Parasite virulence and the maintenance of outcrossing in the Caenorhabditis elegans and Serratia marcescens system Público

Abstract

The Red Queen hypothesis (RQH) predicts that coevolving parasites can impose persistent selection against common host genotypes, and ultimately select against uniparental reproduction. Thus, coevolving parasites are predicted to constrain self-fertilizing lineages from invading obligately outcrossing host populations, despite the inherent costs of biparental reproduction. However, this prediction is contingent on parasite virulence. Only highly virulent parasites are expected to maintain obligate outcrossing, which may limit the scope of the RQH as a broad explanation for the maintenance of outcrossing. Therefore, we tested whether the invasion of self-fertilization within obligately outcrossing host populations was dependent on the virulence of coevolving parasites. We introduced wild-type Caenorhabditis elegans hermaphrodites, capable of both self-fertilization and outcrossing, into C. elegans populations fixed for a mutant allele conferring obligate outcrossing. Replicate C. elegans populations were exposed to one of four strains of Serratia marcescens parasites (Sm2170, SmD1, Sm933, or Db11), varying in virulence, for 24 generations under three treatments: a control (avirulent) parasite treatment, a fixed (non-evolving) parasite treatment, and a copassaged (potentially coevolving) parasite treatment. As predicted, self-fertilization invaded C. elegans host populations in the control and fixed-parasite treatments for all bacterial strains. In the copassaged treatment, self-fertilization invaded host populations in the two least virulent strains (Sm933 and Db11), but remained rare in the two most virulent bacterial strains. Indeed, the obligate outcrossing allele was maintained at high frequencies in multiple populations copassaged with the two most virulent parasite strains, but decreased in frequency in all other populations. Consistent with the RQH, highly virulent coevolving parasites are capable of impeding the invasion of self-fertilization in outcrossing host populations. However, selection imposed by more moderately virulent coevolving parasites was not sufficient to maintain obligate outcrossing. Therefore, the RQH may not be sufficient as the sole explanation for the maintenance of biparental sex.

Table of Contents

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

Methods............................................................................................. 4

Establishment of obligately outcrossing and
mixed mating C. elegans host populations...................................... 4

Serratia marcescens parasite strains.............................................. 5

Serratia Selection Plates (SSPs).................................................... 5

Treatment groups........................................................................ 6

Passage of host populations for experimental evolution.................... 7

Survival assays and parasite virulence........................................... 8

Calculating selfing rates............................................................... 8

Measuring mixed-mating allele frequencies..................................... 9

Statistical methods...................................................................... 9

Results.............................................................................................. 10

Parasite virulence over time......................................................... 10

Figure 1............................................................................ 11

Figure 2............................................................................ 12

Table 1.............................................................................. 13

Selfing rate................................................................................ 13

Table 2.............................................................................. 15

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

Figure 3............................................................................. 16

Obligate outcrossing and mixed-mating genotypic frequencies........... 17

Figure 4............................................................................. 18

Discussion........................................................................................... 18

References.......................................................................................... 24

About this Honors Thesis

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School	Emory College
Department	Biology
Degree	BS
Submission	Honors Thesis
Language	English
Research Field	Biology, Evolution and Development Biology, Parasitology Biology, General
Palavra-chave	Self-fertilization Serratia marcescens Host Parasite Red Queen Hypothesis Outcrossing Caenorhabditis elegans
Committee Chair / Thesis Advisor	Morran, Levi T, Emory University
Committee Members	Gerardo, Nicole Marie, Emory University Weissman, Daniel B, Emory University

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