What we do in the Lab

Research in the Lazzaro Lab is focused on the evolutionary genetics of insect-pathogen interactions. We use population genetic, quantitative genetic, and genomic approaches to study how insects defend themselves against infection. We are particularly interested in understanding the genetic and environmental reasons why individuals vary in their susceptibility to infection, how interactions with pathogens shape the evolution of host immunity, and how immune defense is intertwined with other components of host physiology. We do most of our experimental research on the genetic model insect, Drosophila melanogaster, which allows us to combine functional genetic manipulation with studies of natural genetic variation. We also do population genetic and genomic work in Anopheles mosquitoes that vector malaria in sub-Saharan Africa.

Individual Variation in Defense

Experimental infection of a
D. melanogaster with bacteria.
A main emphasis in our lab is understanding how genetic and environmental factors contribute to variation in defense against infection. Just like some people get sick more often than others, individual insects in natural populations vary in their susceptibility to infection. Much of this variation has a genetic basis, and we use quantitative genetic mapping in D. melanogaster to identify the genes responsible. Our early experiments focused on candidate genes in the immune system, but we have more recently moved to unbiased genome-wide associate studies (GWAS) that can identify the contributions of any gene in the genome to variation in defense.

We are additionally finding that the quality of immune defense is affected by a variety of environmental factors, including ambient temperature, dietary nutrition and reproductive activity. Some individuals are genetically much more sensitive to environment than others, as detailed below. Such genotype-specific reactions to environment (Genotype-by-Environment interactions, or GxE) are critically important in the evolution of defense and underlie the clinical concept of personalized medicine.

Effects of nutrition on immunity

Comparison of systemic bacterial load after infection of 170 D. melanogaster genotypes provided with either high-sugar or low-sugar diets reveals GxE for resistance to infection.
Genetically identical flies provided with diets that vary in sugar, protein and fat content differ substantially in their ability to survive bacterial infection and suppress pathogen proliferation. Importantly, however, genetically distinct individuals differ in the magnitude with which diet impacts immunological capability. We are employing functional and quantitative genetic approaches to determine how diet and metabolism shape immune performance. This includes mapping of genes that influence defense under different dietary conditions, recognizing that the mapped genes might include not just genes in the immune system but also genes involved in nutritional uptake and metabolism. We are testing the hypothesis that diet alters defense through direct effects on immune system function, but are also evaluating the degree to which nutritional status determines the host's ability to withstand or tolerate infection without necessarily affecting immune system activity.

Effects of reproduction on defense
We have found that female D. melanogaster become significantly less able to resist infection upon mating, suffering higher mortality and reduced ability to regulate pathogen growth. This effect is not due simply to the act of copulation, but seems to result from a physiological change in the female that presumably reflects a shift from virgin homeostasis to active production of mature eggs. Thus, reproduction and defense against infection are antagonistically linked in a physiological tradeoff. Notably, female D. melanogsater vary genetically in the degree to which they become immunocompromised after mating (although males show no variation in their ability to immunosuppress their mates). Working in collaboration with Mariana Wolfner, we are currently working to identify the molecular mechanisms for post-mating immunosuppression in D. melanogaster females and the evolutionary consequences of this life history tradeoff.

Key papers from the lab in this area

Short, S.M. and B.P. Lazzaro (2013) Reproductive status alters transcriptomic response to infection in female Drosophila melanogaster G3: Genes, Genomes, Genetics 3:827-840. [pdf]

Short, S.M., M.F. Wolfner and B.P. Lazzaro (2012) Female Drosophila melanogaster suffer reduced defense against infection due to seminal fluid components. Journal of Insect Physiology 58:1192-1201. [pdf]

Short, S.M. and B.P. Lazzaro. (2010) Female and male genetic contributions to female post-mating susceptibility to infection in Drosophila melanogaster. Proceedings of the Royal Society, Biological Sciences, 277:3649-3657. [pdf]

Fellous, S. and B.P. Lazzaro. (2010) Larval food quality affects adult (but not larval) immune gene expression independent of effects on general condition. Molecular Ecology, 19:1462-1468. [pdf]

