Unilateral Incompatibility Research Articles

A pollen selection system links self and interspecific incompatibility in the Brassicaceae.

Self-incompatibility and recurrent transitions to self-compatibility have shaped the extant mating systems underlying the nonrandom mating critical for speciation in angiosperms. Linkage between self-incompatibility and speciation is illustrated by the shared pollen rejection pathway between self-incompatibility and interspecific unilateral incompatibility (UI) in the Brassicaceae. However, the pollen discrimination system that activates this shared pathway for heterospecific pollen rejection remains unknown. Here we show that Stigma UI3.1, the genetically identified stigma determinant of UI in Arabidopsis lyrata × Arabidopsis arenosa crosses, encodes the S-locus-related glycoprotein 1 (SLR1). Heterologous expression of A. lyrata or Capsella grandiflora SLR1 confers on some Arabidopsis thaliana accessions the ability to discriminate against heterospecific pollen. Acquisition of this ability also requires a functional S-locus receptor kinase (SRK), whose ligand-induced dimerization activates the self-pollen rejection pathway in the stigma. SLR1 interacts with SRK and interferes with SRK homomer formation. We propose a pollen discrimination system based on competition between basal or ligand-induced SLR1-SRK and SRK-SRK complex formation. The resulting SRK homomer levels would be sensed by the common pollen rejection pathway, allowing discrimination among conspecific self- and cross-pollen as well as heterospecific pollen. Our results establish a mechanistic link at the pollen recognition phase between self-incompatibility and interspecific incompatibility.

Intra- and inter-specific reproductive barriers in the tomato clade.

Tomato (Solanum lycopersicum L.) domestication and later introduction into Europe resulted in a genetic bottleneck that reduced genetic variation. Crosses with other wild tomato species from the Lycopersicon clade can be used to increase genetic diversity and improve important agronomic traits such as stress tolerance. However, many species in the Lycopersicon clade have intraspecific and interspecific incompatibility, such as gametophytic self-incompatibility and unilateral incompatibility. In this review, we provide an overview of the known incompatibility barriers in Lycopersicon. We begin by addressing the general mechanisms self-incompatibility, as well as more specific mechanisms in the Rosaceae, Papaveraceae, and Solanaceae. Incompatibility in the Lycopersicon clade is discussed, including loss of self-incompatibility, species exhibiting only self-incompatibility and species presenting both self-compatibility and self-incompatibility. We summarize unilateral incompatibility in general and specifically in Lycopersicon, with details on the 'self-compatible x self-incompatible' rule, implications of self-incompatibility in unilateral incompatibility and self-incompatibility-independent pathways of unilateral incompatibility. Finally, we discuss advances in the understanding of compatibility barriers and their implications for tomato breeding.

Genetic relatedness among interspecific hybrids in the genus Capsicum and their implications in breeding King chilli

The present study explores the potentiality of interspecific crosses using three different Capsicum species, comprising of two C. annuum (Krishna and Capsicum), five C. chinense (King chilli A1, A5, A18, A19, A20) and one C. frutescens (Mem) genotypes, with the objective to identify the incompatibility barriers, accomplish the morpho-molecular characterization of parents and hybrids and assess the genetic relatedness between the genotypes.Amongst the three species used for the crossing programme, 'Krishna' (C. annuum) and 'Mem' (C. frutescens) are local cultivars of Assam (India) and possess excellent adaptability. Guinness World Records certified King chilli, or 'Bhut Jolokia' (C. chinense), as the world's hottest chilli (10,001,304 SHU) in September 2007. King chilli has tremendous export potential but is prone to viral diseases, thereby necessitating the transfer of desirable disease resistance from local cultivars.It was observed from the study that the reciprocal crosses involving C. chinense and C. frutescens were compatible, but C. frutescens (Mem) was crossable with only accession A5, suggesting genotype specificity in the interspecific crosses. The usual Euclidean distance between Mem and A5 was 716.605Similarly reciprocal crosses involving C. annuum and C. chinense were partially incompatible. Both the C. annuum (Krishna, Capsicum) genotypes successfully crossed with King chilli accession A18 thereby exhibiting genotype specificity. It was observed that in the cross A18 x Krishna the F1 seeds failed to germinate and in the cross A18 x Capsicum the F1 plants exhibited dwarfism. Hence, unilateral incompatibility was observed when King chilli accession A18 was used as the female parent. In the reciprocal crosses, germination and plant development was normal.The pollen viability ranged from 72.64 to 94.92 % for parents and 23.55 to 81.02 % for the hybrids. Thus, hybrids exhibited lower pollen viability compared to their parents. Significant morphological differences were visible between the parents and their combinations; some features were similar to the male or female parent, and others were either intermediate or novel.Parental polymorphism was detected using thirty random SSR primers, and the markers showing polymorphic bands were used to confirm the hybridity of F1s. Therefore, an attempt was made in this study to assess the possibility of interspecific crosses to transfer disease resistance from the related species to enhance the quality of King chilli, thus improving its commercial valve.

