Image by Alison Larsen

1. The Red Death Meets the Abdominal Bristle: Polygenic Mutation for Susceptibility to a Bacterial Pathogen in Caenorhabditiselegans Veronique Etienne1, Erik C. Andersen2, Jose Miguel Ponciano1, and Charles F. Baer1 1- Dept. of Biology, University of Florida, Gainesville, FL 2- Dept. of Molecular Biosciences, Northwestern University, Evanston, IL Image by Alison Larsen

2. 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?

3. 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?

4. 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

5. Quantitative Genetics of Pathogen Susceptibility in C. elegans • 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

6. The Mutation Accumulation (MA) Process N=1 N=1 N=1

7. Evolution under MA conditions 1 Fitness Genetic Variance (slope=VM) 0 0 Generations of MA

8. Evolution under MA conditions: Mutational Bias (ΔM) Trait Y (e.g., time to maturity) 1 Trait X (e.g., size at maturity) Fitness 0 0 Generations of MA

9. The Phenotype: Susceptibility of C. elegansto the pathogenic effects of Pseudomonas aeruginosa • Inoculate a 35 mm plate w/ 5 ul of saturated P. aeruginosa (PA14) • Introduce ~30 L4-stage juvenile C. elegans • Record mortality at 12 hr intervals for 128 hrs • Median time of death (LT50) is the measure of susceptibility to the pathogen

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

11. Results (2): LT50Pa vs. the Dmel Bristle

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

13. Results (4): Mutational Bias (ΔM)

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

15. 4NeVM Neutral Trait s=.001 s=.01 s=.1 s=1 Dominant Lethal

16. Results (6): Comparison with standing genetic variation; at MSB, VM/VG ~ s VG x 105 VM x 105

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

18. Conclusions: • 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 • Mutations that affect susceptibility to P. aeruginosaare under asymmetric stabilizing selection of a few tenths of a percent. • A model of large target/small-to-modest effects fits the data better than a model with a small mutational target and large effects.

19. Thanks! • NIH R01GM072639 (CFB) • Ruth Kirschstein NRSA F32-GM089007 (ECA) • NCI Training Grant T32-CA009528 (ECA) • Joanna Chan • Sarah Eaton • Nick Martinez • Andy Mills • Joanna Tran-Nguyen • Matt Vasquez Postdoc Wanted!