Jeffrey A. Riffell* ABSTRACT behaviors. However, there are significant gaps in our knowledge Mosquitoes are best known for their proclivity towards biting humans about the degree to which various types of plasticity and learning and transmitting bloodborne pathogens, but there are over 3500 contribute to host preferences. One clue comes from epidemiological species, including both blood-feeding and non-blood-feeding taxa. analyses of disease vectors, including mosquitoes, suggesting that The diversity of host preference in mosquitoes is exemplified by the only 20% of a host population can account for 80% of transmission feeding habits of mosquitoes in the genus Malaya that feed on ant potential (Woolhouse et al., 1997). This implies that, within a host regurgitation or those from the genus Uranotaenia that favor species, a subset of individuals is attracting more mosquitoes than the amphibian hosts. Host preference is also by no means static, but is rest (Kelly, 2001). One explanation for this heterogeneity is that characterized by behavioral plasticity that allows mosquitoes to sensory cues emitted from hosts might be highly variable within a switch hosts when their preferred host is unavailable and by learning species, and mosquito receptors are tuned to respond to only a subset host cues associated with positive or negative experiences. Here we of such cues. Another is that mosquitoes can learn and remember review the diverse range of host-preference behaviors across the sensory information associated with the best (and worst) hosts and family Culicidae, which includes all mosquitoes, and how adaptations change their behavior based on experience. In this Review, we in neural circuitry might affect changes in preference both within the explore the neural mechanisms and other factors underlying host- life history of a mosquito and across evolutionary time-scales. seeking and host preferences, both genetically determined as well as those subject to plasticity (Fig. 2). KEY WORDS: Insect, Sensory processing, Learning, Sensory systems, Neurobiology Host preferences in specialists and generalists Researchers rely on two general methods to determine the hosts Introduction preferred by mosquitoes: behavioral observation and blood meal Mosquitoes that prefer to bite humans have long been studied in analyses. Behavioral observation can be performed in the field or their capacity as disease vectors, commanding significant influence laboratory, using traps baited with potential host-related scents or by over global ecosystems, epidemiology and economies by their placing traps in tents or huts (Mclver, 1968; Service, 1993; Busula impact on human health and welfare (WHO, 2016). However, over et al., 2015). The first study to show human host preference in the 3500 species of mosquito have been described in the family African malaria mosquito Anopheles gambiae in Tanzania (Gillies, Culicidae, inhabiting every continent except Antarctica (Knight and 1964) employed an experimental hut enclosed in a mosquito net. Stone, 1977). Preferred hosts vary widely across mosquito species, Mosquitoes were released to make a choice between two chambers: ranging from humans and other mammals to reptiles, birds and one containing a sleeping human volunteer and one containing a arthropods (Fig. 1) (Harris et al., 1969; Tempelis, 1975). Some sleeping ox calf. In other laboratory studies, wind tunnels were used species may be specialists or opportunists and some have evolved to observe mosquito behavior in response to host odors presented loss of blood-feeding entirely, including all members of the genera upwind and two-choice olfactometers (see Glossary) or ‘Y-mazes’ Malaya, Topomyia and Toxorhynchites (Clements, 1999; Day, were used to calculate preferences (Knols et al., 1994; Eiras and 2005; Steffan and Evenhuis, 1981). Where blood-feeding has Jepson, 1991; Cooperband et al., 2008; Vinauger et al., 2014). A been conserved, host preference does not track the phylogeny of Y-maze consists of an entrance and two arms with air flowing Culicidae, and different species within a genus may prefer different towards the entrance. In either arm, researchers can present host sources of blood. An intriguing issue relates to how mosquitoes odors or clean-air controls and record the proportion of mosquitoes choose their hosts. that chose one arm over the other. In blood meal analyses, To answer this question about host preference and behavior, we researchers collect mosquitoes either in the field or semi-field and must examine the mosquito sensory nervous system that is used to extract gut contents of fed mosquitoes to characterize host