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Self-incompatibility (SI) is a complex mechanism that prevents plants from self-fertilizing to preserve and promote genetic variability. The angiosperm species have developed two different SI systems, the sporophytic (SSI) and the gametophytic (GSI) systems. SI is a significant impediment to steady fruit production in fruit tree species of the Rosaceae. In Rosaceae, GSI is genetically regulated via a single locus, named the 'S-locus', which includes a minimum of two polymorphic and relatively intercorrelated S genes: a pistil-expressed S-RNase gene and several pollen-expressed SFBB (S-locus F-Box Brothers) or SFB (S haplotype-specific F-box protein). This necessitates the interaction of S-RNases with the male determinants. Although genetic and molecular analyses of S genes have shown that mutations in both pistils and pollen-specific components induce self-compatibility in many species and cultivars, other genes or molecules outside the S-locus can co-participate in the male gamete rejection in GSI. However, we highlight and synthesize the most recent knowledge on different mechanisms of GSI in Rosaceae in this current review. [ABSTRACT FROM AUTHOR]

Summary: Self‐incompatibility (SI) is a widespread prezygotic mechanism for flowering plants to avoid inbreeding depression and promote genetic diversity. Citrus has an S‐RNase‐based SI system, which was frequently lost during evolution. We previously identified a single nucleotide mutation in Sm‐RNase, which is responsible for the loss of SI in mandarin and its hybrids. However, little is known about other mechanisms responsible for conversion of SI to self‐compatibility (SC) and we identify a completely different mechanism widely utilized by citrus. Here, we found a 786‐bp miniature inverted‐repeat transposable element (MITE) insertion in the promoter region of the FhiS2‐RNase in Fortunella hindsii Swingle (a model plant for citrus gene function), which does not contain the Sm‐RNase allele but are still SC. We demonstrate that this MITE plays a pivotal role in the loss of SI in citrus, providing evidence that this MITE insertion prevents expression of the S‐RNase; moreover, transgenic experiments show that deletion of this 786‐bp MITE insertion recovers the expression of FhiS2‐RNase and restores SI. This study identifies the first evidence for a role for MITEs at the S‐locus affecting the SI phenotype. A family‐wide survey of the S‐locus revealed that MITE insertions occur frequently adjacent to S‐RNase alleles in different citrus genera, but only certain MITEs appear to be responsible for the loss of SI. Our study provides evidence that insertion of MITEs into a promoter region can alter a breeding strategy and suggests that this phenomenon may be broadly responsible for SC in species with the S‐RNase system. [ABSTRACT FROM AUTHOR]

Self-incompatibility (SI) is a complex mechanism that prevents plants from self-fertilizing to preserve and promote genetic variability. The angiosperm species have developed two different SI systems, the sporophytic (SSI) and the gametophytic (GSI) systems. SI is a significant impediment to steady fruit production in fruit tree species of the Rosaceae. In Rosaceae, GSI is genetically regulated via a single locus, named the ‘S-locus’, which includes a minimum of two polymorphic and relatively intercorrelated S genes: a pistil-expressed S-RNase gene and several pollen-expressed SFBB (S-locus F-Box Brothers) or SFB (S haplotype-specific F-box protein). This necessitates the interaction of S-RNases with the male determinants. Although genetic and molecular analyses of S genes have shown that mutations in both pistils and pollen-specific components induce self-compatibility in many species and cultivars, other genes or molecules outside the S-locus can co-participate in the male gamete rejection in GSI. However, we highlight and synthesize the most recent knowledge on different mechanisms of GSI in Rosaceae in this current review.

