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Life at the edge: local adaptation and range limits for two estuarine sea slugs. Hanna Koch Dr. Patrick Krug California State University, Los Angeles. Local Adaptation. Selection for low salinity tolerance. Selection for high thermal tolerance. Migration (gene flow). Env. 1 - Los Angeles. - PowerPoint PPT Presentation
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Life at the edge:local adaptation and range limits for two estuarine
sea slugs
Hanna Koch Dr. Patrick Krug
California State University, Los Angeles
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
Range Limits
• Represent species’ limits to adaptability
• Useful for ID’ing & testing limiting factors that shape distributions/abundances of organisms
• Geographic barriers• Abiotic (temperature, salinity) • Genetic Constraints (what inhibits adaptation) • Biotic Interactions (competitive exclusion)
Intertidal ecosystem
• Model system:– Ideal for range limit studies– Easily tracked along 1-D, narrow shoreline
• North & south endpoints
– Distribution spatially & temporally restricted by tides• Exposed to extreme temps & salinities• No subtidal refuge• Climate change can lead to range shifts
Alderia modesta - Alderia willowi
•Estuarine sea slugs
•Live on exposed mudflats of salt marshes along North America’s Pacific coast
•Live & feed on alga that grows on the mudflats
Vaucheria longicaulis
Northern sp.
Dynamic Range Boundary
Alderia modesta
Alderia willowiSouthern sp.
willowi, modesta
Bodega Bay
Tomales Bay (HI)
Tomales Bay (OF)
SF Bay
range edge vs. range center• Tomales Bay (stressful):
– range edge– least freshwater input compared to surrounding bays (Bodega & SF)
• Tectonically formed
• Los Angeles (optimal):– range center – More stable environment than TB
• Warmer temps• 4-fold less precipitation• Preferred higher salinities
Tomales BayRange edge
LARange center
A. willowi
Hypotheses
1. The range-edge population is more locally adapted to low salinity stress than range center
2. Local adaptation over the rainy season is more pronounced at the range edge
3. Low salinity tolerance is a genetic, heritable trait & is favored at the range edge
2 ‰ SW
Vital Staining
Methods- measuring low salinity tolerance (time to death)
Results 1 (summer): Results 1 (summer): local adaptation across rangelocal adaptation across range
Mea
n T
ime
to
Dea
th (
min
)
A. willowi
(Range center) (Range edge)
Range edge exhibits greater local adaptation to low salinity stress, even during summer before the winter rains
ANOVA: F2,57 = 10.85, p < 0.001
B > A,p < 0.001
Los Angeles Bolinas Tomales Bay
A B B
Results 2: local adaptation over growing seasonResults 2: local adaptation over growing season
Range Center
Mea
n T
ime
to D
eath
(m
in)
Range EdgeANOVA: F1,38 = 6.0, p = 0.019
A. willowiANOVA: F1,38 = 244.12, p < 0.0001A. willowi
Populations become more adapted over rainy season & even more so @ range edge
3.7 hrs
5.4 hrs
3 hrs
2 days
September ’10 December ‘10 September ’10 December ‘10
Results 3: Results 3: low salinity tolerance is a heritable traitlow salinity tolerance is a heritable trait
Mea
n T
ime
to D
eath
(m
in) A. willowi
Range CenterRange Edge
Generation 0 Generation 1 Generation 2
Low salinity tolerance is (A) genetically based (B) favored at the range edge
A. willowi
Results 4:Results 4:comparing low salinity comparing low salinity
tolerance between sister tolerance between sister species species
Mea
n T
ime
to D
eath
(m
in)
A. modesta A. willowi
ANOVA: F1,38 = 10.4, p < 0.005
~2 days
~3 days
modesta’s ability to withstand lower
salinities for longer provides greater local
adaptation & competitive advantage
over willowi in SF
Conclusions
1. Gradients in salinity drive variations in local adaptation for A. willowi across its range– @ range center, LA: nat’l slxn on low S tolerance is relaxed
• Slight local adaptation, only seen over rainy season
– @ range edge, Tomales Bay: strong slxn on low S tolerance• Strong local adaptation, Low S tolerance conferred to offspring
2. A. willowi northern range limit currently fixed @ TB– Genetic constraints
3.3. In SF, the most adapted In SF, the most adapted willowiwillowi still never reach same level of still never reach same level of tolerance to very low salinity as modesta tolerance to very low salinity as modesta doesdoes
Overall Study Significance
• If we can better understand how tolerance for the physical environment limits the geographical range of species, then we can
make better predictions of potential ecosystem responses to climate change, which is critical to effective management and
conservation
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
Climate Change
• As global temperatures rise:– Changes in salinity & rainfall patterns– Worldwide range limit shifts
• Previous studies suggest poleward (upper) bounds set by physiological limits; equatorial (lower) bounds set by interspecific competition
Daily exposure to 2‰ water Daily exposure to heat stress
Population Dynamics in Mill Valley - SF
Seasonal shifts in
presence / abundance…
…correlated with
seasonal shifts in
temperature & salinity
Abiotic Stress
• Salinity gradients can determine distributions w/in estuaries
• Salinity has not yet been linked to N-S range endpoints in a intertidal animal, yet is critical for estuarine taxa dist. Along coastlines w/ strong latitudinal gradient in precipitation like NE Pacific