Transcript Document

The Evolution of Selfing in
Arabidopsis thaliana
Tang et al., 2007
Evolution of flowering plants
 Transition from outcrossing to selfing

Occurred independently in numerous lineages

Important for colonization
 A. thaliana highly selfing

Closest relative A. lyrata (self-incompatible)

Separated by ~5 million years
Role of the S locus in the transition to
selfing
 Self-incompatibility (SI) locus (S) tightly linked main
components:

S-locus receptor kinase (SRK)
•

Encodes female specificity
determinants of SI
S-locus cysteine-rich protein (SCR)
•
Encodes male specificity determinants
of SI
•
Pollen ligand for SRK
SI Model in Brassicaceae
 SCR is a ligand for SRK
 In self-pollination, SCR protein delivered to stigma,
binds to ectodomain of “self” SRK, activates SRK
kinase

Triggers signal transduction pathway resulting
in inhibition of fertilization
 In cross-pollination, SCR protein cannot bind or
activate “non-self” SRK
 SRK and SCR genes must coevolve to maintain SI
 Generation of novel SI specificity requires
compensatory mutations in receptor and ligand of
same haplotype, so SRK-SCR binding is maintained
S-locus characterized by highly
divergent haplotypes
 SI system in A. thaliana inactivated (SCR and SRK
are psuedogenes in Col-0)
 Inactivation of S-locus could be key step in evolution
•
Transformation with A. lyrata S-locus alleles
restores SI in A. thaliana
 Should show reduced variability compared to
ancestral locus if inactivation only occurred once
Investigating variation at S-locus
 PCR of SCR and SRK - failed
 Whole-genome resequencing data for 20 accessions
with oligonucleotide arrays
•
Designed based on reference sequence
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Many accessions failed to hybridize
 Dideoxy-sequenced BACs of S-locus from 2
accessions
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C24
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Cvi-0
Conclusions
 Found high variation at S-locus allele
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Ancestral balanced polymorphism at S-locus
gradually eroding through genetic drift

Selection for inactivity
 Multiple evolutionary routes to selfing
•
Transformation with A. lyrata SI alleles does
not always restore SI
 Species-wide selfing evolved 1 m.y.a. or more
Specificity determinants and
diversification of the Brassica selfincompatibility pollen ligand
Chookajorn et al., 2004
Research goals
 Use ligands from two haplotypes for structurefunction studies of SCR

SCR6 sequence variants made by swapping
specific domains between SCR6 and SCR13
variants by in vitro mutagenesis
 Used variants to map sequences that determine
recognition specificity in SCR
 Asses if residues largely conserved in SCRs are
important for SCR6 function
 Relate results to hypotheses for the evolution of new
SI specificities
Predicted conservation of overall
structure among highly diverged
SCR variants
 Few residues conserved between most variants
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8 cysteines (Cys-1 through Cys-8)
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Gly-12 in GlyxCys-2 motif
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Tyr-26 in Cys-3xxxTyr/Phe motif
Predicted conservation of overall structure
among highly diverged SCR variants
Generated
structural models of
several SCR
variants
 found to fit a
similar fold
recognition
despite extreme
sequence
variance
Identification of SCR specificity
determinants by domain swapping
 Regions between Cys-3 and Cys-4; Cys-5 and Cys6 are candidate specificity determinants - predicted
to be surface-exposed
 Generated SCR6-SCR13 chimeras by exchanging
various domains
 Only SCR6 chimera (5-6) showed modified specificity

Inactive on SCR6 stigmas

Activated SI on S13 stigmas
 Four specificity determinants: first 4 residues (TDTQ)
from SCR13 C5-C6 region
Accessing the flexibility of SCR6 protein
 Predict that because the 4 specificity residues from
SCR13 function in SCR6 backbone, SCR6 may
tolerate many mutations
 Generated variants of SCR6 by alanine-scanning
mutagenesis of C3-C4 and C5-C6 regions
 Substituted conserved residues with Alanine
 Found Tyr-26 (Y to A mutation) to be required for
SCR6 function
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Mutant failed to activate SI on S6 stigmas despite
high binding affinity
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Uncoupling of SCR binding to SRK from its ability
to activate SI response
Conclusions
 Because of evolutionary adaptability of SCR and the
inferred flexibility of SCR-SRK interaction:

Hypothesize that new SI specificities
generated through SI intermediates by
process that changes pollen and stigma
components of S locus haplotype, but
preserves allelic recognition

New mutations in SCR favored if they
increase activation of SRK allele, or if they
reduce false activation of the mismatching
SRK alleles
Model for generation of new SI
specificities through selfincompatible intermediates
Within each functional haplotype,
variability in SCR and SRK is tolerated
and mutual recognition exists
If, in a subset of alleles (*), SCR and
SRK show stronger affinity with each
other than with corresponding proteins,
then natural selection drives the
strengthening of SRK-SCR and SRK*SCR* interactions
Leads to origination of a novel allele
without producing any SC intermediates