Experimental evolution of parasite host range in the Caenorhabditis elegans and Serratia marcescens system Público

Abstract

Parasites are known to vary in their host range. Some are limited to infecting a single host genotype, while others infect multiple host genotypes, even multiple host species. One hypothesis to explain this variation is that parasite lineages differ in their evolutionary history: parasites that consistently encounter a genetically homogenous host population may experience selection to become specialists, while parasites that often encounter a heterogeneous host population may experience selection to become generalists. Here, we utilized experimental evolution to impose selection on Serratia marcescens bacterial parasites to kill the nematode host Caenorhabditis elegans. Parasites were selected to kill hosts in homogeneous (one host genotype, N2 or CF3) or heterogeneous populations (an equal mix of two genotypes, N2 and CF3). After 20 passages of selection, we assessed the evolution of parasite host range by measuring the parasite populations’ ability to kill hosts of the N2 genotype, CF3 genotype and a novel genotype, JU1395. We predicted that parasites selected to kill homogeneous host populations will evolve a more limited host range than parasites selected to kill heterogeneous host populations. We found mixed support for this prediction. Parasites passaged with N2 exhibited preferential adaptation to N2, in the form of increased mortality rates of N2 hosts. As predicted, these parasites showed reduced ability to kill the novel host genotype relative to control parasites. In contrast, parasites passaged with CF3 did not exhibit preferential adaptation to the CF3 host. Accordingly, these parasites showed no loss of ability to kill the novel host. These results demonstrate that both the diversity and genotypes of local hosts can influence the evolution of host range in parasites.  

Table of Contents

Introduction........................................................................................................1

Methods..............................................................................................................3

Strains used........................................................................................................3

Serratia Selection Plates (SSPs)..........................................................................4

Preparing treatments..........................................................................................5

Passage of parasites............................................................................................6

Survival assay and scoring..................................................................................6

Statistical methods.............................................................................................7

Results................................................................................................................8

N2 Assay.............................................................................................................8

Figure 1..............................................................................................................9

Table 1...............................................................................................................10

CF3 Assay..........................................................................................................10

Figure 2.............................................................................................................10

Table 2..............................................................................................................11

JU1395 Assay.....................................................................................................11

Figure 3.............................................................................................................12

Table 3..............................................................................................................13

Discussion........................................................................................................13

References........................................................................................................17

About this Honors Thesis

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School	Emory College
Department	Biology
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Biology, Evolution and Development
Palavra-chave	experimental evolution S. marcescens niche width parasite C. elegans host range
Committee Chair / Thesis Advisor	Levi Morran, Emory University
Committee Members	Nicole Gerardo, Emory University Jose Soria, Emory University

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