The pollination of self-incompatible diploid sweet cherry is determined by the S-locus alleles. We resolved the S-alleles of 50 sweet cherry cultivars grown in Estonia and determined their incompatibility groups, which were previously unknown for most of the tested cultivars. We used consensus primers SI-19/20, SI-31/32, PaConsI, and PaConsII followed by allele-specific primers and sequencing to identify sweet cherry S-genotypes. Surprisingly, 48% (24/50) of the tested cultivars, including 17 Estonian cultivars, carry the rare S-allele S17, which had initially been described in wild sweet cherries in Belgium and Germany. The S17-allele in Estonian cultivars could originate from 'Leningradskaya tchernaya' (S6|S17), which has been extensively used in Estonian sweet cherry breeding. Four studied cultivars carrying S17 are partly self-compatible, whereas the other 20 cultivars with S17 have not been reported to be self-compatible. The recommended pollinator of seven self-incompatible sweet cherries is of the same S-genotype, including four with S17-allele, suggesting heritable reduced effectiveness of self-infertility. We classified the newly genotyped sweet cherry cultivars into 15 known incompatibility groups, and we proposed four new incompatibility groups, 64–67, for S-locus genotypes S3|S17, S4|S17, S5|S17, and S6|S17, respectively, which makes them excellent pollinators all across Europe. Alternatively, the frequency of S17 might be underestimated in Eastern European populations and some currently unidentified sweet cherry S-alleles might potentially be S17. [ABSTRACT FROM AUTHOR]

There has been an increased interest in true potato seeds (TPS) as planting material because of their advantages over seed tubers. TPS produced from a tetraploid heterozygous bi-parental population produces non-uniform segregating progenies, which have had limited uniformity in yield and quality in commercial cultivation, and, thus, limited success. Inbreeding depression and self-incompatibility hamper the development of inbred lines in both tetraploid and diploid potatoes, impeding hybrid development efforts. Diploid potatoes have gametophytic self-incompatibility (SI) controlled by S-locus, harboring the male-dependent S-locus F-box (SLF/SFB) and female-dependent Stylar-RNase (S-RNase). Manipulation of these genes using biotechnological tools may lead to loss of self-incompatibility. Self-compatibility can also be achieved by the introgression of S-locus inhibitor (Sli) found in the self-compatible (SC) natural mutants of Solanum chacoense. The introgression of Sli through conventional breeding methods has gained much success. Recently, the Sli gene has been cloned from diverse SC diploid potato lines. It is expressed gametophytically and can overcome the SI in different diploid potato genotypes through conventional breeding or transgenic approaches. Interestingly, it has a 533 bp insertion in its promoter elements, a MITE transposon, making it a SC allele. Sli gene encodes an F-box protein PP2-B10, which consists of an F-box domain linked to a lectin domain. Interaction studies have revealed that the C-terminal region of Sli interacts with most of the StS-RNases, except StS-RNase 3, 9, 10, and 13, while full-length Sli cannot interact with StS-RNase 3, 9, 11, 13, and 14. Thus, Sli may play an essential role in mediating the interactions between pollen and stigma and function like SLFs to interact with and detoxify the S-RNases during pollen tube elongation to confer SC to SI lines. These advancements have opened new avenues in the diploid potato hybrid. [ABSTRACT FROM AUTHOR]

There has been an increased interest in true potato seeds (TPS) as planting material because of their advantages over seed tubers. TPS produced from a tetraploid heterozygous bi-parental population produces non-uniform segregating progenies, which have had limited uniformity in yield and quality in commercial cultivation, and, thus, limited success. Inbreeding depression and self-incompatibility hamper the development of inbred lines in both tetraploid and diploid potatoes, impeding hybrid development efforts. Diploid potatoes have gametophytic self-incompatibility (SI) controlled by S-locus, harboring the male-dependent S-locus F-box (SLF/SFB) and female-dependent Stylar-RNase (S-RNase). Manipulation of these genes using biotechnological tools may lead to loss of self-incompatibility. Self-compatibility can also be achieved by the introgression of S-locus inhibitor (Sli) found in the self-compatible (SC) natural mutants of Solanum chacoense. The introgression of Sli through conventional breeding methods has gained much success. Recently, the Sli gene has been cloned from diverse SC diploid potato lines. It is expressed gametophytically and can overcome the SI in different diploid potato genotypes through conventional breeding or transgenic approaches. Interestingly, it has a 533 bp insertion in its promoter elements, a MITE transposon, making it a SC allele. Sli gene encodes an F-box protein PP2-B10, which consists of an F-box domain linked to a lectin domain. Interaction studies have revealed that the C-terminal region of Sli interacts with most of the StS-RNases, except StS-RNase 3, 9, 10, and 13, while full-length Sli cannot interact with StS-RNase 3, 9, 11, 13, and 14. Thus, Sli may play an essential role in mediating the interactions between pollen and stigma and function like SLFs to interact with and detoxify the S-RNases during pollen tube elongation to confer SC to SI lines. These advancements have opened new avenues in the diploid potato hybrid.

