1. The case of Dermo disease
Simulation of the development of disease resistance in oyster populations using a gene-based population dynamics model
The case of Dermo disease
Eric Powell, Eileen Hofmann, John Klinck, Ximing Guo, Susan Ford, Dave Bushek, and others

2. Why does Perkinsus marinus persist as a major source of mortality in oyster populations?
The challenge of developing disease resistance for the eastern oyster Crassostrea virginica

3. Adult Mortality -- 1953-2009 Dermo Era MSX Era
Delaware Bay oysters become
completely MSX resistant
Dermo Era
MSX Era
Partial MSX disease resistance develops

4. The Delaware Bay Scenario: the Dermo era
High-mortality epizootics 1 & 2 level
High-mortality epizootic 3 level
Medium-mortality epizootic level
Low-mortality level

5. The Goal
Investigation of the development of disease resistance in naïve populations newly challenged with disease!
Can the response of the population be modeled such that the future trajectory of genetic response can be inculcated into long-term restoration strategy?

6. The Challenge: Implementation of a gene-based population dynamics model
DyPoGEn (Dynamic Population Genetics Engine), a model designed to simulate population dynamics from the genotype through the phenotype to the population

7. Model Configuration }L1 }L2 G1 G3 G2 G4 10 Chromosome pairs
4 Genes per chromosome
2 Alleles per gene (A, B)
Sex gene identifying protandric animals and permanent males
Genetic subroutines
Meiosis
Crossing-over
Random chromosome sorting
at reproduction
Determination of phenotype
from genotype

8. Model Configuration }L1 }L2 G1 G3 G2 G4 Larval subroutine
Larval survivorship as a function of post-settlement abundance (compensatory broodstock-recruitment)
Adult subroutine
Age
Growth at age
Sex change at size
Determination of parent pairs
Reproduction at size
Mortality at age

9. Model Configuration }L1 }L2 G1 G3 G2 G4
14 Loci with disease-resistant alleles
12 show dominance
2 show underdominance
Distributions: C1-1 C2-1 C3-1
C4-2 C7-3 C8-2
C9-3 C10-1
A allele resistant; B allele sensitive
Naïve population A allele frequency 10%
Naïve population B allele frequency 90%
Whole animal fitness: 14 AA loci; fitness=1
Whole animal fitness: 14 BB loci; fitness~0

10. Imposed Levels of Epizootic Mortality
Gulf Coast representative rate : Level 4 = 40%
Delaware Bay representative rates: Level 3 = 25%
Level 2 = 22%
Limited disease challenge rate:
Level 1 = 17%
Naïve rate:
Level 0 = 13%
Onset of Dermo disease
Challenged population
Unchallenged population

11. Impact of Disease Challenge: Population Abundance Declines Population Average Age Declines

12. The Genetic Response to Disease Challenge: Population Average Fitness Increases
Fitness of naïve population: average = 0.1 on a 0-to-1 scale
Fitness increases over time in challenged population

13. Example Set of Fitness Trajectories
Key modulators of outcome to disease challenge:
1. Increased mortality increases the rate of disease resistance
BUT
2. Selection versus drift: drift can limit the ambit of response through allele loss
Impact of drift at high mortality
Onset of disease challenge

14. Outcome of Selection at 22% Epizootic Mortality
A range in population dynamics affects the outcome very little.
Only the presence of a few controlling alleles permits a much higher level of disease resistance.
Overall, disease resistance develops, but at a very slow rate.
Significant change in disease resistance occurs on half-century time scale

15. Outcome of Selection at 40% Epizootic Mortality
A range in population dynamics affects the outcome considerably.
Population dynamics minimizing drift result in significant increases in disease resistance over years.
Fewer controlling alleles, however, permit nearly complete disease resistance in years.
Reduced genetic determination
High growth, increased fecundity, high abundance

16. The Manager’s Dilemma
A total mortality rate of 15-25% per year produces population decline.
An epizootic mortality rate of 25% per year limits fishing to de minimis rates.
A total mortality rate of 15-25% per year limits development of disease resistance to half-century or longer time scales.
Era of overfishing
Dermo era
reference pt. based
de minimis fishery
40% rule reference pt. MSX partially resistant population

17. The Delaware Bay Scenario
High-mortality epizootics 1 & 2 level
Lower high-mortality level: epizootic 3 ~15 years post-onset
Medium-mortality epizootic level
Low-mortality level

18. Conclusions
Dermo disease resistance develops slowly due to the number of alleles involved.
Time scales are multi-decadal unless total mortality rate is high (e.g., 40%).
Slow rate of disease resistance development in Delaware Bay suggests many alleles are involved and, thus, disease resistance will continue to develop at increasingly slower rates.
Epizootic mortality rates that fail to support rapid disease resistance nevertheless result in significant reductions in abundance and curtailment of anything but a de minimis fishery.
No manipulation of population dynamics through restoration or recruitment enhancement will encourage further development of disease resistance, although it may support a higher level of abundance.