Evaluation of genetic diversity of S-alleles in an almond germplasm collection

Almond [Prunus amygdalus Batsch syn. Prunus dulcis Mill. (D.A. Webb)] has a gametophytic self-incompatibility system (GSI). The GSI is haplotype-specific and controlled by a multi-allelic S-locus that codes for RNases involved in pollen rejection in a specific manner. The different S-alleles can be used for molecular identification and characterisation of the S-haplotypes. PCR analyses with molecular markers, including primers designed from conserved sequences and for the second intron of almond S-RNases, were used to characterise the S-genotype of 39 almond accessions, including 29 Spanish cultivars, eight releases from the CITA almond breeding programme, and two foreign cultivars. The S-allele compositions of 34 of the cultivars have been characterised for the first time, and two cultivars were revised. The use of conserved primers permitted amplification of two putative S-alleles that had not been amplified previously by PCR, for which the genotypes S-31 and S-32 are proposed. Moreover, the S-f allele, responsible for self-compatibility in almond, has been identified for the first time in local cultivars not derived from the Italian region of Puglia. Restriction digestion of PCR products proved efficient in distinguishing and identifying alleles presenting similar-sized bands in agarose gets and/or having high homology. The application of these PCR techniques to study the genetic diversity of S alleles in almond germplasm is discussed.