The population genetics of sporophytic self-incompatibility in Senecio squalidus L. (Asteraceae) I: S allele diversity in a natural population

Drift in small populations predicts mate availability and the breakdown of self-incompatibility in a clonal polyploid

Mate limitation in small populations can reduce reproductive fitness, hinder population growth, and increase extinction risk. Mate limitation is exacerbated in self-incompatible (SI) taxa, where shared S-alleles further restrict mating. Theory suggests genetic drift as a predictor of mate limitation and the breakdown of SI systems. We tested this prediction by evaluating mate availability and S-allele number in populations of a tetraploid herb with gametophytic SI (GSI) spanning a range of effective population sizes. We performed controlled crosses in 13 populations of Argentina anserina to quantify mate availability and S-allele diversity, which were compared with simulations of tetraploid populations with GSI. We further evaluated mechanisms at the pollen-pistil interface contributing to outcross failure and leakiness in self-recognition. Mate availability declined in small populations, and closely fit tetraploid GSI population genetic models where maternal plants receive pollen with diverse S-alleles generated through tetrasomic inheritance. The failure to arrest self-pollen in the style was common in some populations. Specifically, leaky SI was more common in small populations with low mate availability, where it explained higher seed production in natural populations. The restriction of leaky self-recognition to the smallest populations is consistent with mate limitation as a pressure driving the breakdown of self-incompatibility.

Genomic changes and stabilization following homoploid hybrid speciation of the Oxford ragwort Senecio squalidus

Oxford ragwort (Senecio squalidus) is one of only two homoploid hybrid species known to have originated very recently, so it is a unique model for determining genomic changes and stabilization following homoploid hybrid speciation. Here, we provide a chromosome-level genome assembly of S. squalidus with 95% of the assembly contained in the 10 longest scaffolds, corresponding to its haploid chromosome number. We annotated 30,249 protein-coding genes and estimated that ∼62% of the genome consists of repetitive elements. We then characterized genome-wide patterns of linkage disequilibrium, polymorphism, and divergence in S. squalidus and its two parental species, finding that (1) linkage disequilibrium is highly heterogeneous, with a region on chromosome 4 showing increased values across all three species but especially in S. squalidus; (2) regions harboring genetic incompatibilities between the two parental species tend to be large, show reduced recombination, and have lower polymorphism in S. squalidus; (3) the two parental species have an unequal contribution (70:30) to the genome of S. squalidus, with long blocks of parent-specific ancestry supporting a very rapid stabilization of the hybrid lineage after hybrid formation; and (4) genomic regions with major parent ancestry exhibit an overrepresentation of loci with evidence for divergent selection occurring between the two parental species on Mount Etna. Our results show that both genetic incompatibilities and natural selection play a role in determining genome-wide reorganization following hybrid speciation and that patterns associated with homoploid hybrid speciation-typically seen in much older systems-can evolve very quickly following hybridization.

The effect of divergent and parallel selection on the genomic landscape of divergence

While the role of selection in divergence along the speciation continuum is theoretically well understood, defining specific signatures of selection in the genomic landscape of divergence is empirically challenging. Modelling approaches can provide insight into the potential role of selection on the emergence of a heterogenous genomic landscape of divergence. Here, we extend and apply an individual-based approach that simulates the phenotypic and genotypic distributions of two populations under a variety of selection regimes, genotype-phenotype maps, modes of migration, and genotype-environment interactions. We show that genomic islands of high differentiation and genomic valleys of similarity may respectively form under divergent and parallel selection between populations. For both types of between-population selection, negative and positive frequency-dependent selection within populations generated genomic islands of higher magnitude and genomic valleys of similarity, respectively. Divergence rates decreased under strong dominance with divergent selection, as well as in models including genotype-environment interactions under parallel selection. For both divergent and parallel selection models, divergence rate was higher under an intermittent migration regime between populations, in contrast to a constant level of migration across generations, despite an equal number of total migrants. We highlight that interpreting a particular evolutionary history from an observed genomic pattern must be done cautiously, as similar patterns may be obtained from different combinations of evolutionary processes. Modelling approaches such as ours provide an opportunity to narrow the potential routes that generate the genomic patterns of specific evolutionary histories.

