Break-out Session Questions relating to Genetics

• What are the best uses for disease resistant strains (DRS) of oysters?– originally intended for aquaculture– new and improved?

• Can we define a policy for the use of disease resistant strains?

Background

1999 Workshop: Genetic considerations for hatchery-based restoration of oyster reefs

Allen & Hilbish 2000

• Disease perceived as primary obstacle• DRS seed oysters tentatively proposed as

part of the solution:– Higher survivorship could provide greater

reproductive output– Supplementation with DRS may increase

population frequency of alleles related to disease resistance (“genetic rehabilitation”)

Numbers of oysters planted on restoration

reefs in Virginia 1996 - 2006 data from T. Leggett, CBF

Supportive Breeding1. Captive breeding2. Minimize early mortality of juveniles3. Release juveniles into wild

• Genetic impact from single generation of supportive breeding:

e(wild)

2

)e(hatchery

2

e(tot)

11

N

x

N

x

N

• Ne = genetically effective population size

• x = proportional contribution of hatchery bred oysters to recruitment

Recent Findings

Wild oysters with disease tolerance exist in enzootic areas of Chesapeake Bay

Carnegie & Burreson submitted

Chesapeake oyster metapopulationbiophysical models suggest source-sink connections

North et al. submitted

genetic isolation by distance indicates low connectivityRose & Hare 2006

Within Chesapeake tributaries, oyster Ne ~ 103

Rose & Hare 2006

Recent Findings, cont.

DEBY strain oysters are severely bottlenecked genetically, Nb ~ 3

Hare & Rose submitted

Inbreeding depression can be severeFirst cousin matings reduce average oyster weight by 8%

(C. gigas; Evans et al.

2004)

Great Wicomico recruitment in 2002proportion attributable to 2002 DEBY seed oysters (3/4 million) was ~ 5%

Hare et al. 2006

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10

Generation

Ne-

To

tal

Wild brood stock Ne = 50 Closed line Ne = 5

Wild brood stock Ne = 5

0200400600800

1,0001,2001,4001,6001,8002,0002,200

0 1 2 3 4 5 6 7 8 9 10

Ne-

To

tal

Wild brood stock Ne = 50 Closed line Ne = 5

Wild brood stock Ne = 5

Recent Findings, cont.

Using closed line, expect 74% reduction in Ne of supplemented population

Hare & Rose submitted

ModelInitial wild tributary Ne = 420% contribution from seed oysters = 5%

Wild brood stock Nb= 25

Wild brood stock Nb = 2.5

Closed line Nb = 5

Balancing RisksDisease is primary obstacle,

need short cut

• Finding disease-resistant standing stock not practical– most will be highly

susceptible

• DRS seed may increase frequency of alleles related to disease resistance (“genetic rehabilitation”)

• Can’t avoid hatchery bottlenecks even with wild broodstock

Long-term restoration goal, precautionary approach

• DRS are inbred• Inadvertent hatchery

selection lowers fitness in wild

• Supplementation with DRS depresses overall Ne

• compromising adaptive potential

Recommendations• Where long-term restoration goals are primary:

– do not use artificially-selected DRS– do not try to select for an improved restoration oyster– use ‘local’ wild broodstock

Short term goals:

• Put-and-takeIf DRS desirable, make triploid seed to prevent reproductive

contribution

• Aquacultureuse DRS triploids

• Restoration monitoringin restricted areas use artificially selected DRS to enable crucial

genetic monitoring of restoration efficacy

Recommendations, cont.

• Minimize hatchery bottlenecks– as many pair crosses as possible

• target is Nb = 10 – 25

– don’t reuse wild broodstock

• Monitor hatchery bottlenecks– 50 seed oysters sufficient to genetically measure

Nb from a spawn

Thanks to Genetic Working Group

• Stan Allen• Jens Carlsson• Ryan Carnegie• Jan Cordes• Anu Frank-Lawale• Don ‘Mutt’ Meritt• Colin Rose• Jim Wesson

And for additional comments by

• Mark Camara• Pat Gaffney• Dennis Hedgecock• Kim Reece

Population

Number of pairwise locus

comparisons (out of 28) with

significant GD (p < 0.0003)

Harmonic mean

sample size, S

Mean squared allelic

correlation, r2 Nb (95% CI) Ne

Allelic richness

Gene diversity

Primary 5 47.7 0.0788 3.0 (1.6 – 4.7) 6.0 9.60a 0.831ab

GWR02 DEBY 11 87.2 0.0826 1.9 (1.0 – 3.0) 3.8 8.86a 0.790a

LCR02 DEBY 8 77.1 0.1106 2.4 (1.3 – 3.7) 4.8 8.52a 0.796a

LCR04 DEBY 22 96.8 0.0882 2.2 (1.2 – 3.4) 4.4 10.58a 0.798a

Wild LCR02 0 157.7 0.0103 84.7 (45.3 – 135.7) 169 15.65b 0.863b

Wild GWR02 0 413.2 0.0038 243.0 (131.9 – 387.1) 486 16.40b 0.863b

Change in total Ne expected from a single

generation of supportive breeding

(Ryman & Laikre 1991 model)

Annual changes in census size and total Ne resulting from sustained supportive breeding

( Wang and Ryman [2001] models I and II)

0.0E+00

5.0E+06

1.0E+07

1.5E+07

2.0E+07

2.5E+07

0 1 2 3 4 5 6 7 8 9 10

Generation

Cen

sus

N

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10

Generation

Ne-

To

tal

Wild brood stock Ne = 50 Closed line Ne = 5

Wild brood stock Ne = 5

0200400600800

1,0001,2001,4001,6001,8002,0002,200

0 1 2 3 4 5 6 7 8 9 10

Ne-

To

tal

Wild brood stock Ne = 50 Closed line Ne = 5

Wild brood stock Ne = 5

Annual changes in census size and total Ne Model I Model II

Initial Ne

1500

Initial Ne

420

The reduction in average number of alleles over time in 100 simulated populations experiencing a

one time change in Ne from 1500 to150

8

10

12

14

16

18

20

22

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Generation

Av

era

ge

No

. A

lle

les