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

•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

Algae

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 BayRange edge

LARange 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

I’m dead Normal, live slug Dissolving, dead slug

Results 1: Local adaptation to edge conditions

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!

- survival time in continuous 2‰ SW

ANOVA: F2,57 = 10.85, P < 0.001

After onset of winter rains & low salinity stress…RC slugs saw slight increase in survivalMean survival of RE slugs increased from 3 hrs to 2 days…

Suggests rapid local adaptation to edge conditions in RE populations

Results 2: Local adaptation over rainy season

RC: ANOVA: F1,38 = 6.0, P = 0.019RE: ANOVA: F1,38 = 244.12, P < 0.0001 T-TEST INSTEAD

- survival time in continuous 2‰ SW

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?

• Treatment = 20 slugs exposed to successively lower salinities1 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

RC: ANOVA: F1,38 = 0.43, P = 0.5177RE: ANOVA: F1,38 = 9.26, P = 0.0042

- survival time in continuous 2‰ SW

FIX GRAPH

Results 3b: Maternal effects?A. willowi

Control: 32‰ for 6hr

(n=24)

Treatment: 6‰ for 6hr

(n=24)

Low Salinity: 10‰(n=8)

Medium Salinity: 12‰(n=8)

High Salinity: 16‰(n=8)

Treatment @ Level of Mother

Treatment @ Level of Offspring

•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 time2. Performance of offspring

If mother’s experience makes a difference, offspring of

low-salinity mothers will be more resistant to low salinity

themselvesegg mass

Maternal Effects - Development TimeDT= time from when egg mass was laid until 1st larva leaves egg mass (n=8)

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

larva

C = Control: Offspring from 32‰ momT = Treatment: Offspring from 6‰ mom

C T C T C T

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

Mother’s experience did not have effect on offspring performance

C = Control: Offspring from 32‰ momT = Treatment: Offspring from 6‰ mom

C T C T

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; stableSet 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

2 ‰ SW

Vital Staining

Methods- measuring low salinity tolerance (time to death)

I’m REALLY dead

time to death at 2‰ was scored for 20 slugs per collection

I’m alive! I’m dead Normal slug

Maternal Effects - Hatching Success

HS = # of developed embryos (that leave the egg mass) vs. # of undeveloped embryos (that don’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

C T C T C T

C = Control: Offspring from 32‰ momT = Treatment:Offspring from 6‰ mom

modesta

willowi

Dynamic Range Boundary-Ranges consistently overlap between

SF Bay and Bodega Harbor

Bodega Harbor

Hog Island

south Tomales

Bolinas lagoon

MillValley

- monitored at 5 sites from 2003-2011

Prefers cooler temps, lower salinities

Prefers warmer temps, higher salinities

Rainfall correlates with range limits

Bodega receives the most rainfall (modesta advantage)

Tomales consistently gets the least rain (willowi advantage)

Temperature and range limits

Bodega, Mill Valley consistently colder than Tomales sites

max

dai

ly te

mp

(o C)

Local Adaptation

•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

Migration(gene flow)

Selection for high

thermal tolerance

Selection for low salinity tolerance

Env. 1 - Los Angeles Env. 2 – San Fran