Self-incompatibility: a targeted, unexplored pre-fertilization barrier in flower crops of Asteraceae

Asteraceae (synonym as Compositae) is one of the largest angiosperm families among flowering plants comprising one-tenth of all agri-horticultural species grown across various habitats except in Antarctica. These are commercially utilized as cut and loose flowers as well as pot and bedding plants in landscape gardens due to their unique floral traits. Consequently, ineffective seed setting and presence of an intraspecific reproductive barrier known as self-incompatibility (SI) severely reduces the effectiveness of hybridization and self-fertilization by traditional crossing. There have been very few detailed studies of pollen-stigma interactions in this family. Moreover, about 63% of Aster species can barely self-fertilize due to self-incompatibility (SI). The chrysanthemum (Chrysanthemum × morifolium) is one of the most economically important ornamental plants in the Asteraceae family which hugely shows incompatibility. Reasons for the low fertility and reproductive capacity of species are still indefinite or not clear. Hence, the temporal pattern of inheritance of self-incompatibility and its effect on reproductive biology needs to be investigated further to improve the breeding efficiency. This review highlights the self-incompatible (SI) system operating in important Astraceous (ornamental) crops which are adversely affected by this mechanism along with different physiological and molecular techniques involved in breaking down self-incompatibility.

Genome-wide identification and characterization of SRLK gene family reveal their roles in self-incompatibility of Erigeron breviscapus

Self-incompatibility (SI) is a reproductive protection mechanism that plants acquired during evolution to prevent self-recession. As the female determinant of SI specificity, SRK has been shown to be the only recognized gene on the stigma and plays important roles in SI response. Asteraceae is the largest family of dicotyledonous plants, many of which exhibit self-incompatibility. However, systematic studies on SRK gene family in Asteraceae are still limited due to lack of high-quality genomic data. In this study, we performed the first systematic genome-wide identification of S-locus receptor like kinases (SRLKs) in the self-incompatible Asteraceae species, Erigeron breviscapus, which is also a widely used perennial medicinal plant endemic to China.52 SRLK genes were identified in the E. breviscapus genome. Structural analysis revealed that the EbSRLK proteins in E. breviscapus are conserved. SRLK proteins from E. breviscapus and other SI plants are clustered into 7 clades, and the majority of the EbSRLK proteins are distributed in Clade I. Chromosomal and duplication analyses indicate that 65% of the EbSRLK genes belong to tandem repeats and could be divided into six tandem gene clusters. Gene expression patterns obtained in E. breviscapus multiple-tissue RNA-Seq data revealed differential temporal and spatial features of EbSRLK genes. Among these, two EbSRLK genes having high expression levels in tongue flowers were cloned. Subcellular localization assay demonstrated that both of their fused proteins are localized on the plasma membrane. All these results indicated that EbSRLK genes possibly involved in SI response in E. breviscapus. This comprehensive genome-wide study of the SRLK gene family in E. breviscapus provides valuable information for understanding the mechanism of SSI in Asteraceae.

How rapidly do self-compatible populations evolve selfing? Mating system estimation within recently evolved self-compatible populations of Azorean Tolpis succulenta (Asteraceae)

Genome-wide genotyping and Bayesian inference method (BORICE) were employed to estimate outcrossing rates and paternity in two small plant populations of Tolpis succulenta (Asteraceae) on Graciosa island in the Azores. These two known extant populations of T. succulenta on Graciosa have recently evolved self-compatibility. Despite the expectation that selfing would occur at an appreciable rate (self-incompatible populations of the same species show low but nonzero selfing), high outcrossing was found in progeny arrays from maternal plants in both populations. This is inconsistent with an immediate transition to high selfing following the breakdown of a genetic incompatibility system. This finding is surprising given the small population sizes and the recent colonization of an island from self-incompatible colonists of T. succulenta from another island in the Azores, and a potential paucity of pollinators, all factors selecting for selfing through reproductive assurance. The self-compatible lineage(s) likely have high inbreeding depression (ID) that effectively halts the evolution of increased selfing, but this remains to be determined. Like their progeny, all maternal plants in both populations are fully outbred, which is consistent with but not proof of high ID. High multiple paternity was found in both populations, which may be due in part to the abundant pollinators observed during the flowering season.

The loss of self-incompatibility in a range expansion

It is commonly observed that plant species' range margins are enriched for increased selfing rates and, in otherwise self-incompatible species, for self-compatibility (SC). This has often been attributed to a response to selection under mate and/or pollinator limitation. However, range expansion can also cause reduced inbreeding depression, and this could facilitate the evolution of selfing in the absence of mate or pollinator limitation. Here, we explore this idea using spatially explicit individual-based simulations of a range expansion, in which inbreeding depression, variation in self-incompatibility (SI), and mate availability evolve. Under a wide range of conditions, the simulated range expansion brought about the evolution of selfing after the loss of SI in range-marginal populations. Under conditions of high recombination between the self-incompatibility locus (S-locus) and viability loci, SC remained marginal in the expanded metapopulation and could not invade the range core, which remained self-incompatible. In contrast, under low recombination and migration rates, SC was frequently able to displace SI in the range core by maintaining its association with a genomic background with purged genetic load. We conclude that the evolution of inbreeding depression during a range expansion promotes the evolution of SC at range margins, especially under high rates of recombination.‬.