SUMMARY: Self‐incompatibility in Petunia is controlled by the polymorphic S‐locus, which contains S‐RNase encoding the pistil determinant and 16–20 S‐locus F‐box (SLF) genes collectively encoding the pollen determinant. Here we sequenced and assembled approximately 3.1 Mb of the S2‐haplotype of the S‐locus in Petunia inflata using bacterial artificial chromosome clones collectively containing all 17 SLF genes, SLFLike1, and S‐RNase. Two SLF pseudogenes and 28 potential protein‐coding genes were identified, 20 of which were also found at the S‐loci of both the S6a‐haplotype of P. inflata and the SN‐haplotype of self‐compatible Petunia axillaris, but not in the S‐locus remnants of self‐compatible potato (Solanum tuberosum) and tomato (Solanum lycopersicum). Comparative analyses of S‐locus sequences of these three S‐haplotypes revealed potential genetic exchange in the flanking regions of SLF genes, resulting in highly similar flanking regions between different types of SLF and between alleles of the same type of SLF of different S‐haplotypes. The high degree of sequence similarity in the flanking regions could often be explained by the presence of similar long terminal repeat retroelements, which were enriched at the S‐loci of all three S‐haplotypes and in the flanking regions of all S‐locus genes examined. We also found evidence of the association of transposable elements with SLF pseudogenes. Based on the hypothesis that SLF genes were derived by retrotransposition, we identified 10 F‐box genes as putative SLF parent genes. Our results shed light on the importance of non‐coding sequences in the evolution of the S‐locus, and on possible evolutionary mechanisms of generation, proliferation, and deletion of SLF genes. Significance Statement: Sequencing the Petunia S‐locus region of the S2‐haplotype containing 17 S‐locus F‐box (SLF) genes, SLFLike1, and the S‐RNase gene allows comparative analyses of the approximately 3.1 Mb assembled sequence with the S‐locus sequences of other S‐haplotypes of Petunia and S‐locus remnants of several self‐compatible solanaceous species. The results from comparison in both the coding and non‐coding flanking sequences of SLF genes shed light on the genomic organization and evolution of the S‐locus in Petunia. [ABSTRACT FROM AUTHOR]

The Pyrus species exhibit the so-called S-RNase-based gametophytic self-incompatibility system, which is considered to be the most widespread self-incompatibility system among flowering plants. In this study, 57 Iranian pear ( Pyrus communis L.) domestic cultivars and wild genotypes, plus 21 European pear cultivars used as references, were genotyped adopting a PCR-based genotyping assay using consensus and allele-specific primers. The results revealed traces of significant genetic contribution in the Iranian traditional varieties and genotypes from other Pyrus species; the genetic contribution of Japanese pear clearly emerged with the detection of some Pyrus pyrifolia S-RNase alleles. Moreover, our results highlighted the presence of three new S-RNase alleles (named S, S, and S) that were not previously identified in P. communis, possibly introduced in the germplasm of cultivated pear through gene transfer from other cultivated or wild species. [ABSTRACT FROM AUTHOR]

Highlights: [•] We identified the S-genotype of 13 North Western European pear cultivars. [•] All described cultivars differed by their S-genotypes. [•] Several identified S-alleles were rare alleles. [Copyright &y& Elsevier]

