Self-Incompatibility in Apricot: Identifying Pollination Requirements to Optimize Fruit Production

Abstract

In recent years, an important renewal of apricot cultivars is taking place worldwide, with the introduction of many new releases. Self-incompatible genotypes tolerant to the sharka disease caused by the plum pox virus (PPV), which can severely reduce fruit production and quality, are being used as parents in most breeding programs. As a result, the self-incompatibility trait present in most of those accessions can be transmitted to the offspring, leading to the release of new self-incompatible cultivars. This situation can considerably affect apricot management, since pollination requirements were traditionally not considered in this crop and information is lacking for many cultivars. Thus, the objective of this work was to determine the pollination requirements of a group of new apricot cultivars by molecular identification of the S-alleles through PCR amplification of RNase and SFB regions with different primer combinations. The S-genotype of 66 apricot cultivars is reported, 41 for the first time. Forty-nine cultivars were considered self-compatible and 12 self-incompatible, which were allocated in their corresponding incompatibility groups. Additionally, the available information was reviewed and added to the new results obtained, resulting in a compilation of the pollination requirements of 235 apricot cultivars. This information will allow an efficient selection of parents in apricot breeding programs, the proper design of new orchards, and the identification and solution of production problems associated with a lack of fruit set in established orchards. The diversity at the S-locus observed in the cultivars developed in breeding programs indicates a possible genetic bottleneck due to the use of a reduced number of parents.

Keywords: Prunus armeniaca; S-alleles; apricot; pollen tube; pollination; self-incompatibility.

Figure 1

Size of the PCR amplification fragments using different primer pair combinations for the identification of S-alleles. (A) Gene analyzer output for the SRc-(F/R) primers showing the size of the two amplified fragments of the RNase first intron region corresponding to the S-alleles S_c (358 bp, left) and S₁/S₇ (408 bp, right) in apricot cv. ‘Rojo Pasión’. (B) PCR amplification with the AprFBC8-(F/R) primers for identifying S_c- and S₈-alleles in five apricot cultivars (Pa: ‘Paviot’, Gm: ‘Gönci Magyarkajszi’, Co: ‘Corbato’, Ca: ‘Canino’, and Lu: ‘Luizet’). M: 100 bp DNA Ladder.

Figure 2

Pollen germination and pollen tube growth in self-pollinated apricot flowers observed under the microscope. In Gametophytic Self-Incompatibility (GSI), both compatible and incompatible pollen grains germinate on the stigma. The pollen grain carries one of the two S-alleles of the original genotype. In self-incompatible cultivars, if the S-allele of the pollen grain matches one of the two S-alleles of the pistil, pollen tube growth is inhibited in the middle part of the style. (A) Pollen grains (*) germinating at the stigma surface with pollen tubes emerging towards the style the style (arrow) in the self-compatible cultivar ‘Water’. (B) Pollen tubes (arrow) reaching the base of the style (down) in the self-compatible cultivar ‘Water’. (C) Pollen tubes (arrows) growing along the style in the self-compatible cultivar ‘Water’. (D) Pollen tube (arrow) arrested in the middle part of the style in the self-incompatible cultivar ’Samourai’. Aniline blue staining for callose of squash preparations. Scale bars = 100 µm.

Figure 3

Heatmaps of allele frequencies in traditional cultivars (A) and cultivars from breeding programs (B) for each country of origin using “PopGenReport” v. 3.0.4 R package. Cell color indicates the proportion of the total number of alleles, and the numbers within a cell show the percentage of the number of alleles in each country. The frequency of each S-allele was calculated in each country within each group of accessions, showing statistically significant relationships (p < 0.05) between S-alleles and countries in the groups of both traditional cultivars and releases from breeding programs.