Senecio as a model system for integrating studies of genotype, phenotype and fitness

Two major developments have made it possible to use examples of ecological radiations as model systems to understand evolution and ecology. First, the integration of quantitative genetics with ecological experiments allows detailed connections to be made between genotype, phenotype, and fitness in the field. Second, dramatic advances in molecular genetics have created new possibilities for integrating field and laboratory experiments with detailed genetic sequencing. Combining these approaches allows evolutionary biologists to better study the interplay between genotype, phenotype, and fitness to explore a wide range of evolutionary processes. Here, we present the genus Senecio (Asteraceae) as an excellent system to integrate these developments, and to address fundamental questions in ecology and evolution. Senecio is one of the largest and most phenotypically diverse genera of flowering plants, containing species ranging from woody perennials to herbaceous annuals. These Senecio species exhibit many growth habits, life histories, and morphologies, and they occupy a multitude of environments. Common within the genus are species that have hybridized naturally, undergone polyploidization, and colonized diverse environments, often through rapid phenotypic divergence and adaptive radiation. These diverse experimental attributes make Senecio an attractive model system in which to address a broad range of questions in evolution and ecology.

The contribution of hybridization to range-wide population genetic structure in a Pacific coastal dune plant

PREMISE

Interspecific hybridization can cause genetic structure across species ranges if the mating system and degree of sympatry/parapatry with close relatives varies geographically. The coastal dune endemic Camissoniopsis cheiranthifolia (Onagraceae) exhibits genetic subdivisions across its range, some of which are associated with shifts in mating system from outcrossing to selfing, while others are not. For instance, strong differentiation between large-flowered, self-incompatible (LF-SI) and large-flowered, self-compatible (LF-SC) populations occurs without much reduction in outcrossing or obvious barriers to gene flow. We hypothesized that LF-SI diverged from LF-SC via hybridization with the predominantly inland SI sister species C. bistorta.

METHODS

We analyzed spatial proximity using 1460 herbarium records and genetic variation at 12 microsatellites assayed for 805 and 404 individuals from 32 C. cheiranthifolia and 18 C. bistorta populations, respectively. We also assayed nine chloroplast microsatellites for 124 and 111 individuals from 27 and 19 populations, respectively.

RESULTS

Closer parapatry was associated with unexpectedly high genetic continuity between LF-SI C. cheiranthifolia and C. bistorta. LF-SI genotypes clustered with C. bistorta exclusive of other C. cheiranthifolia genotypes. Similarly, pairwise F_ST among SI C. cheiranthifolia and C. bistorta, adjusted for geographic proximity, was not higher between heterospecific than conspecific populations.

CONCLUSIONS

The lack of genetic differentiation between LF-SI C. cheiranthifolia and C. bistorta populations, even those located away from the zone of parapatry, suggests that, instead of hybridizing with C. bistorta, LF-SI C. cheiranthifolia is rather an ecotype of C. bistorta that has adapted to coastal dune habitat independent of other lineages in C. cheiranthifolia proper.

Multilayered dominance hierarchy in plant self-incompatibility

Epigenetic dominance modifier. In polymorphic loci, complex genetic dominance relationships between alleles are often observed. In plants, control of self-incompatibility (SI) expression via allelic interactions in the Brassicaceae is the best-known example of such mechanisms. Here, with emphasis on two recently published papers, we review the progress toward understanding the dominance regulatory mechanism of SI in the Brassicaceae. Multiple small RNA genes linked to the Self-incompatibility (S) locus were found in both Brassica and Arabidopsis genera. Mono-allelic gene expression of the male determinant of SI, SP11/SCR, from a dominant S-allele is under epigenetic control by such small RNA genes. Possible evolutionary trajectories leading to the formation of multilayered dominance hierarchy in Brassicaceae are discussed. We also identify some remaining questions for future studies.

'Comment on Saumitou et al. (2017): Elucidation of the genetic architecture of self-incompatibility in olive: evolutionary consequences and perspectives for orchard management'

The new self-incompatibility system (SI) was presented by Saumitou-Laprade, Vernet, Vekemans et al. (2017). Evolutionary Applications based on 89 crosses between varieties in the olive tree. Four main points are not clear. We are examining here as follows: (i) the assertion that the self-incompatibility system is sporophytic was not sustained by pollen germination data; (ii) surprisingly, the new model does not explain that about one-third of pairwise combinations of olive varieties leads to asymmetric fruit setting; (iii) DNA preparation from one seed may contain two embryos, and thus, embryos should be separated before seed extraction; (iv) although effective self-fertility in olive varieties was reported by many studies, the DSI model fails to explain self-fertility in some olive varieties. Moreover, we cannot discuss result data, as science cannot be verified because variety names were encoded, this does not allow comparison of data with previous works. The DSI model on olive self-incompatibility should explain more features than the model based on four dominance levels shared by six S-alleles. Perspectives for orchard management based on this model may face serious limitations. An olive variety does not have a fifty percent chance of cross-incompatibility, but surely fewer, and thus, the sporophytic system limits fruit production. Evolutionary perspectives of self-incompatibility in Oleaceae should include data from the Jasmineae tribe that displays heterostyly SI.

