Several approaches have been used in evaluating self-compatibility in almond. These include fruit set after self-pollination and bagging, pollen tube growth, and the more recent Sf allele identification by molecular markers and gene sequencing. However, none of these methods have given fully reliable results because they all show advantages and limitations. Pollen contamination may distort pollination results as well as inaccuracies during fruit setting operations. Factors other than self-compatibility such as inbreeding may affect fruit set and pollen tube growth. Detection of S alleles by RNase activity and polymerase chain reaction analysis by consensus primers has not always been conclusive. The differential phenotypic expression of the Sfa and the Sfi alleles has revealed that the presence of the Sf allele is not the only requirement for self-compatibility expression in almond. As a consequence, the coding region of the Sf allele may not be the sole factor involved in that expression, which may be caused by modifier genes outside this region. Missequencing of alleles has also created confusion for allele identification. Thus, self-compatibility evaluation in almond must involve a better knowledge of the plant material as a whole, and not only of its genotype. All factors involved in setting a commercial crop in conditions of solid plantings of a single cultivar must be put together to evaluate almond self-compatibility. This approach is fundamental for the understanding of self-compatibility in almond and for the evaluation of the new selections in a breeding program.
Rafel Socias i Company, Àngel Fernández i Martí, Ossama Kodad, and José M. Alonso
Àngel Fernández i Martí, José M. Alonso, María T. Espiau, María J. Rubio-Cabetas, and Rafel Socias i Company
Genetic diversity of the Spanish national almond (Prunus amygdalus Batsch) collection was characterized with 19 simple sequence repeat (SSR) markers selected because of their polymorphism in almond and other Prunus L. species. A total of 93 almond genotypes, including 63 Spanish cultivars from different growing regions, as well as some international cultivars and breeding releases were analyzed. All primers produced a successful amplification, giving a total of 323 fragments in the genotypes studied, with an average of 17 alleles per SSR, ranging from 4 (EPDCU5100) to 33 (BPPCT038). Allele size ranged from 88 bp at locus PMS40 to 260 bp at locus CPPCT022. The heterozygosity observed (0.72) was much higher not only than in other Prunus species, but also than in other almond pools already studied. The dendrogram generated using the variability observed classified most of the genotypes according to their geographical origin, confirming the particular evolution of different almond ecotypes. The SSR markers have consequently shown their usefulness for cultivar identification in almond, for establishing the genetic closeness among its cultivars, and for establishing genealogical relationships.
Toshio Hanada, Kyoko Fukuta, Hisayo Yamane, Tomoya Esumi, Ryutaro Tao, Thomas M. Gradziel, Abhaya M. Dandekar, Ángel Fernández i Martí, José M. Alonso, and Rafel Socias i Company
Most of the self-compatible (SC) cultivars of almond [Prunus dulcis (Mill.) D.A. Webb. syn. P. amygdalus Batsch] have the Sf haplotype. In this study, we cloned and characterized the S locus region of the Sf haplotype of SC ‘Lauranne’. The relative transcriptional orientation of SFBf and Sf-RNase and the physical distance between them are similar to those of other functional self-incompatible (SI) S haplotypes of Prunus, indicating that the genomic structure of the SC Sf haplotype appears to be intact. Although there is no apparent mutation in the coding sequence of SFBf, the Sf-RNase sequence in this study and previously reported Sf-RNase sequences show discrepancies. First, as opposed to previous indications, the ‘Lauranne’ Sf-RNase sequence encodes a histidine residue in place of a previously reported arginine residue in the conserved C2 region of Prunus S-RNase. Direct sequencing of the polymerase chain reaction products from the Sf-RNase of ‘Tuono’ confirmed that ‘Tuono’ Sf-RNase also encodes the histidine residue. We found another difference in the ‘Lauranne’ Sf-RNase sequence and other reported Sf-RNase sequences. Namely, ‘Lauranne’ Sf-RNase encodes a phenylalanine residue in place of a previously reported leucine residue in the conserved C5 region of Prunus S-RNase. This is also the case for ‘Tuono’ Sf-RNase. Expression analysis of Sf-RNase and SFBf by reverse transcriptase–polymerase chain reaction showed that Sf-RNase transcripts were barely detectable in pistil, whereas SFBf transcripts were accumulated at a similar level to the level that was observed with SFB of other functional SI S haplotypes of almond. We discuss the possible molecular mechanisms of SC observed with the Sf haplotype with special references to the expression of Sf-RNase.