Darwin's 'mystery of mysteries': the role of sexual selection in plant speciation

Sexual selection is considered one of the key processes that contribute to the emergence of new species. While the connection between sexual selection and speciation has been supported by comparative studies, the mechanisms that mediate this connection remain unresolved, especially in plants. Similarly, it is not clear how speciation processes within plant populations translate into large-scale speciation dynamics. Here, we review the mechanisms through which sexual selection, pollination, and mate choice unfold and interact, and how they may ultimately produce reproductive isolation in plants. We also overview reproductive strategies that might influence sexual selection in plants and illustrate how functional traits might connect speciation at the population level (population differentiation, evolution of reproductive barriers; i.e. microevolution) with evolution above the species level (macroevolution). We also identify outstanding questions in the field, and suitable data and tools for their resolution. Altogether, this effort motivates further research focused on plants, which might potentially broaden our general understanding of speciation by sexual selection, a major concept in evolutionary biology.

Divergent evolutionary trajectories shape the postmating transcriptional profiles of conspecifically and heterospecifically mated cactophilic Drosophila females

Postmating-prezygotic (PMPZ) reproductive isolation is hypothesized to result from divergent coevolutionary trajectories of sexual selection and/or sexual conflict in isolated populations. However, the genetic basis of PMPZ incompatibilities between species is poorly understood. Here, we use a comparative framework to compare global gene expression in con- and heterospecifically mated Drosophila mojavensis and D. arizonae female reproductive tracts. We find striking divergence between the species in the female postmating transcriptional response to conspecific mating, including differences in differential expression (DE), alternative splicing (AS), and intron retention (IR). As predicted, heterospecific matings produce disrupted transcriptional profiles, but the overall patterns of misregulation are different between the reciprocal crosses. Moreover, we find a positive correlation between postmating transcriptional divergence between species and levels of transcriptional disruption in heterospecific crosses. This result indicates that mating responsive genes that have diverged more in expression also have more disrupted transcriptional profiles in heterospecifically mated females. Overall, our results provide insights into the evolution of PMPZ isolation and lay the foundation for future studies aimed at identifying specific genes involved in PMPZ incompatibilities and the evolutionary forces that have contributed to their divergence in closely related species.

Comparison of global gene expression patterns in con- and heterospecifically mated Drosophila mojavensis and Drosophila arizonae suggest that mating-responsive genes with divergent expression also exhibit more disrupted transcriptional profiles in heterospecifically mated females, providing further insight into the evolution of postmating-prezygotic reproductive isolation.

Recurrent Co-Option and Recombination of Cytokine and Three Finger Proteins in Multiple Reproductive Tissues Throughout Salamander Evolution

Reproductive proteins evolve at unparalleled rates, resulting in tremendous diversity of both molecular composition and biochemical function between gametes of different taxonomic clades. To date, the proteomic composition of amphibian gametes is largely a molecular mystery, particularly for Urodeles (salamanders and newts) for which few genomic-scale resources exist. In this study, we provide the first detailed molecular characterization of gametes from two salamander species (Plethodon shermani and Desmognathus ocoee) that are models of reproductive behavior. Long-read PacBio transcriptome sequencing of testis and ovary of both species revealed sex-specific expression of many genes common to vertebrate gametes, including a similar expression profile to the egg coat genes of Xenopus oocytes. In contrast to broad conservation of oocyte genes, major testis transcripts included paralogs of salamander-specific courtship pheromones (PRF, PMF, and SPF) that were confirmed as major sperm proteins by mass spectrometry proteomics. Sperm-specific paralogs of PMF and SPF are likely the most abundant secreted proteins in P. shermani and D. ocoee, respectively. In contrast, sperm PRF lacks a signal peptide and may be expressed in cytoplasm. PRF pheromone genes evolved independently multiple times by repeated gene duplication of sperm PRF genes with signal peptides recovered through recombination with PMF genes. Phylogenetic analysis of courtship pheromones and their sperm paralogs support that each protein family evolved for these two reproductive contexts at distinct evolutionary time points between 17 and 360 million years ago. Our combined phylogenetic, transcriptomic and proteomic analyses of plethodontid reproductive tissues support that the recurrent co-option and recombination of TFPs and cytokine-like proteins have been a novel driving force throughout salamander evolution and reproduction.

