Molecular genetic analysis of the candidate gene for MOD, a locus required for self-incompatibility in Brassica rapa

The MIP-MOD (for MOD-locus associated Major Intrinsic Protein) gene encodes an aquaporin-like product, and has been reported to be a candidate for the MOD gene which is required for the self-incompatibility response in Brassica rapa. In an antisense suppression experiment designed to investigate the role of MIP-MOD, we found that levels of MIP-MOD mRNA in the stigmas of fourteen antisense transgenics, as well as in the sell-incompatible cultivar Osome (Osm), were much lower than in the stigmas of the self-incompatible S⁸homozygous (S⁸) strain. Therefore, we analyzed the molecular structure of the MIP-MOD gene in three B. rapa strains: S⁸, Osm, and the self-compatible var. Yellow Sarson (YS). Nucleotide sequence analysis of the MIP-MOD genes isolated from the three strains revealed that all three encode the same amino acid sequence and that YS and Osm contain the same MIP-MOD allele, designated MIP-MOD^YS. Analysis of other self-incompatible B. rapa strains that are homozygous for the MIP-MOD^YSallele indicated that high levels of MIP-MOD transcripts are not essential for the self-incompatibility response. Furthermore, a MOD mutant generated by γ-irradiation was found to contain a wild-type MIP-MOD gene that is expressed at normal levels. These data suggest that MIP-MOD is not MOD itself. We suggest that this gene should be renamed MLM (for MIP gene linked to MOD).