Transcript Life at the edge: local adaptation and range limits for two estuarine

Life at the edge:
local adaptation and northern range limit for an estuarine
sea slug
Hanna Koch
Dr. Patrick Krug
California State University, Los Angeles
Significance of Studying Range Limits
 For basic science, range limits are fundamental to ecology and
evolutionary biology:
– Ecology: What sets distributional limits?
A) Biotic factors: competitive exclusion
B) Abiotic Factors: gradients in temperature, salinity
– Evolution: What prevents adaptation to edge conditions?
A) Gene flow from range center
B) Lack of underlying genetic variation
For applied work, understanding the basis of range limits is critical
to predict how species will respond to climate change
Intertidal Animals
• Model system:
– Ideal for range limit studies
– Easily tracked along linear shoreline distribution
• North & south endpoints
– Exposed @ low tide, submerged @ high tide
• No subtidal refuge
• Exposed to extreme temps & salinities
 “salinity” = concentration of salt in seawater
Alderia willowi
Algae
•Sea slug that lives in estuaries
•Live & feed on algae mats on muddy banks of estuaries
•Range = Baja California, Mexico  Tomales Bay, CA
•Prefers warmer temps, higher salinities
A. willowi - range edge vs. range center
• Tomales Bay (stressful):
– range edge
– more rainfall  lower salinity
– Is northern range limit stable?
• Los Angeles (optimal):
– range center
– more stable environment than TB
• warmer temps
• 4x less precipitation
• higher salinity
Tomales Bay
Range edge
LA
Range center
Question:
Can A. willowi be locally adapted to conditions at the range edge?
Theory
predicts that immigrants from range center will flood the
range edge with mal-adaptive alleles and inhibit adaptation…
Hypotheses:
1. The range-edge population (TB) is more locally adapted to low
salinity stress than range center (LA)
2. Local adaptation occurs over rainy season
3. Low salinity tolerance is a genetic, heritable trait & is favored at the
range edge (TB)
Methods- measuring low salinity tolerance
(time to death)
 time to death at 2‰ scored for 20 slugs per collection
Normal, live slug
Dissolving, dead slug
I’m dead 
Results 1: Local adaptation to edge conditions
- survival time in
continuous 2‰ SW
ANOVA: F2,57 =
10.85, P < 0.001
Before winter onset of low salinity stress, slugs from northern
range limit survive 2x as long as established range-center population
….evidence for local adaptation to low salinity stress @ range edge!
Results 2: Local adaptation over rainy season
- survival time in
continuous 2‰ SW
RC: ANOVA: F1,38 = 6.0, P =
0.019
RE: ANOVA: F1,38 = 244.12,
P < 0.0001
T-TEST INSTEAD
After onset of winter rains & low salinity stress…
RC slugs saw slight increase in survival
Mean survival of RE slugs increased from 3 hrs to 2 days…
 Suggests rapid local adaptation to edge conditions in RE populations
Adaptation to life on the edge?
Is increased low salinity tolerance of Tomales slugs due to…
a) phenotypic plasticity? - slugs acclimatize to increasing stress
b) maternal effects?
- moms under stress buffer their young
c) adaptive evolution?
- low salinity tolerance is heritable trait
Results 3a: Effects of phenotypic plasticity?
- survival time in
continuous 2‰ SW
FIX
GRAPH
RC: ANOVA: F1,38 = 0.43,
P = 0.5177
RE: ANOVA: F1,38 = 9.26,
P = 0.0042
• Treatment = 20 slugs exposed to successively lower salinities
1 hr shock, every 2 days, over 10 days
 shocks: 16, 12, 10, 8, 4‰
•Control = 20 slugs kept @ standard conditions: 32‰, 16o C
•At end, a time to death experiment @ 2‰ was performed
 @ range center, no indication of acclimatization to low salinity stress
 @ range edge, slight evidence of slugs exhibiting phenotypic plasticity
Results 3b: Maternal effects?
