Recent Advances on Self-incompatibility in Almond: A Glance at Genomic and Transcriptomic Levels

Abstract

Self-incompatibility (SI) systems are genetically controlled pollen–pistil interactions that allow the rejection of self-pollen (Nettancourt, Sex Plant Reprod 10:185–199, 1997). Based on the association with floral polymorphism, SI can be classified into two types: heteromorphic and homomorphic. The heteromorphic SI system, with two or three incompatibility types due to different positions of flower parts such as distyly and tristyly, whereas the homomorphic SI system, operates to inhibit self-fertilization regardless of flower morphology with many different incompatibility systems (Charlesworth et al., New Phytol 168:61–69, 2005; Đorđević et al., Genetika 46:411–418, 2014). There are two types of homomorphic SI: sporophytic self-incompatibility (SSI) and gametophytic self-incompatibility (GSI) (Fig. 1). In SSI, the genotype of the diploid parental plant (sporophyte) that acts as the pollen donor determines the incompatibility type (Hiscock and Tabah, Philos Trans R Soc Lond B Biol Sci 358:1037–1045, 2003), and in GSI, the genotype of the haploid pollen itself (gametophyte) determines the incompatibility type. GSI is the most common SI system in the plant kingdom and can be found in Solanaceae, Rosaceae and Plantaginaceae. Almond (Prunus dulcis) is a predominantly out-crossing species, with most cultivars which have been identified as self-incompatible. A few almond cultivars are known to be self-fertile. This phenotype has been attributed to a dominant S-RNase allele, S_f (Grasselly and Olivier, Ann Amélio Plantes 26:107–113, 1976; Socias i Company, Plant breeding reviews, vol 8. Wiley, New York, 1990) or S_fi (S_f-inactive) (Kodad and Socias i Company, Int Soc Hortic Sci (ISHS) 421–424, 2009). Due to poor transcription, plants with the S_f allele seem to lack the active S_f-RNase (Fernández i Martí et al., Sci Hortic 125:685–691, 2010), which arrest the growth of S_f pollen tubes. However, the plants with the S_fa (S_f-active) allele express an active S-RNase and confer SI (Kodad et al., J Am Soc Hortic Sci 134:221–227, 2009). In spite of their contrasting phenotypes, the S_fi- and S_fa-RNase alleles have identical nucleotide sequences and are linked with identical SFB alleles (Fernández i Martí et al., Sci Hortic 125:685–691, 2010). The S_fi and S_fa alleles are differed epigenetically (Fernández i Martí et al., Plant Mol Biol 86:681–689, 2014) by the methylation of a single nucleotide upstream of the coding sequence of the S-RNase, and this variation may determine whether the S_f-RNase allele is expressed. In 2015 Kodad et al. confirmed this experimentally and reported that the plants with S_fiS_fa heterozygotes were fully self-incompatible probably due to their S_fa-RNase arresting the growth of both the S_fi and S_fa pollen tubes.