identity detect hosts and the brain that computes and commands behavioral using forensic methods such as PCR or probing blood with host- actions. To a great extent, the nervous system is hard-wired, with derived antibodies (Tempelis, 1975; Takken and Verhulst, 2013). genetically determined receptors for specific information in the Observations of mosquito host preferences have revealed both environment, allowing mosquitoes to detect cues from some host generalist and specialist strategies across species. Blood meal analysis organisms and not others (McBride et al., 2014; Rinker et al., 2013). of the tiger mosquito Aedes albopictus revealed a generalist strategy, By comparing which receptors are expressed by different mosquito with females feeding on a wide variety of mammals, including species, we can begin to understand differences in host-seeking rabbits, deer and dogs, as well as a variety of birds, including passerines, pigeons and quail (Savage et al., 1993). The crabhole Department of Biology, University of Washington, Seattle, WA 98195, USA. mosquito Deinocerites pseudes will also feed on a range of hosts, including reptiles, mammals and amphibians, but, depending on the *Authors for correspondence ([email protected]; [email protected]) location, proportions of mosquitoes feeding on reptile versus G.H.W., 0000-0002-0075-4975; J.A.R., 0000-0002-7645-5779 mammalian hosts varied dramatically, possibly owing to host Journal of Experimental Biology 1 REVIEW Journal of Experimental Biology (2018) 221, jeb157131. doi:10.1242/jeb.157131 (MacDonald and Traub, 1960). It is unclear, however, how females Glossary of this genus acquire the protein necessary for oviposition if they do Aviphilic not take blood meals. For females in the genus Toxorhynchites, this A preference for birds as hosts. problem is solved by its larvae predating on the larvae of other CRISPR-Cas9 mosquito species, thus acquiring the necessary protein before Genome-editing technology derived from the bacterial immune system. eclosion as adults (Steffan and Evenhuis, 1981). As adults, Dendrite An input region or branch of a neuron. Toxorhynchites feed solely on flower nectars and other sources of De-orphaning plant carbohydrates and do not seek hosts for blood meals, making The process of identifying a receptor’s endogenous ligand. them a particularly interesting group for comparison with blood- Facet feeding mosquitoes (Fig. 1D). Phylogenetic analysis suggests that A lens of a compound eye. Toxorhynchites and Aedes share a sister relationship, with Culex and Gustation Anopheles being more distantly related, and thus that host The chemical sensing of taste. Hygroreception preference does not necessarily track phylogeny (Fig. 1A) (Zhou The ability to detect changes in water content. et al., 2014). With species belonging to these genera having such Ionotropic receptor diverse host preferences, what differences in their nervous systems A receptor protein that directly forms an ion channel. might account for these behaviors? Comparative studies of Lamina drosophilids, moths and other taxa have led to significant The first insect optic lobe neuropil receiving input from the retina. advances in understanding olfactory processing in insects Maxillary palp A head appendage of mosquitoes that is equipped with multiple sensory (Vickers et al., 1998; Dekker et al., 2006; Stacconi et al., 2014). receptors. With thousands of species representing a diverse repertoire of host- Olfactometer seeking behaviors, mosquitoes constitute an excellent study group A maze or apparatus used to measure the detection of, or preference for, for comparative neuroethology that can provide insights into general odors. principles of insect host preferences. Retinotopy The mapping of visual information from the retina onto neurons in the Mosquito sensory system: olfaction brain. Tastant Host-seeking behavior depends on the integration of several sensory A chemical that activates taste receptors. systems, including olfaction, vision, thermoreception, hygroreception Valence (see Glossary) and gustation (see Glossary). Before a mosquito can The intrinsic attractive or aversive quality of a stimulus. see its host or use any of those latter senses, it first detects chemical Zoonotic cues, and olfaction is thought to contribute the most to host seeking Describing a pathogen that can be transmitted from animals to humans. (Takken and Knols, 1999). The olfactory nervous system is highly conserved across insects and even shares similar organization across the animal kingdom between vertebrates and invertebrates. availability (Tempelis, 1975). A similar analysis of blood meals