  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.