Inheritance of Unilateral Incompatibility in the Genus Capsicum

Unilateral incompatibility is reported to occur between the species of Capsicum pubescens complex (Capsicum cardenasii Capsicum. eximium and Capsicum pubescens) and other species of genus Capsicum when the latter are used as the male parent. The relationship between self-incompatibility and unilateral incompatibility are yet to be resolved even in the most intensively investigated crosses within and between species of Lycopersicon and Solanum. Unilateral incompatibility in Capsicum has been less intensively studied compared to some other genus in family Solanaceae. This study was, therefore, conducted to help us to understand the genetic control of unilateral incompatibility in Capsicum. Experiment results showed that while the pistil behaviour of F1 hybrids agrees with the pistil behaviour of F1 hybrids from unilaterally incompatible crosses in other genera of Solanaceae, the pollen behaviour does not agree with data obtained from other genera of Solanaceae. No segregation ratio was obtained for unilateral incompatibility in the backcross progenies, this result does not agree with data from other genera in the Solanaceae either. Possible reasons for not obtaining segregation for unilateral incompatibility may be the small size of backcross progenies and/or distorted segregation ratios. As a conclusion it seemed more probable that, in Capsicum, unilateral incompatibility has arisen as a by-product of genetic divergence between the C. pubescens complex and the other chile peppers, not as a product of natural selection.

Interspecific Crossing between Lilium hansonii Leichtlin and L. brownii var. colchesteri for the Breeding of New Lily Cultivars

This study aimed to generate interspecific hybrids between two remote lily species, L. hansonii Leichtlin and L. brownii var. colchesteri. Reciprocal crosses were performed by conventional and cut-style pollination methods, but viable seeds were only obtained when L. hansonii was used as the female parent, indicating that unilateral incompatibility exists between the two species. In the case of immature seeds, embryos with 2~3 mm were carefully removed from testa for further in vitro culture, and they grew as normal plants. A total of 343 progenies was obtained from the crosses, and hybridity of the progenies were examined using the L9 marker, simple sequence repeat (SSR) marker, at the seedling stage and 92 were confirmed as F1 hybrids. Ploidy level of 76 F1 hybrid was examined and confirmed as diploid. F1 hybrids exhibited intermediate morphologies of the parent in outer tepal and leaf length, but flower shape and color were similar to those of L. hansonii. On the other hand, F1 hybrid plants showed increased flower spots, flower size, and bud numbers, which could be important signatures of the F1 hybrid. This study reports the first attempt to generate an interspecific hybrid between the two species, and therefore, our results from this study would be very informative for future lily breeding.

A Cysteine-Rich Protein, SpDIR1L, Implicated in S-RNase-Independent Pollen Rejection in the Tomato (Solanum Section Lycopersicon) Clade.

Tomato clade species (Solanum sect. Lycopersicon) display multiple interspecific reproductive barriers (IRBs). Some IRBs conform to the SI x SC rule, which describes unilateral incompatibility (UI) where pollen from SC species is rejected on SI species’ pistils, but reciprocal pollinations are successful. However, SC x SC UI also exists, offering opportunities to identify factors that contribute to S-RNase-independent IRBs. For instance, SC Solanum pennellii LA0716 pistils only permit SC Solanum lycopersicum pollen tubes to penetrate to the top third of the pistil, while S. pennellii pollen penetrates to S. lycopersicum ovaries. We identified candidate S. pennellii LA0716 pistil barrier genes based on expression profiles and published results. CRISPR/Cas9 mutants were created in eight candidate genes, and mutants were assessed for changes in S. lycopersicum pollen tube growth. Mutants in a gene designated Defective in Induced Resistance 1-like (SpDIR1L), which encodes a small cysteine-rich protein, permitted S. lycopersicum pollen tubes to grow to the bottom third of the style. We show that SpDIR1L protein accumulation correlates with IRB strength and that species with weak or no IRBs toward S. lycopersicum pollen share a 150 bp deletion in the upstream region of SpDIR1L. These results suggest that SpDIR1L contributes to an S-RNase-independent IRB.

