A relic S-RNase is expressed in the styles of self-compatible Nicotiana sylvestris

Intraspecific breakdown of self-incompatibility in Physalis acutifolia (Solanaceae)

Variation in mating systems is prevalent throughout angiosperms, with many transitions between outcrossing and selfing above and below the species level. This study documents a new case of an intraspecific breakdown of self-incompatibility in a wild relative of tomatillo, Physalis acutifolia. We used controlled greenhouse crosses to identify self-incompatible (SI) and self-compatible (SC) individuals grown from seed sampled across seven sites across Arizona and New Mexico. We measured 14 flower and fruit traits to test for trait variation associated with mating system. We also quantified pollen tube growth in vivo and tested for the presence of the S-RNase proteins in SI and SC styles. We found that seed from six of the seven sites produced SI individuals that terminated self-pollen tubes in the style and showed detectable S-RNase expression. By contrast, seed from one Arizona site produced SC individuals with no S-RNase expression. These SC individuals displayed typical selfing-syndrome traits such as smaller corollas, reduced stigma-anther distances, and a smaller pollen-ovule ratio. We also found plasticity in self-incompatibility as most of the SI individuals became SC and lost S-RNase expression roughly after 6 months in the greenhouse. While fixed differences in mating systems are known among the SI wild species and the often SC domesticated tomatillos, our study is the first to demonstrate intraspecific variation in natural populations as well as variation in SI over an individual's lifespan.

Evolution of self-compatibility by a mutant Sm-RNase in citrus

Self-incompatibility (SI) is an important mechanism that prevents self-fertilization and inbreeding in flowering plants. The most widespread SI system utilizes S ribonucleases (S-RNases) and S-locus F-boxes (SLFs) as S determinants. In citrus, SI is ancestral, and Citrus maxima (pummelo) is self-incompatible, while Citrus reticulata (mandarin) and its hybrids are self-compatible (SC). Here, we identify nine highly polymorphic pistil-specific, developmentally expressed S-RNases from pummelo that segregate with S haplotypes in a gametophytic manner and cluster with authentic S-RNases. We provide evidence that these S-RNases function as the female S determinants in citrus. Moreover, we show that each S-RNase is linked to approximately nine SLFs. In an analysis of 117 citrus SLF and SFL-like (SLFL) genes, we reveal that they cluster into 12 types and that the S-RNases and intra-haplotypic SLF and SLFL genes co-evolved. Our data support the notion that citrus have a S locus comprising a S-RNase and several SLFs that fit the non-self-recognition model. We identify a predominant single nucleotide mutation, S_m-RNase, in SC citrus, which provides a 'natural' loss of function. We show that SI-SC transitions due to the S_m-RNase initially arose in mandarin, spreading to its hybrids and became fixed. Identification of an evolutionarily distant new genus utilizing the S-RNase-based SI system, >100 million years separated from the nearest S-RNase family, is a milestone for evolutionary comparative studies.

NnSR1, a class III non-S-RNase specifically induced in Nicotiana alata under phosphate deficiency, is localized in endoplasmic reticulum compartments

A combined strategy of phosphate (Pi) remobilization from internal and external RNA sources seems to be conserved in plants exposed to Pi starvation. Thus far, the only ribonucleases (RNases) reported to be induced in Nicotiana alata undergoing Pi deprivation are extracellular S-like RNase NE and NnSR1. NnSR1 is a class III non S-RNase of unknown subcellular location. Here, we examine the hypothesis that NnSR1 is an intracellular RNase derived from the self-incompatibility system with specific expression in self-incompatible Nicotiana alata. NnSR1 was not induced in self-compatible Nicotiana species exposed to Pi deprivation. NnSR1 conjugated with a fluorescent protein and transiently expressed in Arabidopsis protoplasts and Nicotiana leaves showed that the fusion protein co-localized with an endoplasmic reticulum (ER) marker. Subcellular fractionation by ultracentrifugation of roots exposed to Pi deprivation revealed that the native NnSR1 migrated in parallel with the BiP protein, a typical ER marker. To our knowledge, NnSR1 is the first class III RNase reported to be localized in ER compartments. The induction of NnSR1 was detected earlier than the extracellular RNase NE, suggesting that intracellular RNA may be the first source of Pi used by the cell under Pi stress.