S-RNase Alleles Associated With Self-Compatibility in the Tomato Clade: Structure, Origins, and Expression Plasticity

The self-incompatibility (SI) system in the Solanaceae is comprised of cytotoxic pistil S-RNases which are countered by S-locus F-box (SLF) resistance factors found in pollen. Under this barrier-resistance architecture, mating system transitions from SI to self-compatibility (SC) typically result from loss-of-function mutations in genes encoding pistil SI factors such as S-RNase. However, the nature of these mutations is often not well characterized. Here we use a combination of S-RNase sequence analysis, transcript profiling, protein expression and reproductive phenotyping to better understand different mechanisms that result in loss of S-RNase function. Our analysis focuses on 12 S-RNase alleles identified in SC species and populations across the tomato clade. In six cases, the reason for gene dysfunction due to mutations is evident. The six other alleles potentially encode functional S-RNase proteins but are typically transcriptionally silenced. We identified three S-RNase alleles which are transcriptionally silenced under some conditions but actively expressed in others. In one case, expression of the S-RNase is associated with SI. In another case, S-RNase expression does not lead to SI, but instead confers a reproductive barrier against pollen tubes from other tomato species. In the third case, expression of S-RNase does not affect self, interspecific or inter-population reproductive barriers. Our results indicate that S-RNase expression is more dynamic than previously thought, and that changes in expression can impact different reproductive barriers within or between natural populations.

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.

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.

Reproductive Proteins Evolve Faster Than Non-reproductive Proteins Among Solanum Species

Elevated rates of evolution in reproductive proteins are commonly observed in animal species, and are thought to be driven by the action of sexual selection and sexual conflict acting specifically on reproductive traits. Whether similar patterns are broadly observed in other biological groups is equivocal. Here, we examine patterns of protein divergence among wild tomato species (Solanum section Lycopersicon), to understand forces shaping the evolution of reproductive genes in this diverse, rapidly evolving plant clade. By comparing rates of molecular evolution among loci expressed in reproductive and non-reproductive tissues, our aims were to test if: (a) reproductive-specific loci evolve more rapidly, on average, than non-reproductive loci; (b) 'male'-specific loci evolve at different rates than 'female'-specific loci; (c) genes expressed exclusively in gametophytic (haploid) tissue evolve differently from genes expressed in sporophytic (diploid) tissue or in both tissue types; and (d) mating system variation (a potential proxy for the expected strength of sexual selection and/or sexual conflict) affects patterns of protein evolution. We observed elevated evolutionary rates in reproductive proteins. However, this pattern was most evident for female- rather than male-specific loci, both broadly and for individual loci inferred to be positively selected. These elevated rates might be facilitated by greater tissue-specificity of reproductive proteins, as faster rates were also associated with more narrow expression domains. In contrast, we found little evidence that evolutionary rates are consistently different in loci experiencing haploid selection (gametophytic-exclusive loci), or in lineages with quantitatively different mating systems. Overall while reproductive protein evolution is generally elevated in this diverse plant group, some specific patterns of evolution are more complex than those reported in other (largely animal) systems, and include a more prominent role for female-specific loci among adaptively evolving genes.

