Broadly, the lab is interested in understanding the genetic basis of naturally occurring phenotypic variation in mosquitoes. We focus primarily on Anopheles mosquitoes that transmit malaria in Sub Saharan Africa. Ultimately, we aim to find the genes, and even the specific mutations, underlying natural variability in phenotypes of medical relevance and/or ecological importance. Identification of these functional polymorphisms and characterization of the evolutionary forces acting upon them may ultimately enable a reduction in the vectorial capacity of natural mosquito populations. Below are some specific projects that are underway.
Functional Evolution of Immune Genes in Malaria Mosquitoes
It has been theorized that the immune system of wild mosquito populations could be manipulated so that they no longer transmit malaria. Indeed, it is clear that substantial natural variation exists in the susceptibility of individual Anopheles mosquitoes to infection with malaria parasites. However, very little is known about why such phenotypic variation exists. Identification of anti-Plasmodium alleles and the forces controlling their frequency may prove useful in the development of unique strategies to increase natural levels of mosquito resistance to malaria. Our preliminary data suggest that genetic variation at a locus on the left arm of the third chromosome (3L) of An. gambiae partially controls its resistance to the deadly human malaria species Plasmodium falciparum. Within this ~150 kilobase (kb) locus is a cluster of ~6 thioester-containing protein (TEP) genes that have been implicated in the innate immune response of the mosquito. Our goal is to identify the specific gene(s) and alleles within this TEP gene cluster that modulate An. gambiae susceptibility to human malaria and to define the evolutionary and ecological pressures responsible for creating and maintaining functionally-relevant polymorphism in these genes. Intriguingly, our preliminary data are not consistent with Plasmodium-mediated evolution of TEP genes, instead suggesting that larval pathogens are shaping variation at these genes. This data challenges the long-held paradigm that the mosquito and the malaria parasite are locked in a co-evolutionary arms race, instead suggesting that natural variation in mosquito susceptibility to malaria may simply be a by-product of evolution against pathogens that are more detrimental to the mosquito.
Project Members: Eric Smith
Recombination in Anopheles
Despite its critical importance in shaping genome-wide patterns of diversity within and between populations, very little is known about recombination rate variation across Anopheline genomes. By using a genotyping-by-sequencing approach to map the crossover breakpoints in individual mosquitoes, we are creating a fine-scale recombination map for Anopheles gambiae. In addition to this fine scale map, we are systematically testing the effects of sex and inversions on recombination. Results from this project will aid in the creation of quantitative models to predict mosquito response to various control interventions, allowing for more effective and efficient vector control.
Project Members: Stephanie Gamez, David Turissini
Speciation between Anopheles gambiae and Anopheles merus
Anopheles gambiae belongs to a complex of at least eight morphologically identical species. We are interested in identifying genes that promote both ecological differentiation and post-zygotic isolation between the sister species An. gambiae and An. merus.
Ecological Differentiation: Unlike An. gambiae, An. merus can complete development in highly saline water. In areas of sympatry, the distribution of An. merus is restricted to coastal waters unfavorable for An. gambiae development. In collaboration with the Besansky Lab at Notre Dame, we are mapping saltwater tolerance genes in An. merus using high throughput QTL mapping and gene expression studies.
Postzygotic Isolation: In reciprocal crosses between An. gambiae and An. merus, F1 males are sterile, but F1 females are fully fertile. By backcrossing F1 females we can create mapping populations of males that range from completely sterile to fully fertile. We are currently using high-throughput QTL mapping, combined with RNA-SEQ to identify the genic incompatibilities responsible for male sterility.
By investigating both prezygotic and postzygotic isolating mechanisms, we will gain insight into how new species are formed and maintained.
Project Members: Raissa Green (Hybrid Sterility)
The Genetic Basis of Insensitivity to DEET
While mosquitoes have rapidly evolved resistance to a variety of insecticides, increased resistance or insensitivity to the commonly used insect repellant DEET has only rarely been observed. However, ongoing efforts to dramatically increase the use of DEET and other repellants may impose strong selection for insensitivity on anthropohilic mosquitoes. Our preliminary experiments clearly demonstrate that An. gambiae harbors substantial natural genetic variation in sensitivity to DEET. We are using an 'evolve and resequence' approach to reveal the specific genetic variants underlying the insensitivity phenotype. By understanding how repellant insensitivity evolves, it will be possible to implement specific formulation and distribution strategies that avoid imposing strong natural selection on field populations.
Project Members: James Ricci
Culex Population Genomics
This is a new project funded by the Mosquito Research Foundation. We plan to explore the statewide population structure of the major Culex vectors of West Nile Virus in California -- a disease that resurged across the state in 2012. By employing a genotyping-by-sequencing methodology we will rapidly characterize cryptic populations within Culex pipiens s.l., Culex tarsalis, and Culex stigmatasoma. This information will allow us to determine if any of these populations play a disproportionate role in the WNV transmission cycle.
Project Members: James Ricci, David Turissini
Cuticle Color Variation
Anopheles gambiae exhibits remarkable variation in both color intensity and pattern on its cuticle. It is not yet known whether this color variation is adaptive. We are mapping the genes responsible for both color pattern and intensity by crossing dark (Cameroon) and light (Mali) colonies. After identifying key genes, we will perform association analysis on field specimens to confirm that our genotype-phenotype inferences hold for natural populations.
Project Members: Stephanie Truong, Colince Kamdem