Identification, genealogical structure and population genetics of S-alleles in Malus sieversii, the wild ancestor of domesticated apple

The self-incompatibility (SI) gene that is specifically expressed in pistils encodes the SI-associated ribonuclease (S-RNase), functioning as the female-specificity determinant of a gametophytic SI system. Despite extensive surveys in Malus domestica, the S-alleles have not been fully investigated for Malus sieversii, the primary wild ancestor of the domesticated apple. Here we screened the M. sieversii S-alleles via PCR amplification and sequencing, and identified 14 distinct alleles in this species. By contrast, nearly 40 are present in its close wild relative, Malus sylvestris. We further sequenced 8 nuclear genes to provide a neutral reference, and investigated the evolution of S-alleles via genealogical and population genetic analyses. Both shared ancestral polymorphism and an excess of non-synonymous substitution were detected in the S-RNases of the tribe Maleae in Rosaceae, indicating the action of long-term balancing selection. Approximate Bayesian Computations based on the reference neutral loci revealed a severe bottleneck in four of the six studied M. sieversii populations, suggesting that the low number of S-alleles found in this species is mainly the result of diversity loss due to a drastic population contraction. Such a bottleneck may lead to ambiguous footprints of ongoing balancing selection detected at the S-locus. This study not only elucidates the constituents and number of S-alleles in M. sieversii but also illustrates the potential utility of S-allele number shifts in demographic inference for self-incompatible plant species.

Population genetics of self-incompatibility in a clade of relict cliff-dwelling plant species

The mating systems of species in small or fragmented populations impact upon their persistence. Small self-incompatible (SI) populations risk losing S allele diversity, responsible for the SI response, by drift thereby limiting mate availability and leading to population decline or SI system breakdown. But populations of relict and/or endemic species have resisted these demographic conditions over long periods suggesting their mating systems have adapted. To address a lack of empirical data on this topic, we studied the SI systems of three relict cliff-dwelling species of Sonchus section Pustulati (Asteraceae): S. masguindalii, S. fragilis and S. pustulatus in the western Mediterranean region. We performed controlled pollinations within and between individuals to measure index of SI (ISI) expression and identify S alleles in multiple population samples. Sonchus masguindalii and S. pustulatus showed strong SI (ISI = 0.6-1.0) compared to S. fragilis (ISI = 0.1-0.7). Just five S alleles were estimated for Spanish S. pustulatus and a moderate 11-15 S alleles for Moroccan S. pustulatus and S. fragilis, respectively. The fact that autonomous fruit set was generally improved by active self-pollination in self-compatible S. fragilis suggests that individuals with weak SI can show a wide range of outcrossing levels dependent on the degree of self or outcross pollen that pollinators bear. We conclude that frequent S allele dominance interactions that mask the incompatibility interactions of recessive S alleles leading to higher mate availability and partial breakdown of SI leading to mixed mating, both contribute to reproductive resilience in this group.

Transcriptomic comparison of the self-pollinated and cross-pollinated flowers of Erigeron breviscapus to analyze candidate self-incompatibility-associated genes

BACKGROUND

Self-incompatibility (SI) is a widespread and important mating system that promotes outcrossing in plants. Erigeron breviscapus, a medicinal herb used widely in traditional Chinese medicine, is a self-incompatible species of Asteraceae. However, the genetic characteristics of SI responses in E. breviscapus remain largely unknown. To understand the possible mechanisms of E. breviscapus in response to SI, we performed a comparative transcriptomic analysis with capitulum of E. breviscapus after self- and cross-pollination, which may provide valuable information for analyzing the candidate SI-associated genes of E. breviscapus.

METHODS

Using a high-throughput next-generation sequencing (Illumina) approach, the transcriptionexpression profiling of the different genes of E. breviscapus were obtained, some results were verified by quantitative real time PCR (qRT-PCR).

