The Red Death Meets the Abdominal Bristle: Polygenic Mutation for Susceptibility to a

Bacterial Pathogen in Caenorhabditis elegans

Image by Alison Larsen

Veronique Etienne1, Erik C. Andersen2, Jose Miguel Ponciano1, and Charles F. Baer1

1- Dept. of Biology, University of Florida, Gainesville, FL2- Dept. of Molecular Biosciences, Northwestern University, Evanston, IL

Question (Age-old):

• When “something is going around”, why do some individuals become deathly ill, other individuals only get mildly sick and other individuals don’t get sick at all?

Question (Age-old):

• When “something is going around”, why do some individuals become deathly ill, other individuals only get mildly sick and other individuals don’t get sick at all?

• That is, what are the sources of variation in the response to exposure to pathogens?

Point of departure:

• Variation in the response to pathogen(s) often has a genetic basis

– In some cases one or a few loci of large effect underlie variation (e.g., β-globin/sickle cell/malaria)

– In other cases the variation appears to be polygenic

Quantitative Genetics of Pathogen Susceptibility in C. elegans

1. How much genetic variance is introduced by mutation?• VM = UQα2

• VM = per-generation increase in genetic variance due to new mutations (“mutational variance”)• U = genome-wide mutation rate• Q = fraction of genome that can affect the trait

(“mutational target”)• α = average effect of a mutation on the trait

The Mutation Accumulation (MA) Process

N=1 N=1 N=1

Evolution under MA conditions

Generations of MA0

0

1

Genetic Variance (slope=VM)

Fitness

Evolution under MA conditions: Mutational Bias (ΔM)

Generations of MA0

0

1

Fitness

Trait X (e.g., size at maturity)

Trait Y (e.g., time to maturity)

The Phenotype: Susceptibility of C. elegans to the pathogenic effects of Pseudomonas aeruginosa

1. Inoculate a 35 mm plate w/ 5 ul of saturated P. aeruginosa (PA14)

2. Introduce ~30 L4-stage juvenile C. elegans

3. Record mortality at 12 hr intervals for 128 hrs

4. Median time of death (LT50) is the measure of susceptibility to the pathogen

Results (1): Mutational Variance, compared with other traits measured in these C. elegans MA lines

10

5

10

15

20

25

30

LT50PaLT50MAW(25)WsortSurvSize

VM (C

V^2

x 1E

5)

Results (2): LT50Pa vs. the Dmel Bristle

10

5

10

15

20

25

30

LT50PaDmel bristles

VM (C

V^2

x 10

x1E

5)

Results (3): Mutational Heritability (VM/VE), compared with other traits in C. elegans

10

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

LT50PaLT50MAW25WsortSurvSizeDmel bristles

h2m

(x 1

E3)

0 50 100 150 200 250 3000.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

LT50PaLinear (LT50Pa)LT50MaLinear (LT50Ma)W25Linear (W25)WsortLinear (Wsort)SurvLinear (Surv)SizeLinear (Size)Generations of MA

Trait

Mean

Results (4): Mutational Bias (ΔM)

Results (5): Selection Gradient – LT50Pa is under asymmetric stabilizing selection

0.00001 0.0001 0.001 0.010.00001

0.0001

0.001

0.01

0.1

1

10

100

1000

VM

VGNeutral TraitNeutral Trait

4NeVM

s=.001

s=1s=.1s=.01

Neutral Trait

Dominant Lethal

Results (6): Comparison with standing genetic variation; at MSB, VM/VG ~ s

VM x 105

VG x

105

Results (7): Distribution of Mutational Effects: Large Target (Q≥0.5%), Modest Effects (|α|~ “a few percent”)

Conclusions:1. Susceptibility to this pathogen (the PA14 strain of P.

aeruginosa) of this host (the PB306 strain of C. elegans) as quantified in this way (LT50) is a typical quantitative trait

2. Mutations that affect susceptibility to P. aeruginosa are under asymmetric stabilizing selection of a few tenths of a percent.

3. A model of large target/small-to-modest effects fits the data better than a model with a small mutational target and large effects.

Thanks!• Joanna Chan• Sarah Eaton • Nick Martinez• Andy Mills• Joanna Tran-Nguyen• Matt Vasquez

• NIH R01GM072639 (CFB)• Ruth Kirschstein NRSA F32-

GM089007 (ECA)• NCI Training Grant T32-

CA009528 (ECA)

Postdoc Wanted!