An S-Locus Independent Pollen Factor Confers Self-Compatibility in ‘Katy’ Apricot

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 S1S2 and S-RNase PCR-typing of selfing and outcrossing populations from ‘Katy’ showed that pollen gametes bearing either the S1- or the S2-haplotype were able to overcome self-incompatibility (SI) barriers. Sequence analyses showed no SNP or indel affecting the SFB1 and SFB2 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.

李属自交亲和品种的花粉S功能丧失，大多与S单倍型特异性F-box（SFB）基因的缺失或插入事件相关。然而，也存在由非S位点因子缺陷引发的自交亲和花粉部分突变体，例如杏（Prunus armeniaca）品种‘卡尼诺（Canino）’。本研究对另一款自交亲和杏品种‘凯蒂（Katy）’开展了遗传与分子分析。经鉴定，‘凯蒂’的S基因型为S1S2；对其自交及异交群体开展S-RNase聚合酶链式反应（PCR）分型结果显示，携带S1或S2单倍型的花粉配子均可突破自交不亲和（SI）屏障。序列分析未在‘凯蒂’的SFB1与SFB2等位基因中发现影响其功能的单核苷酸多态性（SNP）或插入缺失（indel），亦未检测到花粉S基因重复的迹象。综合来看，所得结果支持如下假说：一个在花粉中以配子体方式表达、且与S位点不连锁的因子（M’位点）发生功能丧失，导致了‘凯蒂’的自交不亲和被打破。基于分离偏倚位点的定位策略，将M’位点定位于桃（Prunus persica）基因组中对应约1.29 Mb的3号染色体远端区域，该区域的遗传跨度为9.4 cM。值得注意的是，导致杏品种‘卡尼诺’与‘凯蒂’获得自交亲和（SC）性状的花粉部分突变体（PPMs），在3号染色体上的重叠区域约为273 Kb。目前尚无证据表明二者是否影响同一基因，但分子标记结果显示两个品种在遗传上并无关联，提示这两种花粉部分突变体可能独立起源。要明确‘凯蒂’花粉部分突变体的具体分子本质，仍需开展进一步研究，但本次精细定位已可为自交亲和性状的标记辅助选择提供支持，并为后续目标基因的图位克隆奠定基础。