RESULTS

After assembly, 63,485 gene models were obtained (average gene size 882 bp; N50 = 1485 bp), among which 38,540 unigenes (60.70% of total gene models) were annotated by comparisons with four public databases (Nr, Swiss-Prot, KEGG and COG): 38,338 unigenes (60.38% of total gene models) showed high homology with sequences in the Nr database. Differentially expressed genes were identified among the three cDNA libraries (non-, self- and cross-pollinated capitulum of E. breviscapus), and approximately 230 genes might be associated with SI responses. Several these genes were upregulated in self-pollinated capitulum but downregulated in cross-pollinated capitulum, such as SRLK (SRK-like) and its downstream signal factor, MLPK. qRT-PCR confirmed that the expression patterns of EbSRLK1 and EbSRLK3 genes were not closely related to SI of E. breviscapus.

CONCLUSIONS

This work represents the first large-scale analysis of gene expression in the self-pollinated and cross-pollinated flowers of E. breviscapus. A larger number of notable genes potentially involved in SI responses showed differential expression, including genes playing crucial roles in cell-cell communication, signal transduction and the pollination process. We thus hypothesized that those genes showing differential expression and encoding critical regulators of SI responses, such as MLPK, ARC1, CaM, Exo70A1, MAP, SF21 and Nod, might affect SI responses in E. breviscapus. Taken together, our study provides a pool of SI-related genes in E. breviscapus and offers a valuable resource for elucidating the mechanisms of SI in Asteraceae.

The population genetics of sporophytic self-incompatibility in three hybridizing senecio (asteraceae) species with contrasting population histories

Hybridization generates evolutionary novelty and spreads adaptive variation. By promoting outcrossing, plant self-incompatibility (SI) systems also favor interspecific hybridization because the S locus is under strong negative frequency-dependent balancing selection. This study investigates the SI mating systems of three hybridizing Senecio species with contrasting population histories. Senecio aethnensis and S. chrysanthemifolius native to Sicily, form a hybrid zone at intermediate altitudes on Mount Etna, and their neo-homoploid hybrid species, S. squalidus, has colonized disturbed urban habitats in the UK during the last 150 years. We show that all three species express sporophytic SI (SSI), where pollen incompatibility is controlled by the diploid parental genome, and that SSI is inherited and functions normally in hybrids. Large-scale crossing studies of wild sampled populations allowed direct comparison of SSI between species and found that the main impacts of colonization in S. squalidus compared to Sicilian Senecio was a reduced number of S alleles, increased S allele frequencies, and increased interpopulation S allele sharing. In general, many S alleles were shared between species and the S locus showed reduced intra- and interspecific population genetic structure compared to molecular genetic markers, indicative of enhanced effective gene flow due to balancing selection.

New hypothesis elucidates self-incompatibility in the olive tree regarding S-alleles dominance relationships as in the sporophytic model

Most olive varieties are not strictly self-incompatible, nevertheless, they request foreign pollen to enhance fruit yield, and consequently orchards should contain pollinisers to ensure fruit set of the main variety. The best way to choose pollinisers is to experiment numerous crosses in a diallel design. Here, the genetic mode of inheritance of SI in the olive is deciphered and it does not correspond to the GSI type, but to the SSI type. It leaves S-allele dominance relationship expression in the male (pollen and pollen tube), but not in the female (stigma and style). Thus, a pair-wise combination of varieties may be inter-compatible in one direction (male to female, or female to male) and inter-incompatible in the other direction. Dominance relationships also explain different levels of self-pollination observed in varieties. Little efficient pollinisers were found and predicted in varieties; nevertheless, some new efficient pair-wise allele combinations are predicted and could be created. This model enables one to forecast compatibility without waiting for several years of yield records and to choose pollinisers in silico.

Sporophytic self-incompatibility in Senecio squalidus (Asteraceae): S allele dominance interactions and modifiers of cross-compatibility and selfing rates

Understanding genetic mechanisms of self-incompatibility (SI) and how they evolve is central to understanding the mating behaviour of most outbreeding angiosperms. Sporophytic SI (SSI) is controlled by a single multi-allelic locus, S, which is expressed in the diploid (sporophyte) plant to determine the SI phenotype of its haploid (gametophyte) pollen. This allows complex patterns of independent S allele dominance interactions in male (pollen) and female (pistil) reproductive tissues. Senecio squalidus is a useful model for studying the genetic regulation and evolution of SSI because of its population history as an alien invasive species in the UK. S. squalidus maintains a small number of S alleles (7-11) with a high frequency of dominance interactions. Some S. squalidus individuals also show partial selfing and/or greater levels of cross-compatibility than expected under SSI. We previously speculated that these might be adaptations to invasiveness. Here we describe a detailed characterization of the regulation of SSI in S. squalidus. Controlled crosses were used to determine the S allele dominance hierarchy of six S alleles and effects of modifiers on cross-compatibility and partial selfing. Complex dominance interactions among S alleles were found with at least three levels of dominance and tissue-specific codominance. Evidence for S gene modifiers that increase selfing and/or cross-compatibility was also found. These empirical findings are discussed in the context of theoretical predictions for maintenance of S allele dominance interactions, and the role of modifier loci in the evolution of SI.

