A Cryptic Modifier Causing Transient Self-Incompatibility in Arabidopsis thaliana

SummaryBreakdown of the pollination barrier of self-incompatibility (SI) in older flowers, a phenomenon known as pseudo-self-compatibility or transient SI, has been described as an advantageous reproductive assurance strategy that allows selfing after opportunities for out-crossing have been exhausted [1–9]. Pseudo-self-compatibility is quite prevalent as a mixed mating strategy in nature, but the underlying molecular mechanisms are not known. We had previously shown that Arabidopsis thaliana exhibits cryptic natural variation for pseudo-self-compatibility, which is uncovered by transformation of different accessions with SI specificity-determining SRK and SCR genes from its self-incompatible sister species A. lyrata [10, 11]. Here, by using this transgenic A. thaliana model, we show that pseudo-self-compatibility is caused by a hypomorphic allele of PUB8, an S-locus-linked gene encoding a previously uncharacterized ARM repeat- and U box-containing protein that regulates SRK transcript levels. This is the first gene underlying pseudo-self-compatibility to be identified and the first report in which cryptic natural variation unveiled by a transgene enabled the cloning of a gene for a complex trait

In Vivo Detection of Residues Required for Ligand-Selective Activation of the S-Locus Receptor in Arabidopsis

SummaryThe self-incompatibility response of crucifers is a barrier to fertilization in which arrest of pollen tube development is mediated by allele-specific interactions between polymorphic receptors and ligands encoded by the S-locus haplotype. Activation of stigma-expressed S-locus receptor kinase (SRK) [1] by pollen coat-localized S-locus cysteine-rich (SCR) ligand [2–5] and the resulting rejection of pollen occurs only if receptor and ligand are encoded by the same S haplotype [4, 6–8]. To identify residues within the SRK extracellular domain (eSRK) that are required for its ligand-selective activation, we assayed chimeric receptors and receptor variants containing substitutions at polymorphic sites in Arabidopsis thaliana [9, 10]. We show that only a small number of the ∼100 polymorphic residues in eSRK are required for ligand-specific activation of self-incompatibility in vivo. These essential residues occur in two noncontiguous clusters located at equivalent positions in the two variants tested. They also correspond to sites showing elevated levels of substitutions in other SRKs, suggesting that these residues could define self-incompatibility specificity in most SRKs. The results demonstrate that the majority of eSRK residues that show signals of positive selection and previously surmised to function as specificity determinants are not essential for specificity in the SRK-SCR interaction

Independent S-Locus Mutations Caused Self-Fertility in Arabidopsis thaliana

A common yet poorly understood evolutionary transition among flowering plants is a switch from outbreeding to an inbreeding mode of mating. The model plant Arabidopsis thaliana evolved to an inbreeding state through the loss of self-incompatibility, a pollen-rejection system in which pollen recognition by the stigma is determined by tightly linked and co-evolving alleles of the S-locus receptor kinase (SRK) and its S-locus cysteine-rich ligand (SCR). Transformation of A. thaliana, with a functional AlSRKb-SCRb gene pair from its outcrossing relative A. lyrata, demonstrated that A. thaliana accessions harbor different sets of cryptic self-fertility–promoting mutations, not only in S-locus genes, but also in other loci required for self-incompatibility. However, it is still not known how many times and in what manner the switch to self-fertility occurred in the A. thaliana lineage. Here, we report on our identification of four accessions that are reverted to full self-incompatibility by transformation with AlSRKb-SCRb, bringing to five the number of accessions in which self-fertility is due to, and was likely caused by, S-locus inactivation. Analysis of S-haplotype organization reveals that inter-haplotypic recombination events, rearrangements, and deletions have restructured the S locus and its genes in these accessions. We also perform a Quantitative Trait Loci (QTL) analysis to identify modifier loci associated with self-fertility in the Col-0 reference accession, which cannot be reverted to full self-incompatibility. Our results indicate that the transition to inbreeding occurred by at least two, and possibly more, independent S-locus mutations, and identify a novel unstable modifier locus that contributes to self-fertility in Col-0