Genetic Diversity of Genes Controlling Unilateral Incompatibility in Japanese Cultivars of Chinese Cabbage.

In recent years, unilateral incompatibility (UI), which is an incompatibility system for recognizing and rejecting foreign pollen that operates in one direction, has been shown to be closely related to self-incompatibility (SI) in Brassica rapa. The stigma- and pollen-side recognition factors (SUI1 and PUI1, respectively) of this UI are similar to those of SI (stigma-side SRK and pollen-side SP11), indicating that SUI1 and PUI1 interact with each other and cause pollen-pistil incompatibility only when a specific genotype is pollinated. To clarify the genetic diversity of SUI1 and PUI1 in Japanese B. rapa, here we investigated the UI phenotype and the SUI1/PUI1 sequences in Japanese commercial varieties of Chinese cabbage. The present study showed that multiple copies of nonfunctional PUI1 were located within and in the vicinity of the UI locus region, and that the functional SUI1 was highly conserved in Chinese cabbage. In addition, we found a novel nonfunctional SUI1 allele with a dominant negative effect on the functional SUI1 allele in the heterozygote.

Identification and chromosome doubling of Fragaria mandshurica and F. nilgerrensis

Genetic factors that control various excellent agronomic traits are present in wild strawberries; however, barriers such as ploidy differences and interspecific incompatibility prevent the transmission of these genetic factors to cultivated strawberries. Using the colchicine method to induce doubling, the autotetraploids of Fragaria mandshurica and F. nilgerrensis were obtained, and the biological characteristics of the autotetraploids and the compatibility status distribution in different cross-pollination combinations relating to the two species were further examined. The autotetraploids showed thicker and stronger morphological characteristics than those of their corresponding diploids, especially the F. mandshurica species. The change in ploidy can cause the appearance of new excellent agronomic traits: the autotetraploid of F. nilgerrensis is more resistant to anthracnose disease, caused by Colletotrichum gloeosporioides, than the diploid, and the flowering period is significantly longer, with an increase in flower quantity. There is unilateral incompatibility between the two self-compatible species. The autotetraploids are similar to the corresponding diploid in pollination compatibility; however, due to the decline in pollen viability, the related hybridisation combinations show slightly lower compatibility. The closer the ploidy is, the easier it is to obtain hybrid offspring; the tetraploids of both species crossed with cultivated strawberries, showing possible increases in compatibility after chromosome doubling.

Molecular characteristics of S-RNase alleles as the determinant of self-incompatibility in the style of Fragaria viridis

Strawberry (Fragaria spp.) is a member of the Rosoideae subfamily in the family Rosaceae. The self-incompatibility (SI) of some diploid species is a key agronomic trait that acts as a basic pollination barrier; however, the genetic mechanism underlying SI control in strawberry remains unclear. Two candidate S-RNases (Sa- and Sb-RNase) identified in the transcriptome of the styles of the self-incompatible Fragaria viridis 42 were confirmed to be SI determinants at the S locus following genotype identification and intraspecific hybridization using selfing progenies. Whole-genome collinearity and RNase T2 family analysis revealed that only an S locus exists in Fragaria; however, none of the compatible species contained S-RNase. Although the results of interspecific hybridization experiments showed that F. viridis (SI) styles could accept pollen from F. mandshurica (self-compatible), the reciprocal cross was incompatible. Sa and Sb-RNase contain large introns, and their noncoding sequences (promotors and introns) can be transcribed into long noncoding RNAs (lncRNAs). Overall, the genus Fragaria exhibits S-RNase-based gametophytic SI, and S-RNase loss occurs at the S locus of compatible germplasms. In addition, a type of SI-independent unilateral incompatibility exists between compatible and incompatible Fragaria species. Furthermore, the large introns and neighboring lncRNAs in S-RNase in Fragaria could offer clues about S-RNase expression strategies.