The transmitting tissue of Nicotiana tabacum is not essential to pollen tube growth, and its ablation can reverse prezygotic interspecific barriers

The Nicotiana tabacum transmitting tissue is a highly specialized file of metabolically active cells that is the pathway for pollen tubes from the stigma to the ovules where fertilization occurs. It is thought to be essential to pollen tube growth because of the nutrients and guidance it provides to the pollen tubes. It also regulates gametophytic self-incompatibility in the style. To test the function of the transmitting tissue in pollen tube growth and to determine its role in regulating prezygotic interspecific incompatibility, genetic ablation was used to eliminate the mature transmitting tissue, producing a hollow style. Despite the absence of the mature transmitting tissue and greatly reduced transmitting-tissue-specific gene expression, self-pollen tubes had growth to the end of the style. Pollen tubes grew at a slower rate in the transmitting-tissue-ablated line during the first 24 h post-pollination. However, pollen tubes grew to a similar length 40 h post-pollination with and without a transmitting tissue. Ablation of the N. tabacum transmitting tissue significantly altered interspecific pollen tube growth. These results implicate the N. tabacum transmitting tissue in facilitating or inhibiting interspecific pollen tube growth in a species-dependent manner and in controlling prezygotic reproductive barriers.

NnSR1, a class III non-S-RNase constitutively expressed in styles, is induced in roots and stems under phosphate deficiency in Nicotiana alata

BACKGROUND AND AIMS

Non-S-ribonucleases (non-S-RNases) are class III T2 RNases constitutively expressed in styles of species with S-RNase-based self-incompatibility. So far, no function has been attributed to these RNases. The aim of this work is to examine if NnSR1, a non-S-RNase from Nicotiana alata, is induced under conditions of phosphate (Pi) deprivation. The hypothesis is that under Pi-limited conditions, non-S-RNase functions may resemble the role of S-like RNases. To date, the only RNases reported to be induced by Pi deficiency are class I and class II S-like RNases, which are phylogenetically different from the class III clade of RNases.

METHODS

Gene and protein expression of NnSR1 were assayed in plants grown hydroponically with and without Pi, by combining RT-PCR, immunoblot and enzymatic activity approaches.

KEY RESULTS

NnSR1 transcripts were detected in roots 7 d after Pi deprivation and remained stable for several days. Transcript expression was correlated based on Pi availability in the culture medium. Antiserum against a peptide based on a hypervariable domain of NnSR1 recognized NnSR1 in roots and stems but not leaves exposed to Pi shortage. NnSR1 was not detected in culture medium and was pelleted with the microsomal fraction, suggesting that it was membrane-associated or included in large compartments. The anti-NnSR1 inhibited selectively the enzymatic activity of a 31-kDa RNase indicating that NnSR1 was induced in an enzymatically active form.

CONCLUSIONS

The induction of NnSR1 indicates that there is a general recruitment of all classes of T2 RNases in response to Pi shortage. NnSR1 appears to have regained ancestral functions of class III RNases related to strategies to cope with Pi limitation and also possibly with other environmental challenges. This constitutes the first report for a specific function of class III RNases other than S-RNases.

Proteomics approaches advance our understanding of plant self-incompatibility response

Self-incompatibility (SI) in plants is a genetic mechanism that prevents self-fertilization and promotes out-crossing needed to maintain genetic diversity. SI has been classified into two broad categories: the gametophytic self-incompatibility (GSI) and the sporophytic self-incompatibility (SSI) based on the genetic mechanisms involved in 'self' pollen rejection. Recent proteomic approaches to identify potential candidates involved in SI have shed light onto a number of previously unidentified mechanisms required for SI response. SI proteome research has progressed from the use of isoelectric focusing in early days to the latest third-generation technique of comparative isobaric tag for relative and absolute quantitation (iTRAQ) used in recent times. We will focus on the proteome-based approaches used to study self-incompatibility (GSI and SSI), recent developments in the field of incompatibility research with emphasis on SSI and future prospects of using proteomic approaches to study self-incompatibility.