Expression and inheritance of sporophytic self-incompatibility in synthetic allohexaploid Senecio cambrensis (Asteraceae)

Allopolyploid speciation is common in plants and is frequently associated with shifts from outcrossing, for example self-incompatibility, to inbreeding (i.e. selfing). Senecio cambrensis is a recently evolved allohexaploid species that formed following hybridization between diploid self-incompatible S. squalidus and tetraploid self-compatible S. vulgaris. Studies of reproduction in wild populations of S. cambrensis have concluded that it is self-compatible. Here, we investigated self-compatibility in synthetic lines of S. cambrensis generated via hybridization and colchicine-induced polyploidization and wild S. cambrensis using controlled crossing experiments. Synthetic F(1)S. cambrensis individuals were all self-compatible but, in F(2) and later generations, self-incompatible individuals were identified at frequencies of 6.7-9.2%. Self-incompatibility was also detected in wild sampled individuals at a frequency of 12.2%. The mechanism and genetics of self-incompatibility were tested in synthetic S. cambrensis and found to be similar to those of its paternal parent S. squalidus (i.e. sporophytic). These results show, for the first time, that functional sporophytic self-incompatibility can be inherited and expressed in allopolyploids as early as the second (F(2)) generation. Wild S. cambrensis should therefore be considered as possessing a mixed mating system with the potential for evolution towards either inbreeding or outcrossing.

Effect of variation in self-incompatibility on pollen limitation and inbreeding depression in Flourensia cernua (Asteraceae) scrubs of contrasting density

BACKGROUND AND AIMS

Selection may favour a partial or complete loss of self-incompatibility (SI) if it increases the reproductive output of individuals in the presence of low mate availability. The reproductive output of individuals varying in their strength of SI may also be affected by population density via its affect on the spatial structuring and number of S-alleles in populations. Modifiers increasing levels of self-compatibility can be selected when self-compatible individuals receive reproductive compensation by, for example, increasing seed set and/or when they become associated with high fitness genotypes.

METHODS

The effect of variation in the strength of SI and scrub density (low versus high) on seed set, seed germination and inbreeding depression in seed germination (delta(germ)) was investigated in the partially self-incompatible species Flourensia cernua by analysing data from self-, cross- and open-pollinated florets.

KEY RESULTS

Examination of 100 plants in both high and low scrub densities revealed that 51% of plants were strongly self-incompatible and 49 % varied from being self-incompatible to self-compatible. Seed set after hand cross-pollination was higher than after open-pollination for self-incompatible, partially self-incompatible and self-compatible plants but was uniformly low for strongly self-incompatible plants. Strongly self-incompatible and self-incompatible plants exhibited lower seed set, seed germination and multiplicative female fitness (floral display x seed set x seed germination) in open-pollinated florets compared with partially self-incompatible and self-compatible plants. Scrub density also had an effect on seed set and inbreeding depression: in low-density scrubs seed set was higher after open-pollination and delta(germ) was lower.

CONCLUSIONS

These data suggest that (a) plants suffered outcross pollen limitation, (b) female fitness in partially self-incompatible and self-compatible plants is enhanced by increased mate-compatibility and (c) plants in low-density scrubs received higher quality pollen via open-pollination than plants in high-density scrubs.

Transient SI and the dynamics of self-incompatibility alleles: a simulation model and empirical test

A stochastic computer simulation model was created to compare the combined effects of selection and genetic drift on the dynamics of S-alleles under full sporophytic self-incompatibility (SI) versus transient SI, a form of partial SI in which flowers become self-compatible as they age. S-alleles were lost more rapidly with transient than with full SI, as is expected with weakened frequency-dependent selection. Based on these results, equilibrium S-allele diversity is expected to be lower with partial SI for populations of comparable size and migration rates. Consistent with model results, a comparison of the proportion of incompatible crosses in full diallel experiments for a fully SI and a transiently SI species in the annual genus Leptosiphon suggests that S-allele diversity is lower in the partially SI species. Results of the simulation model indicate that the transmission advantage of self-fertilization can have complex effects on S-allele dynamics in partial SI systems.

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.

