Mapping the Loci Underlying Caenorhabditis elegans Resistance to the Bacterial Parasite Serratia Marcescens Open Access

Abstract

Understanding mechanisms of host defense is critical because parasites are widespread and capable of imposing fitness costs on hosts. Here, we look at the nematode host Caenorhabditis elegans andthe bacterial parasite Serratia marcescens (SM2170), where most strains of C. elegansdie after exposure to the parasite. We see that the N2 strain has a survival percentage of 0.72%, while Hawaiian CB4856 has a much higher survival percentage of 35.68%. From previous studies, it is known that Chromosome V of C. elegansis where many loci responsible for mediating interactions with parasites reside. In this study, our goal was to determine if and where the loci underlying higher CB4856 survival resided on Chromosome V. By utilizing Chromosome V introgression strains of CB4856 with an overall N2 background, we isolated the EWIR 68 strain that despite having only a portion of CB4856 Chromosome V, showed an even higher survival than its CB4856 parent. Thus, the portion of CB4856 Chromosome V present in EWIR 68 underlies increased survival in the presence of S. marcescens(SM2170). Next, we set out to determine the mechanism of defense employed by CB4856 and EWIR 68. Given C. elegansinnate preference for S. marcescens, we tested for a role of parasite avoidance in host defense. We found that CB4856, EWIR 68 and N2, all displayed similar preference towards S. marcescens, meaning that parasite resistance rather than avoidance accounted for increased survival in the CB4856 and EWIR 68 hosts. Knowing this, we set to determine the specific loci underlying resistance on chromosome V. We mated EWIR 68 with N2 males, generating 44 new introgression strains that further split the portion of CB4856 Chromosome V in EWIR 68. Running survival assays on these 44 strains, only two strains, 25 and 30, expressed high survival. These results confirmed that the gene/genes underlying C. elegansresistance against S. marcescens resides on Chromosome V as depicted by EWIR 68. In the future, genotyping strains 25 and 30 will give us better a resolution on a specific Chromosome V region underlying C. elegans resistance.

Table of Contents

Introduction……………………………………………………………………………………………………..1

Methods…………………………………………………………………….…………………………….……….4

1.    Host populations………………………………………………….……………………………..………..4

Figure 1.1…………………………………………………………......…………………………….……………5

Figure 1.2……………………………………………………………………......…………………..…………..6

2.    Bacterial populations……………………………………….….…………………………….…………..6

3.    Survival assay…………………………………………………………………………………..……..……7

4.    Choice index/avoidance assay……………………………….………………………………..……….8

5.    Ramping up male N2 population………………………….………………………………..…………8

6.    Cross between EWIR 68 and N2………………………………..…………………………..………….9

Results…………………………………………………….………………………..……………………..……....9

1.    Survival assay of introgression strains derived from CB4856...........................................9

Figure 2………………………………………………………………………………………….......……….…..11

Table 1………………………………………………………………………………………….…......………….11

2.     Survival assay of introgression strains derived from EWIR…………………..…......……….12

Figure 3………………………………………………………………………………………….…………......…13

Table 2………………………………………………………………………………………….…………......….13

3.     Survival assay of introgression strains 1, 25, and 30…………………………………...……….14

Figure 4………………………………………………………………………………………….………......……15

Table 3…………………………………………………………………………………………………......……..15

4.     Choice index assay between S. marcescens and E. coli……………………………….....……..15

Figure 5………………………………………………………………………………………………......……….17

Table 4………………………………………………………………………………………………......………..17

Discussion…………………………………………………………………………………………………..…….18

References…………………………………………………………………………………………………………22

About this Honors Thesis

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School	Emory College
Department	Biology
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Biology, Genetics Biology, Parasitology Biology, Evolution and Development
Keyword	parasite avoidance genetic mapping host-parasite interaction quantitative trait resistance
Committee Chair / Thesis Advisor	Levi Morran, Emory University
Committee Members	Christine Ristaino, Emory University John Lindo, Emory University

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