PELPIII: the class III pistil-specific extensin-like Nicotiana tabacum proteins are essential for interspecific incompatibility

Pre-zygotic interspecific incompatibility (II) involves an active inhibition mechanism between the pollen of one species and the pistil of another. As a barrier to fertilization, II effectively prevents hybridization and maintains species identity. Transgenic ablation of the mature transmitting tract (TT) in Nicotiana tabacum resulted in the loss of inhibition of pollen tube growth in Nicotiana obtusifolia (synonym Nicotiana trigonophylla) and Nicotiana repanda. The role of the TT in the II interaction between N. tabacum and N. obtusifolia was characterized by evaluating N. obtusifolia pollen tube growth in normal and TT-ablated N. tabacum styles at various post-pollination times and developmental stages. The II activity of the TT slowed and then arrested N. obtusifolia pollen tube growth, and was developmentally synchronized. We hypothesize that proteins produced by the mature TT and secreted into the extracellular matrix inhibit interspecific pollen tubes. When extracts from the mature TT of N. tabacum were injected into the TT-ablated style prior to pollination, the growth of incompatible pollen tubes of N. obtusifolia and N. repanda was inhibited. The class III pistil-specific extensin-like protein (PELPIII) was consistently associated with specific inhibition of pollen tubes, and its requirement for II was confirmed through use of plants with antisense suppression of PELPIII. Inhibition of N. obtusifolia and N. repanda pollen tube growth required accumulation of PELPIII in the TT of N. tabacum, supporting PELPIII function in pre-zygotic II.

A new dual-specific incompatibility allele revealed by absence of glycosylation in the conserved C2 site of a Solanum chacoense S-RNase

The stylar determinant of gametophytic self-incompatibility (GSI) in Solanaceae, Rosaceae, and Plantaginaceae is an S-RNase encoded by a multiallelic S-locus. The primary structure of S-RNases shows five conserved (C) and two hypervariable (HV) regions, the latter forming a domain implicated in S-haplotype-specific recognition of the pollen determinant to SI. All S-RNases are glycosylated at a conserved site in the C2 region, although previous studies have shown that N-linked glycans at this position are not required for S-haplotype-specific recognition and pollen rejection. Here the incompatibility phenotype of three constructs derived from an originally monoglycosylated S11-RNase of Solanum chacoense, that were designed to explore the role of the HV domain in determining pollen recognition and the role of the N-linked glycan in the C2 region, is reported. In one series of experiments, a second glycosylation site was introduced in the HVa region to test for inhibition of pollen-specific recognition. This modification does not impede pollen rejection, although analysis shows incomplete glycosylation at the new site in the HVa region. A second construct, designed to permit complete glycosylation at the HVa site by suppression of the conserved site in the C2 region, did increase the degree of site occupancy, but, again, glycosylation was incomplete. Plants expressing this construct rejected S 11 pollen and, surprisingly, also rejected S 13 pollen, thus displaying an unusual dual specificity phenotype. This construct differs from the first by the absence of the conserved C2 glycosylation site, and thus the dual specificity is observed only in the absence of the C2 glycan. A third construct, completely lacking glycosylation sites, conferred an ability to reject only S 11 pollen, disproving the hypothesis that lack of a conserved glycan would confer a universal pollen rejection phenotype to the plant.