Sackton, T.B, B.P. Lazzaro and A.G. Clark (2010) Genotype and gene expression associations with immune function in Drosophila PLoS Genetics, 6:e1000797. [pdf]

Lazzaro, B.P. and T.J. Little. (2009) Immunity in a variable world. Philosophical Transactions of the Royal Society, series B - Biology, 364:15-26. [pdf]

Lazzaro, B.P., H.A. Flores, J.G. Lorigan and C.P. Yourth. (2008) Genotype by environment interactions and adaptation to local temperature affect immunity and fecundity in Drosophila melanogaster. PLoS Pathogens 4:e1000025. [pdf]

McKean, K.A., C.P. Yourth, B.P. Lazzaro and A.G. Clark. (2008) The evolutionary costs of immunological maintenance and deployment. BMC Evolutionary Biology 8:76. [pdf]

Lazzaro, B.P., T.B. Sackton and A.G. Clark. (2006) Genetic variation in Drosophila melanogaster resistance to infection: a comparison across bacteria. Genetics 174:1539-1554. [pdf]

Lazzaro, B.P., B.K. Sceurman and A.G. Clark. (2004) The genetic basis of natural variation in D. melanogaster antibacterial immunity. Science 303:1873-1876. [pdf]


Molecular evolution of insect immune genes

We use molecular population genetic and comparative genomic analyses establish how natural selection operates on the immune systems of Drosophila melanogaster, Anopheles mosquitoes that vector human malaria, and other insect species. We ask what are the natural selective, demographic and epidemiological pressures that drive evolution of insect immune systems, and how population-level phenomena such as population subdivision, migration, local adaptation, and changes in population size may affect the evolution of defense.

Rapid duplicaiton and deletion in the Cecropin antimicrobial peptide gene family across the genus Drosophila.

Evolution of the Drosophila antibacterial immune system
We and others have shown that immune system genes evolve, on average, more quickly and more adaptively than other genes in the Drosophila genome. We infer that this rapid evolution is driven by co-evolution with pathogens that themselves evolve the capacity to evade or suppress host immune defense. To our surprise, intracellular signaling genes that regulate immune system activity show some of the fastest rates of adaptive evolution. These signaling genes retain highly conserved orthology across insects and even into vertebrates, and we hypothesize that their rapid molecular evolution is driven by pathogen interference with the host’s capacity to activate the immune system. In contrast, Drosophila genes encoding antimicrobial peptides that kill infecting bacteria show little evidence of molecular adaptation, perhaps reflecting simplicity of function that is not easily subject to host-pathogen co-evolution at the amino acid level. Antimicrobial peptide genes show very rapid rates of duplication and deletion across insect species, however, and are among the most dynamic gene families in the genome. Receptors involved in defensive phagocytosis of microbes show both rapid evolution at the amino acid sequence level and rapid gene family turnover. Our current molecular population genetic studies of the D. melanogaster immune system emphasize genome-scale tests for subpopulation structure and local adaptation.

Molecular population genetics of Anopheles mosquitoes
Malaria is a devastating disease, particularly in Africa, with hundreds of millions of clinical cases and nearly a million deaths reported each year. Even within single populations, however, mosquitoes are highly genetic polymorphic for the ability to transmit disease. Together with Ken Vernick's group at Institut Pasteur, we have been studying the evolutionary genetics of A. gambiae, the epidemiologically most important vector of human malaria. A. gambiae subpopulations are extremely fragmented, with restricted interbreeding and genetic exchange among them. We have fitted models to population diversity data to determine the demographic history of two major subgroups of A. gambiae, known as the "M" and "S" molecular forms. We find these potential incipient species to be have distinct evolutionary trajectories, possibly including differential selection on the hyperpolymorphic APL1 family of anti-malaria genes. We are currently engaged in multi-locus analyses of evolutoin and diversity of the A. gambiae immune system. We hope that better understanding of the evolution of anti-malaria defense genes in Anopheles will suggest potential targets for intervention, and that appreciating demographic structure in mosquito populations will lead to more effective control, ultimately reducing malaria transmission and the human disease burden.