Ornithine decarboxylase genes contribute to S-RNase-independent pollen rejection.

Unilateral incompatibility (UI) manifests as pollen rejection in the pistil, typically when self-incompatible (SI) species are pollinated by self-compatible (SC) relatives. In the Solanaceae, UI occurs when pollen lack resistance to stylar S-RNases, but other, S-RNase-independent mechanisms exist. Pistils of the wild tomato Solanum pennellii LA0716 (SC) lack S-RNase yet reject cultivated tomato (Solanum lycopersicum, SC) pollen. In this cross, UI results from low pollen expression of a farnesyl pyrophosphate synthase gene (FPS2) in S. lycopersicum. Using pollen from fps2-/- loss-of-function mutants in S. pennellii, we identified a pistil factor locus, ui3.1, required for FPS2-based pollen rejection. We mapped ui3.1 to an interval containing 108 genes situated on the IL 3-3 introgression. This region includes a cluster of ornithine decarboxylase (ODC2) genes, with four copies in S. pennellii, versus one in S. lycopersicum. Expression of ODC2 transcript was 1,034-fold higher in S. pennellii than in S. lycopersicum styles. Pistils of odc2-/- knockout mutants in IL 3-3 or S. pennellii fail to reject fps2 pollen and abolish transmission ratio distortion (TRD) associated with FPS2. Pollen of S. lycopersicum express low levels of FPS2 and are compatible on IL 3-3 pistils, but incompatible on IL 12-3 × IL 3-3 hybrids, which express both ODC2 and ui12.1, a locus thought to encode the SI proteins HT-A and HT-B. TRD observed in F2 IL 12-3 × IL 3-3 points to additional ODC2-interacting pollen factors on both chromosomes. Thus, ODC2 genes contribute to S-RNase independent UI and interact genetically with ui12.1 to strengthen pollen rejection.

Restoring Fertility for Novel Interspecific Hybrids between Kalanchoe garambiensis and K. nyikae Using Colchicine Treatment.

Interspecific hybridization is an effective strategy in Kalanchoe breeding programs for the introduction of new traits. Wild species within the Kalanchoe genus are valuable genetic resources for providing new horticulture traits and to improve environmental adaptations. However, reproductive barriers associated with fertilization and hybrid sterility must be overcome to produce fertile hybrid progenies. To approach the breeding objectives for Kalanchoe cut flower cultivars with long stem traits and adaptation to tropical/subtropical regions, a tropical species endemic to Taiwan, Kalanchoe garambiensis Kudo, was used as a parent to cross with other long stem Kalanchoe species. Reciprocal crossing was effective in overcoming interspecific unilateral incompatibility in our crossed pairs. One superior hybrid, ‘103-1’, produced capsules without seeds by selfing and backcrossing with pollens from either parent. Other than the seedless trait, failure of pollen releasing from anther, pollen aggregation and no pollen germination in ‘103-1’ suggested its F1 sterility. Colchicine treatments on apical buds of ‘103-1’ successfully overcame potential meiotic abnormalities by doubling ploidy. For the first time, fertile interspecific hybrids of K. garambiensis and K. nyikae Engler were generated. The fertile hybrid has further produced progeny populations by crossing with K. nyikae or K. blossfeldiana von Poelln, ‘Ida’.

Intraspecific Genetic Variation Underlying Postmating Reproductive Barriers between Species in the Wild Tomato Clade (Solanum sect. Lycopersicon)