Modes and rates of selfing and associated inbreeding depression in the self-incompatible plant Senecio squalidus (Asteraceae): a successful colonizing species in the British Isles

The strength of the self-incompatibility (SI) response in Senecio squalidus was measured across its British range. Geographic variation in SI was investigated and the extent and inheritance of pseudo-self-compatibility (PSC) and inbreeding depression were determined. Mean self-fruit-set per capitulum was calculated for individuals and sample populations. The heritability of PSC and the magnitude of inbreeding depression were assessed by comparing selfing rates and fitness trait values between SI and PSC parent-progeny lines. SI was found to be strongly expressed in S. squalidus throughout its British range, with only 3.1% of the individuals sampled showing PSC. This PSC had relatively low heritability with stronger expression of SI in selfed progeny relative to PSC parents. Inbreeding depression was shown to be great in S. squalidus, with mean life history stage values ranging from 0.18 to 0.25. The strength of SI in S. squalidus appears not to have weakened in response to its rapid colonization of Britain. The avoidance of inbreeding depression is likely to be the primary factor maintaining strong SI in this successful colonizing species.

Isolation and characterization of a polymorphic stigma-specific class III peroxidase gene from Senecio squalidus L. (Asteraceae)

A novel stigma-specific class III peroxidase gene, SSP (Stigma-Specific Peroxidase), has been isolated from the self-incompatible daisy Senecio squalidus L. (Asteraceae). Expression of SSP in flower buds is developmentally regulated, with maximal levels of expression coinciding with anthesis, when stigmas are most receptive to pollen and when self-incompatibility is fully developed. In situ hybridization revealed SSP expression to be localized exclusively to the specialized secretory epidermal cells (papillae) of the stigma, which receive and discriminate pollen. SSP is therefore the first tissue-specific and cell-specific peroxidase gene identified in a plant. SSP belongs to a distinct clade of class III plant peroxidases that possess two introns, instead of the more normal situation of three conserved introns. The deduced amino acid sequence of SSP revealed a 27 amino acid signal peptide, suggesting that the SSP protein is secreted to the cell wall of the stigmatic papillae. In-gel peroxidase activity assays showed that SSP has relatively low peroxidase activity compared to other, as yet uncharacterized, peroxidases present in stigmatic extracts. Six SSP alleles have been cloned from different lines of S. squalidus carrying a range of self-incompatibility (S)-alleles but there was no consistent association between the presence of a particular SSP allele and S-genotype indicating that SSP is not the female determinant of SSI in S. squalidus. Nevertheless, the precise expression of SSP in stigmatic papillae suggests that it may have a more general function in pollen-stigma interactions, or alternatively in protection of stigmas from pathogen attack. Extensive database screens have identified homologues of SSP in other plant species, but available expression data for these genes indicates that none are flower-specific, suggesting that SSP represents a new functional type of class III peroxidase specific to the stigma. We discuss the possible function(s) of S. squalidus SSP in pollen-stigma interactions and in protection of stigmas from pathogen attack.

Inheritance and dominance of self-incompatibility alleles in polyploid Arabidopsis lyrata

Natural populations of diploid Arabidopsis lyrata exhibit the sporophytic type of self-incompatibility system characteristic of Brassicaceae, in which complicated dominance interactions among alleles in the diploid parent determine self-recognition phenotypes of both pollen and stigma. The purpose of this study was to investigate how polyploidy affects this already complex system. One tetraploid population (Arabidopsis lyrata ssp kawasakiana from Japan) showed complete self-compatibility and produced viable selfed progeny for at least three generations subsequent to field collection. In contrast, individuals from a second tetraploid population (A. lyrata ssp petraea from Austria) were strongly self-incompatible (SI). Segregation of SI genotypes in this population followed Mendelian patterns based on a tetrasomic model of inheritance, with two to four alleles per individual, independent segregation of alleles, and little evidence of dosage effects of alleles found in multiple copies. Similar to results from diploids, anomalous compatibility patterns involving particular combinations of individuals occurred at a low frequency in the tetraploids, suggesting altered dominance in certain genetic backgrounds that could be due to the influence of a modifier locus. Overall, dominance relationships among S-alleles in self-incompatible tetraploid families were remarkably similar to those in related diploids, suggesting that this very important and complicated locus has not undergone extensive modification subsequent to polyploidization.

Plant self-incompatibility in natural populations: a critical assessment of recent theoretical and empirical advances

Self-incompatibility systems in plants are genetic systems that prevent self-fertilization in hermaphrodites through recognition and rejection of pollen expressing the same allelic specificity as that expressed in the pistils. The evolutionary properties of these self-recognition systems have been revealed through a fascinating interplay between empirical advances and theoretical developments. In 1939, Wright suggested that the main evolutionary force driving the genetic and molecular properties of these systems was strong negative frequency-dependent selection acting on pollination success. The empirical observation of high allelic diversity at the self-incompatibility locus in several species, followed by the discovery of very high molecular divergence among alleles in all plant families where the locus has been identified, supported Wright's initial theoretical predictions as well as many of its later developments. In the last decade, however, advances in the molecular characterization of the incompatibility reaction and in the analysis of allelic frequencies and allelic divergence from natural populations have stimulated new theoretical investigations that challenged some important assumptions of Wright's model of gametophytic self-incompatibility. We here review some of these recent empirical and theoretical advances that investigated: (i) the hypothesis that S-alleles are selectively equivalent, and the evolutionary consequences of genetic interactions between alleles; (ii) the occurrence of frequency-dependent selection in female fertility; (iii) the evolutionary genetics of self-incompatibility systems in subdivided populations; (iv) the evolutionary implications of the self-incompatibility locus's genetic architecture; and (v) of its interactions with the genomic environment.