TCMP-2 affects tomato flowering and interacts with BBX16, a homolog of the arabidopsis B-box MiP1b

Flowering and fruiting are processes subject to complex control by environmental and endogenous signals. Endogenous signals comprise, besides classical phytohormones, also signaling peptides and miniproteins. Tomato cystine-knot miniproteins (TCMPs), which belong to a Solanaceous-specific group of Cys-rich protein family, have been recently involved in fruit development. TCMP-1 and TCMP-2 display a highly modulated expression pattern during flower and fruit development. A previous study reported that a change in the ratio of the two TCMPs affects the timing of fruit production. In this work, to investigate TCMP-2 mode of action, we searched for its interacting partners. One of the interactors identified by a yeast two hybrid screen, was the B-box domain-containing protein 16 (SlBBX16), whose closest homolog is the Arabidopsis microProtein 1b implicated in flowering time control. We demonstrated the possibility for the two proteins to interact in vivo in tobacco epidermal cells. Arabidopsis plants ectopically overexpressing the TCMP-2 exhibited an increased level of FLOWERING LOCUS T (FT) mRNA and anticipated flowering. Similarly, in previously generated transgenic tomato plants with increased TCMP-2 expression in flower buds, we observed an augmented expression of SINGLE-FLOWER TRUSS gene, the tomato ortholog of FT, whereas the expression of the antiflorigen SELF-PRUNING was unchanged. Consistently, these transgenic plants showed alterations in the flowering pattern, with an accelerated termination of the sympodial units. Overall, our study reveals a novel function for TCMP-2 as regulatory factor that might integrate, thanks to its capacity to interact with SlBBX16, into the signaling pathways that control flowering, and converge toward florigen regulation.

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.

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.

Standard Deviations: The Biological Bases of Transmission Ratio Distortion

The rule of Mendelian inheritance is remarkably robust, but deviations from the equal transmission of alternative alleles at a locus [a.k.a. transmission ratio distortion (TRD)] are also commonly observed in genetic mapping populations. Such TRD reveals locus-specific selection acting at some point between the diploid heterozygous parents and progeny genotyping and therefore can provide novel insight into otherwise-hidden genetic and evolutionary processes. Most of the classic selfish genetic elements were discovered through their biasing of transmission, but many unselfish evolutionary and developmental processes can also generate TRD. In this review, we describe methodologies for detecting TRD in mapping populations, detail the arenas and genetic interactions that shape TRD during plant and animal reproduction, and summarize patterns of TRD from across the genetic mapping literature. Finally, we point to new experimental approaches that can accelerate both detection of TRD and characterization of the underlying genetic mechanisms.

Pollination Type Recognition from a Distance by the Ovary Is Revealed Through a Global Transcriptomic Analysis

Sexual reproduction in flowering plants involves intimate contact and continuous interactions between the growing pollen tube and the female reproductive structures. These interactions can trigger responses in distal regions of the flower well ahead of fertilization. While pollination-induced petal senescence has been studied extensively, less is known about how pollination is perceived at a distance in the ovary, and how specific this response is to various pollen genotypes. To address this question, we performed a global transcriptomic analysis in the ovary of a wild potato species, Solanum chacoense, at various time points following compatible, incompatible, and heterospecific pollinations. In all cases, pollen tube penetration in the stigma was initially perceived as a wounding aggression. Then, as the pollen tubes grew in the style, a growing number of genes became specific to each pollen genotype. Functional classification analyses revealed sharp differences in the response to compatible and heterospecific pollinations. For instance, the former induced reactive oxygen species (ROS)-related genes while the latter affected genes associated to ethylene signaling. In contrast, incompatible pollination remained more akin to a wound response. Our analysis reveals that every pollination type produces a specific molecular signature generating diversified and specific responses at a distance in the ovary in preparation for fertilization.

Global Transcriptional Insights of Pollen-Pistil Interactions Commencing Self-Incompatibility and Fertilization in Tea [Camellia sinensis (L.) O. Kuntze]