A goal of speciation genetics is to understand how the genetic components underlying interspecific reproductive barriers originate within species. Unilateral incompatibility (UI) is a postmating prezygotic barrier in which pollen rejection in the female reproductive tract (style) occurs in only one direction of an interspecific cross. Natural variation in the strength of UI has been observed among populations within species in the wild tomato clade. In some cases, molecular loci underlying self-incompatibility (SI) are associated with this variation in UI, but the mechanistic connection between these intra- and inter-specific pollen rejection behaviors is poorly understood in most instances. We generated an F2 population between SI and SC genotypes of a single species, Solanum pennellii, to examine the genetic basis of intraspecific variation in UI against other species, and to determine whether loci underlying SI are genetically associated with this variation. We found that F2 individuals vary in the rate at which UI rejection occurs. One large effect QTL detected for this trait co-localized with the SI-determining S-locus. Moreover, individuals that expressed S-RNase-the S-locus protein involved in SI pollen rejection-in their styles had much more rapid UI responses compared with those without S-RNase protein. Our analysis shows that intraspecific variation at mate choice loci-in this case at loci that prevent self-fertilization-can contribute to variation in the expression of interspecific isolation, including postmating prezygotic barriers. Understanding the nature of such intraspecific variation can provide insight into the accumulation of these barriers between diverging lineages.

Inheritance of breeding system in Cakile (Brassicaceae) following hybridization: implications for plant invasions.

Hybridization is commonly assumed to aid invasions through adaptive introgression. In contrast, a recent theoretical model predicted that there can be non-adaptive demographic advantages from hybridization and that the population consequences will depend on the breeding systems of the species and the extent to which subsequent generations are able to interbreed and reproduce. We examined cross-fertilization success and inheritance of breeding systems of two species in order to better assess the plausibility of the theoretical predictions. Reciprocal artificial crosses were made to produce F1, F2 and backcrosses between Cakile maritima (self-incompatible, SI) and Cakile edentula (self-compatible, SC) (Brassicaceae). Flowers were emasculated prior to anther dehiscence and pollen was introduced from donor plants to the recipient's stigma. Breeding system, pollen viability, pollen germination, pollen tube growth and reproductive output were then determined. The results were used to replace the assumptions made in the original population model and new simulations were made. The success rate with the SI species as the pollen recipient was lower than when it was the pollen donor, in quantitative agreement with the 'SI × SC rule' of unilateral incompatibility. Similar outcomes were found in subsequent generations where fertile hybrids were produced but lower success rates were observed in crosses of SI pollen donors with SC pollen recipients. Much lower proportions of SC hybrids were produced than expected from a single Mendelian allele. When incorporated into a population model, these results predicted an even faster rate of replacement of the SC species by the SI species than previously reported. Our study of these two species provides even clearer support for the feasibility of the non-adaptive hybridization hypothesis, whereby the colonization of an SI species can be assisted by transient hybridization with a congener. It also provides novel insight into reproductive biology beyond the F1 generation.

Reproductive systems and low outbreeding barriers between Jacaranda cuspidifolia and J. mimosifolia (Jacarandeae, Bignoniaceae)

The genus Jacaranda shows notable karyotype stability and a prevailing self‐sterile breeding system with evidence of late‐acting self‐incompatibility in several species. However, some studies have indicated self‐compatibility in J. mimosifolia, a species cultivated worldwide in tropical and subtropical areas. Jacaranda cuspidifolia is a closely related species with natural distribution broadly overlapping with that of J. mimosifolia, and manual heterospecific pollination studies have indicated that these species are interfertile, but there is no report on the breeding system of the former. In this study, we used hand‐pollination experiments, pistil longevity, epi‐fluorescence and histological analysis of post‐pollination events to determine the breeding system of J. cuspidifolia. We also employed intra and interspecific crosses and seed germination tests to reevaluate the breeding system of J. mimosifolia and the inter‐fertility between the two species. Some fruits were initiated from self‐pollinated pistils in J. mimosifolia, but none of them reached maturity. On the other hand, complete absence of fruit development by self‐pollination was verified in J. cuspidifolia, while in situ pollen tube growth and histological analysis of post‐pollination events in selfed pistils revealed the characteristic ovule penetration, fertilization and endosperm initiation also observed in other bignoniaceous species with late‐acting self‐incompatibility. Low outbreeding barriers seem to operate between these species because reciprocal interspecific crosses and hybrid seed germination tests indicated they are bilaterally interfertile. However, fruit/seed production and seed germinability were significantly lower when pistils of J. cuspidifolia were pollinated with pollen of J. mimosifolia, compared with crosses in the opposite direction, which indicates a partial unilateral incompatibility. This result is discussed in the context of the possible occurrence of self‐compatibility in J. mimosifolia. The low level of incongruity operating between the two species also points to their recent evolutionary divergence.