Molecular and genetic characterization of novel S-RNases from a natural population of Nicotiana alata

Self-incompatibility in the Solanaceae is mediated by S-RNase alleles expressed in the style, which confer specificity for pollen recognition. Nicotiana alata has been successfully used as an experimental model to elucidate cellular and molecular aspects of S-RNase-based self-incompatibility in Solanaceae. However, S-RNase alleles of this species have not been surveyed from natural populations and consequently the S-haplotype diversity is poorly known. Here the molecular and functional characterization of seven S-RNase candidate sequences, identified from a natural population of N. alata, are reported. Six of these candidates, S ( 5 ), S ( 27 ), S ( 70 ), S ( 75 ), S ( 107 ), and S ( 210 ), showed plant-specific amplification in the natural population and style-specific expression, which increased gradually during bud maturation, consistent with the reported S-RNase expression. In contrast, the S ( 63 ) ribonuclease was present in all plants examined and was ubiquitously expressed in different organs and bud developmental stages. Genetic segregation analysis demonstrated that S ( 27 ), S ( 70 ), S ( 75 ), S ( 107 ), and S ( 210 ) alleles were fully functional novel S-RNases, while S ( 5 ) and S ( 63 ) resulted to be non-S-RNases, although with a clearly distinct pattern of expression. These results reveal the importance of performing functional analysis in studies of S-RNase allelic diversity. Comparative phylogenetic analysis of six species of Solanaceae showed that N. alata S-RNases were included in eight transgeneric S-lineages. Phylogenetic pattern obtained from the inclusion of the novel S-RNase alleles confirms that N. alata represents a broad sample of the allelic variation at the S-locus of the Solanaceae.

Petunia nectar proteins have ribonuclease activity

Plants requiring an insect pollinator often produce nectar as a reward for the pollinator's visitations. This rich secretion needs mechanisms to inhibit microbial growth. In Nicotiana spp. nectar, anti-microbial activity is due to the production of hydrogen peroxide. In a close relative, Petunia hybrida, limited production of hydrogen peroxide was found; yet petunia nectar still has anti-bacterial properties, suggesting that a different mechanism may exist for this inhibition. The nectar proteins of petunia plants were compared with those of ornamental tobacco and significant differences were found in protein profiles and function between these two closely related species. Among those proteins, RNase activities unique to petunia nectar were identified. The genes corresponding to four RNase T2 proteins from Petunia hybrida that show unique expression patterns in different plant tissues were cloned. Two of these enzymes, RNase Phy3 and RNase Phy4 are unique among the T2 family and contain characteristics similar to both S- and S-like RNases. Analysis of amino acid patterns suggest that these proteins are an intermediate between S- and S-like RNases, and support the hypothesis that S-RNases evolved from defence RNases expressed in floral parts. This is the first report of RNase activities in nectar.

RNase-based self-incompatibility: puzzled by pollen S

Many plants have a genetically determined self-incompatibility system in which the rejection of self pollen grains is controlled by alleles of an S locus. A common feature of these S loci is that separate pollen- and style-expressed genes (pollen S and style S, respectively) determine S allele identity. The long-held view has been that pollen S and style S must be a coevolving gene pair in order for allelic recognition to be maintained as new S alleles arise. In at least three plant families, the Solanaceae, Rosaceae, and Plantaginaceae, the style S gene has long been known to encode an extracellular ribonuclease called the S-RNase. Pollen S in these families has more recently been identified and encodes an F-box protein known as either SLF or SFB. In this perspective, we describe the puzzling evolutionary relationship that exists between the SLF/SFB and S-RNase genes and show that in most cases cognate pairs of genes are not coevolving in the expected manner. Because some pollen S genes appear to have arisen much more recently than their style S cognates, we conclude that either some pollen S genes have been falsely identified or that there is a major problem with our understanding of how the S locus evolves.