Population genetics of sporophytic self-incompatibility in Senecio squalidus L. (Asteraceae) II: a spatial autocorrelation approach to determining mating behaviour in the presence of low S allele diversity

We recently estimated that as few as six S alleles represent the extent of S locus diversity in a British population of the self-incompatible (SI) coloniser Senecio squalidus (Oxford Ragwort). Despite the predicted constraints to mating imposed by such a low number of S alleles, S. squalidus maintains a strong sporophytic self-incompatibility (SSI) system and there is no evidence for a breakdown of SSI or any obvious negative reproductive consequences for this highly successful coloniser. The present paper assesses mating behaviour in an Oxford S. squalidus population through observations of its effect on spatial patterns of genetic diversity and thus the extent to which it is responsible for ameliorating the potentially detrimental reproductive consequences of low S allele diversity in British S. squalidus. A spatial autocorrelation (SA) treatment of S locus and allozyme polymorphism data for four loci indicates that mating events regularly occur at all the distance classes examined from 60 to 480 m throughout the entire sample population. Less SA is observed for S locus data than for allozyme data in accordance with the hypothesis that SSI and low diversity at the S locus are driving these large-scale mating events. The limited population structure at small distances of 60 m and less observed for SA analysis of the Me-2 locus and by F-statistics for all the allozyme data, is evidence of some local relatedness due to limited seed and pollen dispersal in S. squalidus. However, the overall impression of mating dynamics in this S. squalidus population is that of ample potential mating opportunities with many individuals at large population scales, indicating that reproductive success is not seriously affected by few S alleles available for mating interactions.

The population genetics of sporophytic self-incompatibility in Senecio squalidus L. (Asteraceae): avoidance of mating constraints imposed by low S-allele number

Senecio squalidus L. (Asteraceae) has been the subject of several ecological and population genetic studies due to its well-documented history of introduction, establishment and spread throughout Britain in the past 300 years. Our recent studies have focused on identifying and quantifying factors associated with the sporophytic self-incompatibility (SSI) system of S. squalidus that may have contributed to its success as a colonist. These findings are of general biological interest because they provide important insights into the short-term evolutionary dynamics of a plant mating system. The number of S-alleles in populations and their dominance interactions were investigated in eight wild British populations using cross-diallel studies. The numbers of S-alleles in British S. squalidus populations are typically low (average of 5.3 S-alleles) and the entire British population is estimated to possess no more than 7-11 S-alleles. Such low numbers of S-alleles are most probably a consequence of population bottlenecks associated with introduction and colonization. Potential evolutionary impacts on SSI caused by a paucity of S-alleles, such as restricted mate availability, are discussed, and we suggest that increased dominance interactions between S-alleles may be an important short-term means of increasing mate availability when S-allele numbers are low.

The different mechanisms of sporophytic self-incompatibility

Flowering plants have evolved a multitude of mechanisms to avoid self-fertilization and promote outbreeding. Self-incompatibility (SI) is by far the most common of these, and is found in ca. 60% of flowering plants. SI is a genetically controlled pollen-pistil recognition system that provides a barrier to fertilization by self and self-related pollen in hermaphrodite (usually co-sexual) flowering plants. Two genetically distinct forms of SI can be recognized: gametophytic SI (GSI) and sporophytic SI (SSI), distinguished by how the incompatibility phenotype of the pollen is determined. GSI appears to be the most common mode of SI and can operate through at least three different mechanisms, two of which have been characterized extensively at a molecular level in the Solanaceae and Papaveraceae. Because molecular studies of SSI have been largely confined to species from the Brassicaceae, predominantly Brassica species, it is not yet known whether SSI, like GSI, can operate through different molecular mechanisms. Molecular studies of SSI are now being carried out on Ipomoea trifida (Convolvulaceae) and Senecio squalidus (Asteraceae) and are providing important preliminary data suggesting that SSI in these two families does not share the same molecular mechanism as that of the Brassicaceae. Here, what is currently known about the molecular regulation of SSI in the Brassicaceae is briefly reviewed, and the emerging data on SSI in I. trifida, and more especially in S. squalidus, are discussed.