Introgression lines of Solanum sitiens, a wild nightshade of the Atacama Desert, in the genome of cultivated tomato.

The wild tomato relative Solanum sitiens is a xerophyte endemic to the Atacama Desert of Chile and a potential source of genes for tolerance to drought, salinity and low-temperature stresses. However, until recently, strong breeding barriers prevented its hybridization and introgression with cultivated tomato, Solanum lycopersicum L. We overcame these barriers using embryo rescue, bridging lines and allopolyploid hybrids, and synthesized a library of introgression lines (ILs) that captures the genome of S.sitiens in the background of cultivated tomato. The IL library consists of 56 overlapping introgressions that together represent about 93% of the S.sitiens genome: 65% in homozygous and 28% in heterozygous (segregating) ILs. The breakpoints of each segment and the gaps in genome coverage were mapped by single nucleotide polymorphism (SNP) genotyping using the SolCAP SNP array. Marker-assisted selection was used to backcross selected introgressions into tomato, to recover a uniform genetic background, to isolate recombinant sub-lines with shorter introgressions and to select homozygous genotypes. Each IL contains a single S.sitiens chromosome segment, defined by markers, in the genetic background of cv. NC 84173, a fresh market inbred line. Large differences were observed between the lines for both qualitative and quantitative morphological traits, suggesting that the ILs contain highly divergent allelic variation. Several loci contributing to unilateral incompatibility or hybrid necrosis were mapped with the lines. This IL population will facilitate studies of the S.sitiens genome and expands the range of genetic variation available for tomato breeding and research.

Production of male fertile interspecific hybrides between cultivated potatoes and valuable for breeding allotetraploid species Solanum stoloniferum

The germplasm of valuable for breeding wild allotetraploid potato species Solanum stoloniferumis rarely used because of pre- and postzygotic reproductive barriers with cultivated potatoes. One of the factors that complicate crosses between S. stoloniferum and S. tuberosumis unilateral incompatibility (UI).Here, we present the results of application of original SvSv-lines for overcoming UI in crosses with S. stoloniferum and of generating male fertile hybrids derived from this species. SvSv-lines are F2 S. tuberosum dihaploid× S. verrucosum that are male fertile and have D/γ-type cytoplasm. Since they are hybrids on homozygous for Svgene from S. verrucosum, they do not form SvSv-lines and have the same ability for elimination of prezygotic incompatibility as this species.As a result of pollination seven SvSv-lines were pollinated by 26 accessions of S. stoloniferum and a lot of hybrid seeds have been produced.In spite of low percentage of germination (1.9 %), formed 40 seedlings of interspecific hybrids. The experiment on hybridization between SvSv-lines and S. stoloniferum has been reproduced with the accession PI205522 of the wild species, which had DNA markers of PVY and LB resistance genes and “sterile” type cytoplasm W/γ: 950 hybrid seeds and 12 viable seedlings were produced. The genome of the seedlings was doubled by colchicine treatment, which generated hexaploids (F1) that formed highly fertile pollen and set seeds from self-pollination. We were able to cross them as females with the variety Katahdin. Produced pentaploid hybrids (BC1) were readily backcrossed by potato variety Quarta. Seedlings of BC2 were then backcrossed by potato varieties as female and, some of them, as male parents. The substantial part of F1, BC1 and BC2 plants of interspecific hybrids were male fertile (produced a lot functionally fertile pollen).

The Molecular and Cellular Regulation of Brassicaceae Self-Incompatibility and Self-Pollen Rejection.

In flowering plants, sexual reproduction is actively regulated by cell-cell communication between the male pollen and female pistil, and many species possess self-incompatibilitysystems for the selective rejection of self-pollen to maintain genetic diversity. The Brassicaceae self-incompatibility pathway acts early on when pollen grains have landed on the stigmatic papillae at the top of the pistil. Extensive studies have revealed that self-pollen rejection in the Brassicaceae is initiated by an S-haplotype-specific interaction between two polymorphic proteins: the pollen S-locus protein 11/S cysteine-rich (SP11/SCR) ligand and the stigma S receptor kinase (SRK). While the different S-haplotypes are typically codominant, there are several examples of dominant-recessive interactions, and a small RNA-based regulation of SP11/SCR expression has been uncovered as a mechanism behind these genetic interactions. Recent research has also added to our understanding of various cellular components in the pathway leading from the SP11/SCR-SRK interaction, including two signaling proteins, the M-locus protein kinase (MLPK) and the ARM-repeat containing 1 (ARC1) E3 ligase, as well as calcium fluxes and induction of autophagy in the stigmatic papillae. Finally, a better understanding of the compatible pollen responses that are targeted by the self-incompatibility pathway is starting to emerge, and this will allow us to more fully understand how the Brassicaceae self-incompatibility pathway causes self-pollen rejection. Here, we provide an overview of the field, highlighting recent contributions to our understanding of Brassicaceae self-incompatibility, and draw comparisons to a recently discovered unilateral incompatibility system.