An S-RNase-based gametophytic self-incompatibility system evolved only once in eudicots

It has been argued that the common ancestor of about 75% of all dicots possessed an S-RNase-based gametophytic self-incompatibility (GSI) system. S-RNase genes should thus be found in most plant families showing GSI. The S-RNase gene (or a duplicate) may also acquire a new function and thus genes belonging to the S-RNase lineage may also persist in plant families without GSI. Nevertheless, sequences that belong to the S-RNase lineage have been found in the Solanaceae, Scrophulariaceae, Rosaceae, Cucurbitaceae, and Fabaceae plant families only. Here we search for new sequences that may belong to the S-RNase lineage, using both a phylogenetic and a much faster and simpler amino acid pattern-based approach. We show that the two methods have an apparently similar false-negative rate of discovery (approximately 10%). The amino acid pattern-based approach produces about 15% false positives. Genes belonging to the S-RNase lineage are found in three new plant families, namely, the Rubiaceae, Euphorbiaceae, and Malvaceae. Acquisition of a new function by genes belonging to the S-RNase lineage is shown to be a frequent event. A putative S-RNase sequence is identified in Lotus, a plant genus for which molecular studies on GSI are lacking. The hypothesis of a single origin for S-RNase-based GSI (before the split of the Asteridae and Rosidae) is further supported by the finding of genes belonging to the S-RNase lineage in some of the oldest lineages of the Asteridae and Rosidae, and by Baysean constrained tree analyses.

A tale of two continents: Baker's rule and the maintenance of self-incompatibility in Lycium (Solanaceae)

Over 50 years ago, Baker (1955, 1967) suggested that self-compatible species were more likely than self-incompatible species to establish new populations on oceanic islands. His logic was straightforward and rested on the assumption that colonization was infrequent; thus, mate limitation favored the establishment of self-fertilizing individuals. In support of Baker's rule, many authors have documented high frequencies of self-compatibility on islands, and recent work has solidified the generality of Baker's ideas. The genus Lycium (Solanaceae) has ca. 80 species distributed worldwide, and phylogenetic studies suggest that Lycium originated in South America and dispersed to the Old World a single time. Previous analyses of the S-RNase gene, which controls the stylar component of self-incompatibility, have shown that gametophytically controlled self-incompatibility is ancestral within the genus, making Lycium a good model for investigating Baker's assertions concerning reproductive assurance following oceanic dispersal. Lycium is also useful for investigations of reproductive evolution, given that species vary both in sexual expression and the presence of self-incompatibility. A model for the evolution of gender dimorphism suggests that polyploidy breaks down self-incompatibility, leading to the evolution of gender dimorphism, which arises as an alternative outcrossing mechanism. There is a perfect association of dimorphic gender expression, polyploidy, and self-compatibility (vs. cosexuality, diploidy, and self-incompatibility) among North American Lycium. Although the association between ploidy level and gender expression also holds for African Lycium, to date no studies of mating systems have been initiated in Old World species. Here, using controlled pollinations, we document strong self-incompatibility in two cosexual, diploid species of African Lycium. Further, we sequence the S-RNase gene in 15 individuals from five cosexual, diploid species of African Lycium and recover 24 putative alleles. Genealogical analyses indicate reduced trans-generic diversity of S-RNases in the Old World compared to the New World. We suggest that genetic diversity at this locus was reduced as a result of a founder event, but, despite the bottleneck, self-incompatibility was maintained in the Old World. Maximum-likelihood analyses of codon substitution patterns indicate that positive Darwinian selection has been relatively strong in the Old World, suggesting the rediversification of S-RNases following a bottleneck. The present data thus provide a dramatic exception to Baker's rule, in addition to supporting a key assumption of the Miller and Venable (2000) model, namely that self-incompatibility is associated with diploidy and cosexuality.

Expression of 10 S-class SLF-like genes in Nicotiana alata pollen and its implications for understanding the pollen factor of the S locus

The S locus of Nicotiana alata encodes a polymorphic series of ribonucleases (S-RNases) that determine the self-incompatibility (SI) phenotype of the style. The pollen product of the S locus (pollen S) in N. alata is unknown, but in species from the related genus Petunia and in self-incompatible members of the Plantaginaceae and Rosaceae, this function has been assigned to an F-box protein known as SLF or SFB. Here we describe the identification of 10 genes (designated DD1-10) encoding SLF-related proteins that are expressed in N. alata pollen. Because our approach to cloning the DD genes was based on sequences of SLFs from other species, we presume that one of the DD genes encodes the N. alata SLF ortholog. Seven of the DD genes were exclusively expressed in pollen and a low level of sequence variation was found in alleles of each DD gene. Mapping studies confirmed that all 10 DD genes were linked to the S locus and that at least three were located in the same chromosomal segment as pollen S. Finally, the different topologies of the phylogenetic trees produced using available SLF-related sequences and those produced using S-RNase sequences suggests that pollen S and the S-RNase have different evolutionary histories.

