Optimizing Production in the New Generation of Apricot Cultivars: Self-incompatibility, S-RNase Allele Identification, and Incompatibility Group Assignment

Abstract

Apricot (Prunus armeniaca L.) is a species of the Rosaceae that was originated in Central Asia, from where it entered Europe through Armenia. The release of an increasing number of new cultivars from different breeding programs is resulting in an important renewal of plant material worldwide. Although most traditional apricot cultivars in Europe are self-compatible, the use of self-incompatible cultivars as parental genotypes for breeding purposes is leading to the introduction of a number of new cultivars that behave as self-incompatible. As a consequence, there is an increasing need to interplant those new cultivars with cross-compatible cultivars to ensure fruit set in commercial orchards. However, the pollination requirements of many of these new cultivars are unknown. In this work, we analyze the pollination requirements of a group of 92 apricot cultivars, including traditional and newly-released cultivars from different breeding programs and countries. Self-compatibility was established by the observation of pollen tube behavior in self-pollinated flowers under the microscope. Incompatibility relationships between cultivars were established by the identification of S-alleles by PCR analysis. The self-(in)compatibility of 68 cultivars and the S-RNase genotype of 74 cultivars are reported herein for the first time. Approximately half of the cultivars (47) behaved as self-compatible and the other 45 as self-incompatible. Identification of S-alleles in self-incompatible cultivars allowed allocating them in 11 incompatibility groups, six of them reported here for the first time. The determination of pollination requirements and the incompatibility relationships between cultivars is highly valuable for the appropriate selection of apricot cultivars in commercial orchards and of parental genotypes in breeding programs. The approach described can be transferred to other woody perennial crops with similar problems.

Identification of Self-Incompatibility Alleles by Specific PCR Analysis and S-RNase Sequencing in Apricot

“…However, while the sequence of S 1 has been previously reported [ 41 ] (AY587561), the S 7 allele has only been identified by PCR analysis and no sequence has been yet included in the NCBI database. Thus, using the primers Pru-C2 and PruC4R, we cloned and sequenced the fragment of 900 bp that was obtained from the cultivar ‘Charisma’ ( S c / S 7 , [ 24 ]), which corresponds to the S 7 allele [ 33 ]. Surprisingly, the cloned sequence resulted in a fragment of 915 bp ( Figure S2 ) that showed a 99% identity with the partial coding sequence (790 bp, EF062341, unpublished), of the S 13 allele and with a fragment of 830 bp that corresponds to the unpublished S 46 allele (HQ342876).…”

“…According to their S -allele composition, 13 self-incompatible cultivars have been allocated in nine previously determined incompatibility groups [ 24 , 37 , 38 , 39 , 40 ]. “Primaya” ( S 1 S 6 ), “A106” ( S 1 S 9 ), and “C009” ( S 6 S 8 ) have been assigned to XXIV-XXVI groups, three new incompatibility groups that have been reported herein for the first time.…”

“…Thus, in order to minimize environmental effects, the (in)compatibility phenotype can be evaluated in semi-in vivo culture of flowers in laboratory-controlled self- and cross-pollinations, followed by the subsequent observation of pollen tube behavior through the style under a fluorescence microscopy. This approach has been successfully used to determine self-(in)compatibily and incompatibility relationships between cultivar, in apricot [ 24 ] and other Prunus species [ 25 ].…”

“…Thirty-three alleles have been reported so far in apricot ( S 1 to S 20 , S 22 to S 30 , S 52 , S 53 , S v , S x ), including one allele for self-compatibility ( S c ) [ 32 , 33 , 34 , 35 , 36 ]. Additional alleles have been included in the NCBI database but not yet published showing misidentification and the appearance of numerous homologies [ 24 , 35 ]. S- genotyping has allowed for classifying the cultivars in their corresponding incompatibility group according to their compatibility relationships.…”

“…Thus, those self-incompatible cultivars with the same S -alleles are inter-incompatible and are allocated in the same incompatibility group, whereas cultivars from different groups with at least one different S -allele are inter-compatible [ 35 ]. In apricot, 23 incompatibility groups have been stablished [ 24 , 37 , 38 , 39 , 40 ], which provides useful information to apricot growers and breeders.…”