This study explicates molecular insights commencing Self-Incompatibility (SI) and CC (cross-compatibility/fertilization) in self (SP) and cross (CP) pollinated pistils of tea. The fluorescence microscopy analysis revealed ceased/deviated pollen tubes in SP, while successful fertilization occurred in CP at 48 HAP. Global transcriptome sequencing of SP and CP pistils generated 109.7 million reads with overall 77.9% mapping rate to draft tea genome. Furthermore, concatenated de novo assembly resulted into 48,163 transcripts. Functional annotations and enrichment analysis (KEGG & GO) resulted into 3793 differentially expressed genes (DEGs). Among these, de novo and reference-based expression analysis identified 195 DEGs involved in pollen-pistil interaction. Interestingly, the presence of 182 genes [PT germination & elongation (67), S-locus (11), fertilization (43), disease resistance protein (30) and abscission (31)] in a major hub of the protein-protein interactome network suggests a complex signaling cascade commencing SI/CC. Furthermore, tissue-specific qRT-PCR analysis affirmed the localized expression of 42 DE putative key candidates in stigma-style and ovary, and suggested that LSI initiated in style and was sustained up to ovary with the active involvement of csRNS, SRKs & SKIPs during SP. Nonetheless, COBL10, RALF, FERONIA-rlk, LLG and MAPKs were possibly facilitating fertilization. The current study comprehensively unravels molecular insights of phase-specific pollen-pistil interaction during SI and fertilization, which can be utilized to enhance breeding efficiency and genetic improvement in tea.

Do changes in gene expression contribute to sexual isolation and reinforcement in the house mouse?

Expression divergence, rather than sequence divergence, has been shown to be important in speciation, particularly in the early stages of divergence of traits involved in reproductive isolation. In the two European subspecies of house mice, Mus musculus musculus and Mus musculus domesticus, earlier studies have demonstrated olfactory-based assortative mate preference in populations close to their hybrid zone. It has been suggested that this behaviour evolved following the recent secondary contact between the two taxa (~3,000 years ago) in response to selection against hybridization. To test for a role of changes in gene expression in the observed behavioural shift, we conducted a RNA sequencing experiment on mouse vomeronasal organs. Key candidate genes for pheromone-based subspecies recognition, the vomeronasal receptors, are expressed in these organs. Overall patterns of gene expression varied significantly between samples from the two subspecies, with a large number of differentially expressed genes between the two taxa. In contrast, only ~200 genes were found repeatedly differentially expressed between populations within M. m. musculus that did or did not display assortative mate preferences (close to or more distant from the hybrid zone, respectively), with an overrepresentation of genes belonging to vomeronasal receptor family 2. These receptors are known to play a key role in recognition of chemical cues that handle information about genetic identity. Interestingly, four of five of these differentially expressed receptors belong to the same phylogenetic cluster, suggesting specialization of a group of closely related receptors in the recognition of odorant signals that may allow subspecies recognition and assortative mating.

Two Loci Contribute Epistastically to Heterospecific Pollen Rejection, a Postmating Isolating Barrier Between Species

Recognition and rejection of heterospecific male gametes occurs in a broad range of taxa, although the complexity of mechanisms underlying these components of postmating cryptic female choice is poorly understood. In plants, the arena for postmating interactions is the female reproductive tract (pistil), within which heterospecific pollen tube growth can be arrested via active molecular recognition and rejection. Unilateral incompatibility (UI) is one such postmating barrier in which pollen arrest occurs in only one direction of an interspecific cross. We investigated the genetic basis of pistil-side UI between Solanum species, with the specific goal of understanding the role and magnitude of epistasis between UI QTL. Using heterospecific introgression lines (ILs) between Solanum pennellii and S. lycopersicum, we assessed the individual and pairwise effects of three chromosomal regions (ui1.1, ui3.1, and ui12.1) previously associated with interspecific UI among Solanum species. Specifically, we generated double introgression (‘pyramided’) genotypes that combined ui12.1 with each of ui1.1 and ui3.1, and assessed the strength of UI pollen rejection in the pyramided lines, compared to single introgression genotypes. We found that none of the three QTL individually showed UI rejection phenotypes, but lines combining ui3.1 and ui12.1 showed significant pistil-side pollen rejection. Furthermore, double ILs (DILs) that combined different chromosomal regions overlapping ui3.1 differed significantly in their rate of UI, consistent with at least two genetic factors on chromosome three contributing quantitatively to interspecific pollen rejection. Together, our data indicate that loci on both chromosomes 3 and 12 are jointly required for the expression of UI between S. pennellii and S. lycopersicum, suggesting that coordinated molecular interactions among a relatively few loci underlie the expression of this postmating prezygotic barrier. In addition, in conjunction with previous data, at least one of these loci appears to also contribute to conspecific self-incompatibility (SI), consistent with a partially shared genetic basis between inter- and intraspecific mechanisms of postmating prezygotic female choice.