Variation in the self-incompatibility response within and among populations of the tropical shrub Witheringia solanacea (Solanaceae)

Breakdown of genetically enforced self-incompatibility (SI), an extremely common and important evolutionary transition in plants, has conventionally been conceived as a qualitative rather than a quantitative change. We evaluated qualitative and quantitative variation in SI for four populations of Witheringia solanacea in Costa Rica, examining growth of self-pollen tubes in pollinations of buds and mature flowers. We also measured levels of RNase production in styles to determine whether enzyme production was correlated with differences in self-rejection. The two small populations contained both self-compatible (SC) individuals and obligate outcrossers (female or SI). Plants in the two large populations were uniformly SI as revealed by pollen tube growth, although several of these individuals sporadically set seed autogamously. Stylar RNase activity did not differ significantly between bud and mature flowers, but self-pollen tube growth did differ, suggesting that a gene product in addition to S-RNase is responsible for developmental onset of SI. Population-level differences in RNase activity were consistent with differences in the strength of the rejection response in bud pollinations, suggesting that a threshold level of S-RNase, in combination with other factors, is necessary for SI. Our results support a growing body of evidence that not only qualitative variation in SI, but also quantitative variation may be functionally significant.

Ancient polymorphism reveals unidirectional breeding system shifts

Loss of complex characters is thought to be irreversible (Dollo's law). However, hypotheses of irreversible evolution are remarkably difficult to test, especially when character transitions are frequent. In such cases, inference of ancestral states, in the absence of fossil evidence, is uncertain and represents the single greatest constraint for reconstructing the evolutionary history of characters. Breeding system character transitions are of particular interest because they affect the amount and distribution of genetic variation within species. Transitions from obligate outcrossing to partial or predominant self-fertilization are thought to represent one of the most common trends in flowering plants. We use the unique molecular genetic properties (manifested as deep persistent polymorphisms) of the locus that enforces outcrossing to demonstrate that its loss is irreversible in the plant family Solanaceae. We argue that current phylogenetic methods of reconstruction are potentially inadequate in cases where ancestral state information is inferred by using only the phylogeny and the distribution of character states in extant taxa. This study shows in a statistical framework that a particular character transition is irreversible, consistent with Dollo's law.

Identification of a non-S RNase, a possible ancestral form of S-RNases, in Prunus

This study identifies and characterizes a basic non-S RNase in the styles with stigmas of sweet cherry (Prunus avium L.), a member of the Rosaceae subfamily Amygdaloideae, which has an RNase-based gametophytic self-incompatibility system. Internal sequences of putative non-S RNases (RNase PA1 and PA2) were determined, and a cDNA for PA1 was obtained. The deduced amino acid sequence of PA1 contained two conserved sequence motifs essential for T2/ S-type RNase activity. PA1 shows 20-30% sequence identity to S-RNases of Rosaceae, Solanaceae and Scrophulariaceae, and non-S RNases of higher plants. Transcription of the PA1 gene was specific to the styles with stigmas, and the gene was not expressed in other tissues. Although PA1 resembles RNase X2, a non-S RNase from Petunia inflata, the placement of PA1 and RNase X2 in the phylogenetic tree was quite different. Placement of PA1 was also distinct from that of rosaceous S-RNases, while RNase X2 was incorporated in the clade of S-RNases from the Solanaceae. The sole intron in the PA1 gene is located at a position equivalent to that of the second intron of amygdaloid S-RNase genes, and that of the only intron in most other S-RNase genes. Genomic Southern analysis revealed the presence of sequences homologous to PA1 in all of the other four Prunus species tested, suggesting that PA1 has an important physiological function. The significance of the discovery of PA1 is discussed in terms of the origin and evolution of S-RNases and self-incompatibility in Rosaceae.

