Self-(in)compatibility of the almonds P. dulcis and P. webbii: detection and cloning of 'wild-type Sf ' and new self-compatibility alleles encoding inactive S-RNases

Abstract

Prunus dulcis, the almond, is a predominantly self-incompatible (SI) species with a gametophytic self-incompatibility system mediated by S-RNases. The economically important allele Sf, which results in self-compatibility in P. dulcis, is said to have arisen by introgression from Prunus webbii in the Italian region of Apulia. We investigated the range of self-(in)compatibility alleles in Apulian material of the two species. About 23 cultivars of P. dulcis (14 self-compatible (SC) and nine SI) and 33 accessions of P. webbii (16 SC, two SI and 15 initially of unknown status), all from Apulia, were analysed using PCR of genomic DNA to amplify S-RNase alleles and, in most cases, IEF and staining of stylar protein extracts to detect S-RNase activity. Some amplification products were cloned and sequenced. The allele Sf was present in nearly all the SC cultivars of P. dulcis but, surprisingly, was absent from nearly all SC accessions of P. webbii. And of particular interest was the presence in many SI cultivars of P. dulcis of a new active allele, labelled S30, the sequence of which showed it to be the wild-type of Sf so that Sf can be regarded as a stylar part mutant S30 degrees . These findings indicate Sf may have arisen within P. dulcis, by mutation. One SC cultivar of P. dulcis, 'Patalina', had a new self-compatibility allele lacking RNase activity, Sn5, which could be useful in breeding programmes. In the accessions of P. webbii, some of which were known to be SC, three new alleles were found which lacked RNase activity but had normal DNA sequences.