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

Abstract

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 S-C-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 m(0)-haplotype. Results indicate that m(0)-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 S-C 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 m(0). Genotyping results showed that four cultivars classified as self-compatible do not have neither the S-C-nor the m(0)-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 m(0)-haplotype associated with SC is shared by 'Canino', 'Katy' and many other cultivars. Its origin is still unknown but phylogenetic analysis supports that m(0) arose later in time than S-C from a widely distributed M-haplotype. Lastly, other mutants putatively carrying new mutations conferring SC have also been identified deserving future research.