Key papers from the lab in this area

Crawford, J.E., S.M. Rottschaefer, B. Coulibaly, M. Sacko, O. Niaré, M.M. Riehle, S.F. Traore, K.D. Vernick and B.P. Lazzaro (2013) No evidence for positive selection at two potential targets for malaria transmission-blocking vaccines in Anopheles gambiae s.s. Infection, Genetics, and Evolution 16:97-92 [pdf]

Rottschaefer, S., M.M. Riehle, B. Coulibaly, M. Sacko, O. Niare, I. Morlais, S.F. Traore, K.D. Vernick and B.P. Lazzaro. (2011) Exceptional diversity, maintenance of polymorphism, and recent directional selection on the APL1 malaria resistance genes of Anopheles gambiae. PLoS Biology 9:e1000600. [pdf]

Crawford, J.E., W.M. Guelbeogo, A. Sanou, A. Traore, K.D. Vernick, N. Sagnon and B.P. Lazzaro. (2010) De novo transcriptome sequencing in Anopheles funestus using Illumina RNA-seq technology. PLoS One, 5:314202. [pdf]

Juneja, P. and B.P. Lazzaro. (2010) Haplotype structure and expression divergence at the Drosophila cellular immune gene eater. Mol. Biol. Evol., 27:2284-2299.
[pdf]

Crawford, J. and B.P. Lazzaro (2010) The demographic histories of the molecular forms of Anopheles gambiae s.s. Molecular Biology and Evolution, 27:1739-1744. [pdf]

Lazzaro, B.P. (2008) Natural Selection on the Drosophila innate immune system. Current Opinion in Microbiology, 11:284-289. [pdf]

Sackton, T.B., B.P. Lazzaro, T.A. Schlenke, J.D. Evans, D. Hultmark and A.G. Clark. (2007) Dynamic evolution of the innate immune system in Drosophila. Nature Genetics 39:1461-1468. [pdf]

Lazzaro, B.P. (2005) Elevated polymorphism and divergence in the class C scavenger receptors of Drosophila melanogaster and D. simulans. Genetics 169:2023-2034. [pdf]

Lazzaro, B.P. and A.G. Clark. (2003) Molecular population genetics of inducible antibacterial peptide genes in Drosophila melanogaster. Molecular Biology and Evolution 20:914-923. [pdf]

Lazzaro, B. P. and A.G. Clark. (2001) Evidence for recurrent paralogous gene conversion and exceptional allelic divergence in the Attacin genes of Drosophila melanogaster. Genetics 159:659-671. [pdf]


Infection genomics in bacteria

Comparative genomics and pathology of bacteria recovered as natural infections of D. melanogaster.
The majority of the bacteria we work with in our program are ones that we have isolated in the field as natural infections of D. melanogaster. When we have done such field surveys, we have not found microbes that appear to be co-evolved obligate pathogens of D. melanogaster, but rather we find a diverse suite of seemingly opportunistic infectors. Nonetheless, many of these bacteria are well able to infect Drosophila and other insects if introduced into a wound. We have sequenced the genomes of several of our field-isolated bacteria. This sequencing reveals that they possess bona fide virulence mechanisms, as well as other physiological differences that might influence infection capability. We are developing the tools to genetically manipulate the bacteria, which we can combine with comparative pathology to elucidate mechanisms of infection and the basis for diversity in infection capability. When combined with analysis of host genetic diversity, we should be able to elucidate the genetic basis for variation in host-pathogen "compatibility", or host-genotype by pathogen-genotype interactions (GH*GP). These interactions can of course be logically extended to include effects of the abiotic environmental (for instance, effects of host dietary nutrition), yielding insight into GH*GP*E effects that shape that evolution of hosts and pathogens.

Key papers from the lab in this area

Galac, M.R. and B.P. Lazzaro (2012) Comparative genomics of bacteria in the genus Providencia isolated from wild Drosophila melanogaster. BMC Genomics 13:612. [pdf]

Galac, M. and B.P. Lazzaro. (2011) Comparative pathology of bacteria in the genus Providencia to a natural host, Drosophila melanogaster. Microbes and Infection13:673-683.[pdf]

Lazzaro, B.P., T.B. Sackton and A.G. Clark. (2006) Genetic variation in Drosophila melanogaster resistance to infection: a comparison across bacteria. Genetics 174:1539-1554. [pdf]