MIK2 is a candidate gene of the S-locus for sporophytic self-incompatibility (SSI) in chicory (Cichorium intybus, Asteraceae)

The Cichorium genus offers a unique opportunity to study the sporophytic self incompatibility (SSI) system, being composed of species characterized by highly efficient SI (C. intybus) and complete self compatibility (C. endivia). The chicory genome was used to map 7 previously identified SSI locus-associated markers. The region containing the S locus was restricted to an 4 M bp window on chromosome 5. Among the genes predicted in this region, MDIS1 INTERACTING RECEPTOR LIKE KINASE 2 (MIK2) was promising as a candidate for SSI. Its ortholog in Arabidopsis is involved in pollen stigma recognition reactions, and its protein structure is similar to that of S-receptor kinase (SRK), a key component of the SSI in the Brassica genus. The sequencing of MIK2 in chicory and endive accessions revealed two contrasting scenarios. In C. endivia, MIK2 was fully conserved even comparing different botanical varieties (smooth and curly). In C. intybus, 387 SNPs and 3 INDELs were identified when comparing accessions of different biotypes from the same botanical variety (radicchio). The SNP distribution throughout the gene was uneven, with hypervariable domains preferentially localized in the LRR-rich extracellular region, putatively identified as the receptor domain. The gene was hypothesized to be under positive selection, as the nonsynonymous mutations were more than double the synonymous ones (dN / dS = 2.17). An analogous situation was observed analyzing the first 500 bp of the MIK2 promoter: no SNPs were observed among the endive samples, whereas 44 SNPs and 6 INDELs were detected among the chicory samples. Further analyses are needed to confirm the role of MIK2 in SSI and to demonstrate whether the 23 species-specific nonsynonymous SNPs in the CDS and/or the species-specific 10 bp INDEL found in a CCAAT box region of the promoter are responsible for the contrasting sexual behaviors of the two species.