adaptation or exaptation? an experimental test of hypotheses on the origin of salinity tolerance in...

Adaptation or Exaptation?An experimental test of hypotheses on

the origin of salinity tolerance in Bufo calamita

Ivan Gomez-mestre and Miguel Tejedo

2005

• Introduction

• Materials and methods

• Results

• Discussion

• Conclution

• Introduction• Materials and methods

• Results

• Discussion

• Conclution

Adaptation

Differences in phenotype among populations if have a genetic basis.

Confer fitness advantage in environment.

(Endler 1986; Sinervo and Basolo 1996).

Exaptation

As a trait evolved for other usages, and later co-opted for its current function.

feather

Gould & Vrba(1982)

Trait

Some traits may share a developmental history in spite of metamorphosis

Postmetamorphic traits may be affected by the environment experienced during premetamorphic stages

(Goater, 1994; Tejedo et al., 2000;Relyea, 2001; Relyea & Hoverman, 2003)

• A trait under certain environmental in one life stage could be affected by selection acting on an associated trait from another life stage experiencing a different environment.

(Deban & Marks, 2002)

Local adaptation of salinity tolerance

• linked to fitness

• genetic basis underlying the trait

(Gomez-Mestre & Tejedo 2003)

Adaptation or Exaptation?

Salinity tolerance may have – direct action of selection– a correlated response to selection acting on s

ome other trait

• Spain populations (even freshwater populations)have shown higher embryonic salinity tolerance than any of the UK populations

• steep South-to-North decreasing gradient in genetic diversity

(Beebee,1985)

North-to-South in western Europe

• decreasing rainfall

• increasing evapotranspiration

• increasing summer drought

• freshwater is the standard larval environment

• drought is a more common selective pressure than salinity

Exaptation

salinity tolerance during embryonic and larval phases

drought tolerance during the terrestrial phases

• The salinity tolerance traits could have evolved as an exaptation.

(Arnold, 1994)

Hypothesis

• relies on association between – salinity tolerance in the aquatic stages,– drought tolerance of the terrestrial juvenile an

d adult stages.

• Introduction

• Materials and methods• Results

• Discussion

• Conclution

Bufo calamita

http://www.herpetofauna.at/amphibien/bufo_calamita.php

Three populations

• freshwater environments– Donana– Pedroso

• brackish environments– Jarales

• Exposed juveniles to either humid or dry conditions for 5 weeks– survival– growth – behaviour： burying and prey capture

Two hydric potentials

• Humid (12 replicates per population)

-150 kPa

• Dry (15 replicates per population)

-1150 kPa.

vermiculite

Burying and prey capture

• Introduction

• Materials and methods

• Results• Discussion

• Conclution

Survival

• Weight at metamorphosis significantly affected survival

• Neither population of origin nor humidity significantly affected survival

Growth

• Growth was not the same across treatments.

• The dry environment significantly reduced growth rate.

• However, neither population of origin nor its interaction with humidity affected growth rate.

Burying

• Humidity had a very significant effect on proportion of time spent buried.

• Toadlets under dry conditions spent more time buried.

Morphological changes

• Body length relative to body mass did not vary significantly between humidity treatments

• Population of origin did not affect relative changes in morphology.

Prey capture success

Toadlets from the humid treatment – more attempts at prey capture– higher efficiency

• Introduction

• Materials and methods

• Results

• Discussion• Conclution

Decreased growth rates

• physiological adjustments,

• shifts in behaviour

• osmotic stress interfere with the control of tongue

Decreased growth rates

• smaller size

• reduced ability to capture prey

Lack of association between drought and salinity tolerance

• the reaction of the three populations across environments were remarkably paralle

• high similarity in drought tolerance among populations

Explain why

• First– the level of drought used in the experiment m

ay not have been stressful enough

– may be expressed only at the hardest conditions

• Secondly– populations are not isolated

– substantial variation in salinity tolerance within freshwater populations

(Gomez-Mestre & Tejedo, 2003)

• Third– different pathways for osmoregulatory physiol

ogy may simply

– however,dismiss the coupling hypothesis

• the information available on tadpole osmoregulation is scarce

• aquatic and terrestrial stages may have different physiological responses to osmotic stress

• each evolving independently of the other.

• Introduction

• Materials and methods

• Results

• Discussion

• Conclution

Conclution

• Salinity tolerance in the aquatic phase of

B. calamita is more likely to have evolved in these populations as an adaptation, rather than an exaptation from drought tolerance.

• Thanks for your attention

• Environmental heterogeneity tends to increase the phenotypic plasticity of traits (when populations exchange migrants)

• However, when migration is restricted, selection under extreme conditions tends to favor local adaptation over plasticity

(Pigliucci 2001)

(Rawson and Hilbish 1991; Sultan and Spencer 2002)

common garden experiments