A. willowi
Treatment @ Level
of Mother
Treatment @ Level
of Offspring
Control:
32‰ for 6hr
(n=24)
Low Salinity:
10‰
(n=8)
Treatment:
6‰ for 6hr
(n=24)
Medium Salinity:
12‰
(n=8)
High Salinity:
16‰
(n=8)
•After treatment, mothers lay 1 egg mass each
•Eggs masses from control and treatment mothers then divided into 3 treatments (10, 12, 16‰)
•For each egg mass, kept @ designated salinity, I measured:
1. Development time
 If mother’s experience
2. Performance of offspring
makes a difference, offspring of
egg mass
low-salinity mothers will be
more resistant to low salinity
themselves
Maternal Effects - Development Time
DT= time from when egg mass was laid until 1st larva leaves egg mass (n=8)
C = Control:
Offspring from 32‰ mom
T = Treatment:
Offspring from 6‰ mom
C
T
C
T
C
T
larva
Across all 3 salinity levels, NO significant effect of whether the mother experienced
low salinity stress or not on the development time of her offspring
Mother’s experience did not have effect on offspring’s ability to tolerate low salinity stress
Maternal Effects - Performance of Offspring
• Shock offspring @ 2‰ for 4 hrs, scored % mortality afterwards
– 10 largest slugs in each dish of 8 replicates per salinity treatment
C = Control: Offspring
from 32‰ mom
T = Treatment: Offspring
from 6‰ mom
C
T
C
T
 Mother’s experience did not have effect on offspring performance
Results 3c: Adaptive evolution?
- slugs collected @ same time from range edge, center (gen 0)
-offspring reared for 2 generations in lab
-time to death experiment performed on all 3 generations
-superior low salinity tolerance exhibited in TB offspring
 low salinity tolerance is genetically based, i.e. local adaptation
Conclusions
 For basic science, range limits are fundamental to ecology and
evolutionary biology:
Ecology: What sets distributional limits?
A. willowi’s northern range limit set @ TB; stable
Set by abiotic factor: salinity
Evolution: What inhibits adaptation to edge conditions?
Gradients in salinity drive variations in local adaptation for A. willowi across range
@ range center, LA: natural selection on low S tolerance is relaxed
•
Slight local adaptation, only seen over rainy season
@ range edge, TB: strong selection on low S tolerance
• Slight effect of phenotypic plasticity
• No evidence for maternal effects
• Low salinity tolerance conferred to offspring
• Strong evidence for adaptive evolution
 Can A. willowi be locally adapted to range edge conditions?
YES!
Thank You
• The Krug Lab:
Dr. Patrick Krug, Dr. Jann Vendetti, Betsy Shimer,
Dominique Gordon, Matthew Garchow, Angela
Llaban, Julia Vo, Diane Rico, John Martin, Zar Phyo
Methods- measuring low salinity tolerance
(time to death)
Vital Staining
 time to death at 2‰ was scored
for 20 slugs per collection
2 ‰ SW
Normal slug
I’m alive! 
I’m dead 
I’m REALLY dead
Maternal Effects - Hatching Success
HS = # of developed embryos (that leave the egg mass) vs. # of undeveloped
embryos (that don’t)
C = Control:
Offspring from 32‰ mom
T = Treatment:
Offspring from 6‰ mom
C
T
C
T
C
T
Across all 3 salinity levels, NO significant effect of whether mother experiences low salinity
stress or not on the # of embryos that fully develop & hatch out vs. the # of undeveloped ones
 Mother’s experience did not have effect on offspring’s ability to tolerate low salinity stress
Prefers cooler temps, lower
salinities
modesta
Dynamic Range BoundaryRanges consistently overlap between
SF Bay and Bodega Harbor
Bodega Harbor
Hog Island
south Tomales
willowi
Prefers warmer temps,
higher salinities
- monitored at 5 sites
from 2003-2011
Mill
Valley
Bolinas lagoon
Rainfall correlates with range limits
Mean Total Monlthy Rainfall (cm)
25
Bodega Harbor
Tomales Bay
Mill Valley
20
15
10
5
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
 Bodega receives the most rainfall (modesta advantage)
 Tomales consistently gets the least rain (willowi advantage)
Nov
Dec
max daily temp (oC)
Temperature and range limits
 Bodega, Mill Valley consistently colder than Tomales sites
Local Adaptation
Selection
for low
salinity
tolerance
Selection
for high
thermal
tolerance
Migration
(gene flow)
Env. 1 - Los Angeles
Env. 2 – San Fran
•Natural selection produces adaptation, but:
• Slxn doesn’t always favor same traits in every habitat
• Animals don’t always stay in one place
•Adaptation results from selection on a heritable trait
• Gene flow opposes adaptation