Self-(in)compatibility in apricot germplasm is controlled by two major loci, S and M.

BACKGROUND: Apricot (Prunus armeniaca L.) exhibits a gametophytic self-incompatibility (GSI) system and it is mostly considered as a self-incompatible species though numerous self-compatible exceptions occur. These are mainly linked to the mutated SC -haplotype carrying an insertion in the S-locus F-box gene that leads to a truncated protein. However, two S-locus unlinked pollen-part mutations (PPMs) termed m and m' have also been reported to confer self-compatibility (SC) in the apricot cultivars 'Canino' and 'Katy', respectively. This work was aimed to explore whether other additional mutations might explain SC in apricot as well.

RESULTS: A set of 67 cultivars/accessions with different geographic origins were analyzed by PCR-screening of the S- and M-loci genotypes, contrasting results with the available phenotype data. Up to 20 S-alleles, including 3 new ones, were detected and sequence analysis revealed interesting synonymies and homonymies in particular with S-alleles found in Chinese cultivars. Haplotype analysis performed by genotyping and determining linkage-phases of 7 SSR markers, showed that the m and m' PPMs are linked to the same m0- haplotype. Results indicate that m0 -haplotype is tightly associated with SC in apricot germplasm being quite frequent in Europe and North-America. However, its prevalence is lower than that for SC in terms of frequency and geographic distribution. Structures of 34 additional M-haplotypes were inferred and analyzed to depict phylogenetic relationships and M1-2 was found to be the closest haplotype to m0. Genotyping results showed that four cultivars classified as self-compatible do not have neither the SC - nor the m0 -haplotype.

CONCLUSIONS: According to apricot germplasm S-genotyping, a loss of genetic diversity affecting the S-locus has been produced probably due to crop dissemination. Genotyping and phenotyping data support that self-(in)compatibility in apricot relies mainly on the S- but also on the M-locus. Regarding this latter, we have shown that the m0 -haplotype associated with SC is shared by 'Canino', 'Katy' and many other cultivars. Its origin is still unknown but phylogenetic analysis supports that m0 arose later in time than SC from a widely distributed M-haplotype. Lastly, other mutants putatively carrying new mutations conferring SC have also been identified deserving future research.