Transcriptomic analysis links gene expression to unilateral pollen-pistil reproductive barriers

BACKGROUND

Unilateral incompatibility (UI) is an asymmetric reproductive barrier that unidirectionally prevents gene flow between species and/or populations. UI is characterized by a compatible interaction between partners in one direction, but in the reciprocal cross fertilization fails, generally due to pollen tube rejection by the pistil. Although UI has long been observed in crosses between different species, the underlying molecular mechanisms are only beginning to be characterized. The wild tomato relative Solanum habrochaites provides a unique study system to investigate the molecular basis of this reproductive barrier, as populations within the species exhibit both interspecific and interpopulation UI. Here we utilized a transcriptomic approach to identify genes in both pollen and pistil tissues that may be key players in UI.

RESULTS

We confirmed UI at the pollen-pistil level between a self-incompatible population and a self-compatible population of S. habrochaites. A comparison of gene expression between pollinated styles exhibiting the incompatibility response and unpollinated controls revealed only a small number of differentially expressed transcripts. Many more differences in transcript profiles were identified between UI-competent versus UI-compromised reproductive tissues. A number of intriguing candidate genes were highly differentially expressed, including a putative pollen arabinogalactan protein, a stylar Kunitz family protease inhibitor, and a stylar peptide hormone Rapid ALkalinization Factor. Our data also provide transcriptomic evidence that fundamental processes including reactive oxygen species (ROS) signaling are likely key in UI pollen-pistil interactions between both populations and species.

CONCLUSIONS

Gene expression analysis of reproductive tissues allowed us to better understand the molecular basis of interpopulation incompatibility at the level of pollen-pistil interactions. Our transcriptomic analysis highlighted specific genes, including those in ROS signaling pathways that warrant further study in investigations of UI. To our knowledge, this is the first report to identify candidate genes involved in unilateral barriers between populations within a species.

HT proteins contribute to S-RNase-independent pollen rejection in Solanum

Plants have mechanisms to recognize and reject pollen from other species. Although widespread, these mechanisms are less well understood than the self-incompatibility (SI) mechanisms plants use to reject pollen from close relatives. Previous studies have shown that some interspecific reproductive barriers (IRBs) are related to SI in the Solanaceae. For example, the pistil SI proteins S-RNase and HT protein function in a pistil-side IRB that causes rejection of pollen from self-compatible (SC) red/orange-fruited species in the tomato clade. However, S-RNase-independent IRBs also clearly contribute to rejecting pollen from these species. We investigated S-RNase-independent rejection of Solanum lycopersicum pollen by SC Solanum pennellii LA0716, SC. Solanum habrochaites LA0407, and SC Solanum arcanum LA2157, which lack functional S-RNase expression. We found that all three accessions express HT proteins, which previously had been known to function only in conjunction with S-RNase, and then used RNAi to test whether they also function in S-RNase-independent pollen rejection. Suppressing HT expression in SC S. pennellii LA0716 allows S. lycopersicum pollen tubes to penetrate farther into the pistil in HT suppressed plants, but not to reach the ovary. In contrast, suppressing HT expression in SC. Solanum habrochaites LA0407 and in SC S. arcanum LA2157 allows S. lycopersicum pollen tubes to penetrate to the ovary and produce hybrids that, otherwise, would be difficult to obtain. Thus, HT proteins are implicated in both S-RNase-dependent and S-RNase-independent pollen rejection. The results support the view that overall compatibility results from multiple pollen-pistil interactions with additive effects.