Although self-compatibility was discovered in almond as early as 1945 (Almeida, 1945), no attention was paid to the issue until the 1970s. The importance of self-compatibility in almond-growing and in breeding for new self-compatible cultivars was then fully understood (Socias i Company, 1978). The first attempts for self-compatibility identification involved fruit set evaluation after artificial self-pollinations (Almeida, 1945). This approach is based on the horticultural importance of almond self-compatibility, that is, to obtain commercial yields after an acceptable fruit set (Socias i Company et al., 2009).
Several approaches, each one showing advantages and limitations, have been used to assess the level of self-compatibility in almond. Effective self-compatibility implies, first of all, pollen tube growth after self-pollination similar to that after cross-pollination with cross-compatible pollen. Second, this good pollen tube growth after self-pollination should result in similar fruit sets, which may not always be the case. Third, these fruit sets must reach the level of a commercial crop. From a horticultural point of view, there is a fourth requirement, because these fruit sets must be obtained by autogamy, and that is the ability of a genetically self-compatible cultivar to pollinate itself in the absence of insects (Weinbaum, 1985). Additionally, a good cultivar must always be productive with a crop of good kernel quality.
Identification of S alleles was first attempted to establish cross-incompatibility groups by test pollination crosses (Kester et al., 1994). However, this approach could not allow the identification of the Sf allele. Only after Bošković et al. (1999) found no RNase activity for the Sf allele could an efficient identification of this allele be initiated.
More recently, once the genetic structure of the Sf allele was further understood, the detection of self-compatibility was also undertaken by molecular markers. Gametophytic self-compatibility such as that found in almond is controlled by a single polymorphic locus containing at least two tightly linked genes, one specifically expressed in the pistil and the other in the pollen (Kao and Tsukamoto, 2004). The pistil component of this gene codes for an S-RNase responsible for the pollen tube growth inhibition in the styles (Bošković et al., 1997). The candidate gene for the S pollen component (SFB) has been identified by Ushijima et al. (2003) showing a tight linkage with the S-RNase gene (Ikeda et al., 2005). Undoubtedly, the knowledge of the molecular basis for self-compatibility in the rosaceous fruit species has advanced significantly in recent years (Tao and Iezzoni, 2010; Yamane and Tao, 2009).
However, this information is only genetic and not horticultural. The final evaluation of self-compatibility of a cultivar or selection is its productivity under field conditions. This implies solid blocks of one clone isolated from any other almond clone and even in the absence of pollinating insects. Thus, our objective was to review the different physiological and genetic aspects of almond self-compatibility. This approach is required to better understand how these aspects are evaluated and how the results of their evaluation may be applied efficiently in a breeding program. This wider approach has become more necessary, especially after stating that some confusing results have been reported recently. These results refer to the Sf allele identification by molecular markers and gene sequencing as well as to the presence of modifier genes affecting the expression of self-compatibility in almond.
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