dudley sse2011

From elegant to slender, does phenotypic selection on leaf physiological traits predict the divergence between Clarkia sister species, C. unguiculata and C. exilis?

Dr. Leah S. Dudley*, Alisa A. Hove, and Dr. Susan J. MazerDept. of Ecology, Evolution and Marine BiologyUniversity of California, Santa Barbara20 June 2011; Evolution 2011; Norman, Oklahoma

Mating System

• Who is your sexual partner?

• For animals▫ Monogamy▫ Polygamy

• For plants▫ Outcrossing▫ Selfing

Vector-mediated pollination

Autogamous, no vector necessary

Mating System Evolution

•Relative to their outcrossing progenitors or relatives, selfing taxa often exhibit▫Reduced corolla size ▫Lower pollen: ovule▫Shorter anther-stigma distance

(herkogamy)▫Closer dehiscence and receptivity

(dichogamy) ▫Faster development rates▫Earlier flowering

Clarkia ssp

•Selfing evolved numerous times•Outcrossing-selfing diploid sister pairs•Flowers large, numerous and easily

manipulated•Maintain many of the trait divergence

patterns

Phenotypic differences (greenhouse)

Life history Morphology

• C. exilis flowers earlier than C. unguiculata

• C. exilis is protogynous; C. unguiculata is protandrous

• (Dudley et al 2007)

• C. exilis smaller at senescence• C. exilis has less pollen/flower• C. exilis produces more

ovules/ovary• (Mazer et al 2009)

• C. exilis smaller anther-stigma distanceC.

exilisC. unguiculata

Physiological traits

• Instantaneous gas-exchange▫Photosynthetic rate (A)▫Transpiration rate (E)▫Water use efficiency (WUE)

•Twice during the growing season in 2008▫Early, Vegetative▫Late, Reproductive

•Several populations per species (n=24-56 plants/pop)

▫Sierra Nevada▫Lake Isabella and vicinity

Colleague A. Hove warming up LiCor 6400

Accounting for microclimatic variation• Linear regression for each population• Leaf position within the plant• Air temperature at time of measurement

Node number Air temperature (°C)

Tra

nsp

irati

on

(μ

mol

H2O

m-2

leaf

are

a s

ec-

1) C. unguiculata Early transpiration

Phenotypic means

LSM

Pho

tosy

nthe

sis

LSM

Tra

nspi

ratio

n

LSM

Inst

anta

neou

s w

ater

use

effi

cien

cy

Also see Mazer et al 2010

SpeciesP=0.001

1

Early Late

Early Late

ns

Species x Period

P=0.0042a

b

ab

ab

C. exilisC. unguiculata

Maturity index (Mi)•Late season only•Relative measure of reproductive maturity•Total flower and fruit production•Statistically control for plant size

▫Plant height at time of gas-exchange sampling▫Above ground stem biomass at senescence

Flo

wer

an

d f

ruit

p

rod

uct

ion

(L

og

10 x

i+1

)

Main stem height (cm) Senescent plant stem mass (g)

Phenotypic means, Maturity index

LS

M M

atu

rity

in

dex

C. exilis C. unguiculata

SpeciesP<0.000

1

Natural selection

•Goal: to detect evidence that natural selection may contribute to or reinforce the observed phenotypic divergence between sister species.

•Prediction: Direction or strength of natural selection should be consistent with the directon of phenotypic divergence between sister taxa

Models•Relative fitness

▫w=xi (x -1)

•Standardized trait value▫z=(xi - x)(SDx)-1

• wearly=βAzA+βEzE+βAE(zA*zE)+βAAz2A+βEEz2

E+ε

• wearly=βWuezWue+βWueWuez2Wue+ε

• wlate=βAzA+βEzE+βMizMi+βAE(zA*zE)+βAMi(zA*zMi)+βEMi(zE*zMi)+βAAz2A+βEEz2

E+βMiMiz2Mi+ε

• wlate=βWuezWue+βMizMi+βWueMi(zWue*zMi)+βWueWuez2Wue+βMiMiz2

Mi+ε

Models• Relative fitness

▫ w=xi (x -1)

• Standardized trait value▫ z=(xi - x)(SDx)-1

• wearly=βAzA+βEzE+ε

• wearly=βWuezWue+ε

• wlate=βAzA+βEzE+βMizMi+ε

• wlate=βWuezWue+βMizMi+ε

• Direct selection, linear slope estimates▫ βA, βE, βMi, βWue

Direct selection, early sampling period

C. e

xilis

freq

uenc

yC.

ung

uciu

lata

freq

uenc

y

Photosynthesis Transpiration Instantaneous water use efficiency

Granite Willow Springs Woody RoadJack and Stage Live Oak Stark Creek

Cow FlatPopulations:

C. exilis > C. unguiculata

C. exilis > C. unguiculata

Direct selection, late sampling period

C. e

xilis

freq

uenc

yC.

ung

uciu

lata

freq

uenc

y

Photosynthesis Transpiration Instantaneous water use efficiency

Granite Willow Springs Woody RoadJack and Stage Live Oak Stark Creek

Cow FlatPopulations:

C. exilis > C. unguiculata C. exilis > C. unguiculata

Also see Mazer et al 2010

Maturity index

Conclusions

•Photosynthesis is the trait under the most consistent direct selection (positive)

•Selection on WUEi is in the direction expected▫C. exilis < C. unguiculata▫Direct selection favors low WUE

•Selection on the maturity index opposes the phenotypic mean differences▫C. exilis >C. unguiculata▫Selection favors low MI in C. exilis