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 M _1-2 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.

Keywords: Apricot; M-locus; Modifiers; Prunus; S-alleles; S-locus; Self-(in)compatibility.

Fig. 1

Apricot M-locus haplotypes structure. The peach syntenic region at the distal end of chr. 3 (black box) comprising the M-locus (grey box) is zoomed twice and shows SSRs (dashed lines) PCR-amplified in apricot cultivars ‘Goldrich’ (G), ‘Canino’ (Ca) and ‘Katy’ (K). SSR positions in peach genome (Kb) and allele sizes (bp) determined in apricot are indicated. White, black, diagonal striped and grey thick lines represent apricot m ₀, M ₁, M ₂ and M ₃ haplotypes, respectively. SSR anchoring positions are shown in centimorgans (cM) (boxed numbers) according to the available mapping populations (‘G × Ca’, ‘K × K’ and ‘G × K’)

Fig. 2

S- and M-locus haplotypes distribution according to geographic areas. Apricot accessions analyzed in this study were grouped in four arbitrarily defined geographic areas represented by the bottom map: Western Europe (WE), North America (NA), Southern Europe and North Africa (SE/NAf) and Eastern Europe (EE). Accordingly, relative frequencies for S- and M-haplotypes (pie charts) are shown for each area (clonal sibs from ‘Canino’ were excluded from estimations). For the sake of simplicity M-locus haplotypes are represented in their ‘main classes’. Question marks (?) designate not defined haplotypes. Number of accessions showing self-(in)compatible phenotype are encircled (green color means SC, red SI and blue undetermined phenotype)

Fig. 3

Clustering analysis of apricot M-locus haplotypes based on genetic distances. a Clustering obtained by Neighbor-Joining algorithm using Jaccard’s distance. b Clustering obtained by Neighbor-Joining algorithm using Bruvo’s distance. Colors represent geographic areas where the distinct M-locus haplotypes were detected (see legend)