The molecular bases of the gametophytic self-incompatibility (GSI) system of species of the subtribe Pyrinae (Rosaceae), such as apple and pear, have been widely studied in the last two decades. The characterization of S-locus genes and of the mechanisms underlying pollen acceptance or rejection have been topics of major interest. Besides the single pistil-side S determinant, the S-RNase, multiple related S-locus F-box genes seem to be involved in the determination of pollen S specificity. Here, we collect and review the state of the art of GSI in the Pyrinae. We emphasize recent genomic data that have contributed to unveiling the S-locus structure of the Pyrinae, and discuss their consistency with the models of self-recognition that have been proposed for Prunus and the Solanaceae. Experimental data suggest that the mechanism controlling pollen–pistil recognition specificity of the Pyrinae might fit well with the collaborative ‘non-self’ recognition system proposed for Petunia (Solanaceae), whereas it presents relevant differences with the mechanism exhibited by the species of the closely related genus Prunus, which uses a single evolutionarily divergent F-box gene as the pollen S determinant. The possible involvement of multiple pollen S genes in the GSI system of Pyrinae, still awaiting experimental confirmation, opens up new perspectives to our understanding of the evolution of S haplotypes, and of the evolution of S-RNase-based GSI within the Rosaceae family. Whereas S-locus genes encode the players determining self-recognition, pollen rejection in the Pyrinae seems to involve a complex cascade of downstream cellular events with significant similarities to programmed cell death. [ABSTRACT FROM AUTHOR]

European pear, as well as its close relatives Japanese pear and apple, exhibits S-RNase-based gametophytic self-incompatibility. The male determinant of this self-incompatibility mechanism is a pollen-expressed protein containing an F-box domain; in the genera Petunia (Solanaceae), Antirrhinum (Plantaginaceae), and Prunus (Rosaceae), a single F-box gene determines the pollen S. In apple and Japanese pear, however, multiple S-locus F-box genes were recently identified as candidates for the pollen S, and they were named S-locus F-Box Brothers. These genes were considered good candidates for the pollen S determinant since they exhibit S-haplotype-specific polymorphisms, pollen-specific expression, and linkage to the S-RNase. In the present study, S-locus F-Box Brothers homologs have been cloned from two of the most agronomically important European pear varieties, 'Abbé Fétel' (S/S) and 'Max Red Bartlett' (S/S), and they have been mapped on a genetic linkage map developed on their progeny. Our results suggest that the number of F-box genes linked to the S-locus of the European pear is higher than expected according with previous reports for apple and Japanese pear, since up to five genes were found to be linked to a single S-haplotype. Moreover, two of these genes exhibited an incomplete linkage to the S-RNase, allowing the identification of low-frequency recombinant haplotypes, generated by a crossing-over event between the two genes. These F-box genes are most likely placed in close proximity of the S-locus but do not belong to it, and they can thus be excluded from being responsible for the determination of pollen S function. [ABSTRACT FROM AUTHOR]

Most fruit trees in the Rosaceae exhibit self-incompatibility, which is controlled by the pistil S gene, encoding a ribonuclease (S-RNase), and the pollen S gene at the S-locus. The pollen S in Prunus is an F-box protein gene (SLF/SFB) located near the S-RNase, but it has not been identified in Pyrus and Malus. In the Japanese pear, various F-box protein genes (PpSFBB-α–γ) linked to the S-RNase are proposed as the pollen S candidate. Two bacterial artificial chromosome (BAC) contigs around the S-RNase genes of Japanese pear were constructed, and 649 kb around S4-RNase and 378 kb around S2-RNase were sequenced. Six and 10 pollen-specific F-box protein genes (designated as PpSFBB4-u1–u4, 4-d1–d2 and PpSFBB2-u1–u5, 2-d1–d5, respectively) were found, but PpSFBB4-α–γ and PpSFBB2-γ were absent. The PpSFBB4 genes showed 66.2–93.1% amino acid identity with the PpSFBB2 genes, which indicated clustering of related polymorphic F-box protein genes between haplotypes near the S-RNase of the Japanese pear. Phylogenetic analysis classified 36 F-box protein genes of Pyrus and Malus into two major groups (I and II), and also generated gene pairs of PpSFBB genes and PpSFBB/Malus F-box protein genes. Group I consisted of gene pairs with 76.3–94.9% identity, while group II consisted of gene pairs with higher identities (>92%) than group I. This grouping suggests that less polymorphic PpSFBB genes in group II are non-S pollen genes and that the pollen S candidates are included in the group I PpSFBB genes. [ABSTRACT FROM PUBLISHER]

