Abstract
Loss of pollen-S function in Prunus self-compatible cultivars has been mostly associated with deletions or insertions in the S-haplotype-specific F-box (SFB) genes. However, self-compatible pollen-part mutants defective for non-S-locus factors have also been found, for instance, in the apricot (Prunus armeniaca) cv. ‘Canino’. In the present study, we report the genetic and molecular analysis of another self-compatible apricot cv. termed ‘Katy’. S-genotype of ‘Katy’ was determined as S 1 S 2 and S-RNase PCR-typing of selfing and outcrossing populations from ‘Katy’ showed that pollen gametes bearing either the S 1- or the S 2-haplotype were able to overcome self-incompatibility (SI) barriers. Sequence analyses showed no SNP or indel affecting the SFB 1 and SFB 2 alleles from ‘Katy’ and, moreover, no evidence of pollen-S duplication was found. As a whole, the obtained results are compatible with the hypothesis that the loss-of-function of a S-locus unlinked factor gametophytically expressed in pollen (M’-locus) leads to SI breakdown in ‘Katy’. A mapping strategy based on segregation distortion loci mapped the M’-locus within an interval of 9.4 cM at the distal end of chr.3 corresponding to ∼1.29 Mb in the peach (Prunus persica) genome. Interestingly, pollen-part mutations (PPMs) causing self-compatibility (SC) in the apricot cvs. ‘Canino’ and ‘Katy’ are located within an overlapping region of ∼273 Kb in chr.3. No evidence is yet available to discern if they affect the same gene or not, but molecular markers seem to indicate that both cultivars are genetically unrelated suggesting that every PPM may have arisen independently. Further research will be necessary to reveal the precise nature of ‘Katy’ PPM, but fine-mapping already enables SC marker-assisted selection and paves the way for future positional cloning of the underlying gene.
Highlights
Gametophytic self-incompatibility (GSI) is a widespread mechanism in the plant kingdom that prevents inbreeding [1]
Evidence accumulated in Petunia and Antirrhinum supports a model in which SLFs are components of a SCF E3 ubiquitin ligase complex that interacts with non-self S-RNases leading to their ubiquitination and degradation by the 26S proteasome proteolytic pathway [8,9]
Regarding the pollen-part mutations (PPM), self-compatible mutants with non-functional specific F-box (SFB) genes have been identified in sweet cherry (Prunus avium) [15,16,17], apricot (Prunus armeniaca) [18], sour cherry [19], Japanese apricot (Prunus mume) [15] and peach [14], supporting their role as the pollen-S determinants in this genus
Summary
Gametophytic self-incompatibility (GSI) is a widespread mechanism in the plant kingdom that prevents inbreeding [1]. Regarding the pollen-part mutations (PPM), self-compatible mutants with non-functional SFB genes have been identified in sweet cherry (Prunus avium) [15,16,17], apricot (Prunus armeniaca) [18], sour cherry [19], Japanese apricot (Prunus mume) [15] and peach [14], supporting their role as the pollen-S determinants in this genus. In most of these cases, the self-compatible phenotype was associated with indels in the SFB codifying region causing a frame-shift in translation that produces a non-functional truncated protein [20] This seems to be a specific feature of the S-RNase based GSI system operating in Prunus, since in Solanaceae the only pollen-side mutations found to cause SC are due to the S-heteroallelic pollen effect [21]. Macro- and micro-synteny of this region has been studied by comparing with the M-locus in ‘Canino’ and by analyzing the ORFs comprised in the peach syntenic region according to the peach genome v1.0 (International Peach Genome Initiative - IPGI; http://www.rosaceae.org/peach/ genome)
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