Insights into the evolution of self-compatibility in Lycopersicon from a study of stylar factors

To elucidate the molecular basis of loss of self-incompatibility in Lycopersicon, S-RNases and HT-proteins were analysed in seven self-compatible (SC) and three self-incompatible (SI) taxa. No or low stylar RNase activity was a common feature in most SC taxa examined, in contrast to the uniformly high levels of activity found in all SI species. The S-RNase gene is most likely deleted in the four red-fruited SC taxa (L. esculentum, L. esculentum var. cerasiforme, L. pimpinellifolium and L. cheesmanii) because S-RNase genes could not be amplified from genomic DNA. S-RNase genes could, however, be amplified from the genomes of the three green-fruited SC taxa examined. L. chmielewskii and L. hirsutum f. glabratum show a decreased accumulation of transcripts, possibly reflecting changes in the 5' flanking regions of the S-RNase genes. The remaining green-fruited SC species, L. parviflorum, has a functional S-RNase gene in its genome that is expressed at high levels in the style, suggesting a genetic factor responsible for the low S-RNase activity. Together these results argue for several independent mutations in the S-RNase gene over the course of Lycopersicon diversification, and that loss of S-RNase function is unlikely to the primary cause of the loss of self-incompatibility. We also examined the HT-B genes that play a role in self-incompatibility. HT-B transcripts were markedly reduced in the styles of all the SC taxa examined. A scenario is described where a mutation causing reduced transcription of HT-B in an ancestral SI species was central to the loss of self-incompatibility in Lycopersicon.

Molecular mechanisms underlying the breakdown of gametophytic self-incompatibility

The breakdown of self-incompatibility has occurred repeatedly throughout the evolution of flowering plants and has profound impacts on the genetic structure of populations. Recent advances in understanding of the molecular basis of self-incompatibility have provided insights into the mechanisms of its loss in natural populations, especially in the tomato family, the Solanaceae. In the Solanaceae, the gene that controls self-incompatibility in the style codes for a ribonuclease that causes the degradation of RNA in pollen tubes bearing an allele at the S-locus that matches either of the two alleles held by the maternal plant. The pollen component of the S-locus has yet to be identified. Loss of self-incompatibility can be attributed to three types of causes: duplication of the S-locus, mutations that cause loss of S-RNase activity, and mutations that do not cause loss of S-RNase activity. Duplication of the S-locus has been well studied in radiation-induced mutants but may be a relatively rare cause of the breakdown of self-incompatibility in nature. Point mutations within the S-locus that disrupt the production of S-RNase have been documented in natural populations. There are also a number of mutants in which S-RNase production is unimpaired, yet self-incompatibility is disrupted. The identity and function of these mutations is not well understood. Careful work on a handful of model organisms will enable population biologists to better understand the breakdown of self-incompatibility in nature.

Evolutionary relationships among self-incompatibility RNases

T2-type RNases are responsible for self-pollen recognition and rejection in three distantly related families of flowering plants-the Solanaceae, Scrophulariaceae, and Rosaceae. We used phylogenetic analyses of 67 T2-type RNases together with information on intron number and position to determine whether the use of RNases for self-incompatibility in these families is homologous or convergent. All methods of phylogenetic reconstruction as well as patterns of variation in intron structure find that all self-incompatibility RNases along with non-S genes from only two taxa form a monophyletic clade. Several lines of evidence suggest that the best interpretation of this pattern is homology of self-incompatibility RNases from the Scrophulariaceae, Solanaceae, and Rosaceae. Because the most recent common ancestor of these three families is the ancestor of approximately 75% of dicot families, our results indicate that RNase-based self-incompatibility was the ancestral state in the majority of dicots.