Elisa Visher presents "The Evolution of Host Genotype Specialization in an Insect Pathogen"11/21/2019 Illustration by Alison Feder Evolutionary tradeoffs in an insect-virus model system Last week, graduate student Elisa Visher of the Boots lab (link: https://bootslab.org/) presented details related to her most recent experiment at our Ecology and Evolution of Infectious Diseases seminar. Elisa works in an experimental evolution model system which Mike developed as a graduate student back in the UK. In this system, Elisa manipulates interactions between a host, the Indian meal moth (Plodia interpunctella) and its species-specific pathogen, Plodia interpuntella granulosis virus (PiGV) to explore evolutionary tradeoffs between investment in resistance and growth on the part of the moth and infectivity and productivity on the part of the virus. Experimental evolution in a host-pathogen system typically involves growing a generation of pathogens on a specific host, then transferring this pathogen to a new generation of hosts, such that over time, the experimenter can quantify adaptations of the pathogen to the host environment. In the Boots lab, the experimenter (in this case Elisa) infects a generation of Plodia larvae by supplying them with virus-infected food. At timepoint one, called the first ‘passage’ in a ‘serial passage’ experiment, Elisa mixes virus in sugar and puts it on a petri dish for the larval meal moths to consume. PiVG is an obligate killer which is transmitted via larval cannibalism, so for the next passage, Elisa collects dead cadavers from the first passage, mashes them up, and mixes them in food for the second generation. Over time, the pathogen is influenced by natural selection in that it competes with other genotypes in the virus population. By being the virus genotype that (a) infects the most cadavers and (b) produces the most virions when the insects get ground up, one virus genotype can outcompete the others to be propagated into the future. The Plodia, by contrast, are under selection to resist this virus in order to survive. In past work (i.e. his PhD), Mike has demonstrated that Plodia will develop resistance to PiGV, though at a cost of longer development times (Boots and Begon 1993), meaning that the more effective a larvae is at avoiding infection, the longer it takes to grow into a moth. Mike has further shown that these costs are magnified when resources available to the host are scarce (Boots 2011). More recently, Lewis Bartlett of the Boots lab developed twelve distinct inbred lines of Plodia to demonstrate that this tradeoff between virus resistance and host growth rate is a constitutive trait with a genetic basis (Bartlett et al. 2018). For one chapter of her PhD, Elisa is expanding on these themes, using Lewis’s inbred lines, to show that PiGV can evolve to specialize on distinct genotypes of Plodia. In a serial passage experiment, Elisa evolved the same stock of virus across three different Plodia genotypes for nine generations, then assessed each evolved virus’s fitness on this familiar genotype and the other two genotypes across the time course of serial passage. Critically, Elisa measured “viral fitness” in two different ways—both in terms of a virus’s capacity to infect a Plodia host, as well as its virion productivity after infection. Two of the viruses she passaged evolved to be “infection specialists” for their host genotype, meaning that they were more effective at infecting the host they had evolved with than the other two genotypes. The third virus, by contrast, evolved to be a “productivity specialist,” showing no difference in its ability to infect other Plodia genotypes at the end of the experiment but producing many more virions post-infection when assayed on its host genotype. In ongoing work, Elisa is using this same system to test the so-called ‘monoculture effect’ to explore whether more diverse host environments made up of multiple different Plodia genotypes select for more generalist viruses which can infect hosts more broadly but may be less virulent. In agricultural systems, we fear that the absence of genetic diversity allows for pathogens to specialize on a single host and attain high levels of virulence which result in crop destruction. In the future, Elisa also plans to manipulate the physical arrangement of Plodia genotypes in a microcosm to further explore the effect of spatial structure on these evolutionary tradeoffs in a host-pathogen system. We are excited to hear what she finds out next! Summary by Cara Brook
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