Extensive survey of the S-locus diversity of plant species with RNase-based gametophytic self-incompatibility has failed to identify neutral variation segregating within S-allele specificities. Although this is the expected result according to population genetics theory, it conflicts with recent models of S-allele evolution, which suggest that new specificities might arise by a continuous process of subtle changes that individually do not alter the specificity of the S-genes, but whose cumulative effects result in new S-allele functions. Genomic analysis of S-RNase sequences associated with the S 104 (= S 4, = S b) allele of European pear ( Pyrus communis L.) cultivars yielded two distinct variants (named herein S 104-1 and S 104-2) that differed at five nucleotide positions within the open reading frame, two of which resulted in changes in the predicted protein sequence. Test-cross experiments indicated that the S-alleles associated with the S 104-1 and S 104-2 RNases exhibit the same pollen and pistil functions, suggesting that they are two neutral variants segregating within the S 104 haplotype of European pear. These allelic forms might represent transitional states in the process of generating new specificities in the species, in accordance with models that predict S-function transition through neutral intermediates. This possibility was further evaluated through the pattern of molecular evolution of functionally distinct European pear S-RNases, which indicated that most recent S-allele diversification in this species proceeded in the absence of adaptive selective pressure. [ABSTRACT FROM AUTHOR]

Low sequence divergence within functional alleles is predicted for the self-incompatibility locus because of strong negative frequency-dependent selection. Nevertheless, sequence variation within functional alleles is essential for current models of the evolution of new mating types. We genotyped the stylar self-incompatibility RNase of 20 Sorbus aucuparia from a population in the Pyrenees mountains of France in order to compare alleles found there to those previously sampled in a Belgian population. Both populations returned 20 different alleles from samples of 20 individuals, providing maximum-likelihood estimates of 24.4 (95% CI 20–34) alleles in each. Ten alleles occurred in both samples. The maximum likelihood (ML) estimate of the overlap in the alleles present in both populations was 16, meaning that an estimated eight alleles are private to each population, and a total of 32 alleles occur across the two populations examined. We used Fisher's (1961) missing plot method to estimate that 40 alleles occur in the species. In accord with population genetics theory, we observed at most one synonymous sequence difference between copies of alleles sampled from the different populations and no variation within populations. Phylogenetic analysis shows that nearly every allele in S. aucuparia arose prior to divergence of this species from members of three different genera of the Rosaceae subfamily, Maloideae. Lack of observable sequence variation within alleles, coupled with the slow pace of allelic relative to taxonomic diversification, implies that finding intermediate stages in the process of new allele creation will be difficult in this group. [ABSTRACT FROM AUTHOR]

Biochemical interactions between the pollen and the pistil allow plants fine control over fertilization. S‐RNase‐based pollen rejection is among the most widespread and best understood of these interactions. At least three plant families have S‐RNase‐based self‐incompatibility (SI) systems, and S‐RNases have also been implicated in interspecific pollen rejection. Although S‐RNases determine the specificity of SI, other genes are required for the pollen rejection system to function. Progress is being made toward identifying these non‐S‐RNase factors. HT‐protein, first identified as a non‐S‐RNase factor that was required for SI in Nicotiana alata, has now been implicated in other species as well. In addition, several pistil proteins bind to S‐RNase in vitro. One hypothesis is that S‐RNase forms a complex with these proteins in vivo that is the active form of S‐RNase in pollen rejection. [ABSTRACT FROM PUBLISHER]

The Solanaceae, Rosaceae, and Scrophulariaceae families all possess an RNase‐mediated self‐incompatibility mechanism through which their pistils can recognize and reject self‐pollen to prevent inbreeding. The highly polymorphic S‐locus controls the self‐incompatibility interaction, and the S‐locus of the Solanaceae has been shown to be a multi‐gene complex in excess of 1.3 Mb. To date, the function of only one of the S‐locus genes, the S‐RNase gene, has been determined. This article reviews the current status of the search for the pollen S‐gene and the current models for how S‐haplotype specific inhibition of pollen tubes can be accomplished by S‐RNases. [ABSTRACT FROM PUBLISHER]