the evolution of selfing in arabidopsis thaliana tang et al., 2007
Post on 22-Dec-2015
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TRANSCRIPT
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
• Many accessions failed to hybridize
Dideoxy-sequenced BACs of S-locus from 2 accessions
• C24
• Cvi-0
Conclusions
Found high variation at S-locus allele
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 self-
incompatibility pollen ligand Chookajorn et al., 2004
Research goals
Use ligands from two haplotypes for structure-function 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
8 cysteines (Cys-1 through Cys-8)
Gly-12 in GlyxCys-2 motif
Tyr-26 in Cys-3xxxTyr/Phe motif
Generated structural models of several SCR variants
found to fit a similar fold recognition despite extreme sequence variance
Predicted conservation of overall structure among highly diverged SCR variants
Identification of SCR specificity determinants by domain swapping
Regions between Cys-3 and Cys-4; Cys-5 and Cys-6 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
Mutant failed to activate SI on S6 stigmas despite high binding affinity
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 self-
incompatible 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