Evolution of interspecies unilateral incompatibility in the relatives of Arabidopsis thaliana.

The evolutionary concurrence of intraspecies self-incompatibility (SI) and explosive angiosperm radiation in the Cretaceous have led to the hypothesis that SI was one of the predominant drivers of rapid speciation in angiosperms. Interspecies unilateral incompatibility (UI) usually occurs when pollen from a self-compatible (SC) species is rejected by the pistils of a SI species, while the reciprocal pollination is compatible (UC). Although this SI×SC type UI is most prevalent and viewed as a prezygotic isolation barrier to promote incipient speciation of angiosperms, comparative evidence to support such a role is lacking. We show that SI×SI type UI in SI species pairs is also common in the well-characterized accessions representing the four major lineages of the Arabidopsis genus and is developmentally regulated. This allowed us to reveal a strong correlation between UI strength and species divergence in these representative accessions. In addition, analyses of a SC accession and the pseudo-self-compatible (psc) spontaneous mutant of Arabidopsis lyrata indicate that UI shares, at least, common pollen rejection pathway with SI. Furthermore, genetic and genomic analyses of SI×SI type UI in A.lyrata×A.arenosa species pair showed that two major-effect quantitative trait loci are the stigma and pollen-side determinant of UI, respectively, which could be involved in heterospecies pollen discrimination. By revealing a close link between UI and SI pathway, particularly between UI and species divergence in these representative accessions, our findings establish a connection between SI and speciation. Thus, the pre-existence of SI system would have facilitated the evolution of UI and accordingly promote speciation.

Unilateral incompatibility is linked to reduced pollen expression of a farnesyl pyrophosphate synthase.

Unilateral incompatibility is linked to reduced pollen expression of a farnesyl pyrophosphate synthase.

A farnesyl pyrophosphate synthase gene expressed in pollen functions in S-RNase-independent unilateral incompatibility.

Multiple independent and overlapping pollen rejection pathways contribute to unilateral interspecific incompatibility (UI). In crosses between tomato species, pollen rejection usually occurs when the female parent is self-incompatible (SI) and the male parent self-compatible (SC) (the 'SI×SC rule'). Additional, as yet unknown, UI mechanisms are independent of self-incompatibility and contribute to UI between SC species or populations. We identified a major quantitative trait locus on chromosome 10 (ui10.1) which affects pollen-side UI responses in crosses between cultivated tomato, Solanum lycopersicum, and Solanum pennelliiLA0716, both of which are SC and lack S-RNase, the pistil determinant of S-specificity in Solanaceae. Here we show that ui10.1 is a farnesyl pyrophosphate synthase gene (FPS2) expressed in pollen. Expression is about 18-fold higher in pollen of S. pennellii than in S. lycopersicum. Pollen with the hypomorphic S. lycopersicum allele is selectively eliminated on pistils of the F1 hybrid, leading to transmission ratio distortion in the F2 progeny. CRISPR/Cas9-generated knockout mutants (fps2) in S.pennelliiLA0716 are self-sterile due to pollen rejection, but mutant pollen is fully functional on pistils of S. lycopersicum. F2 progeny of S. lycopersicum×S. pennellii (fps2) show reversed transmission ratio distortion due to selective elimination of pollen bearing the knockout allele. Overexpression of FPS2 in S.lycopersicum pollen rescues the pollen elimination phenotype. FPS2-based pollen selectivity does not involve S-RNase and has not been previously linked to UI. Our results point to an entirely new mechanism of interspecific pollen rejection in plants.