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): the number, frequency, and dominance interactions of S alleles across its British range

Sporophytic self-incompatibility (SSI) was studied in 11 British Senecio squalidus populations to quantify mating system variation and determine how its recent colonization of the United Kingdom has influenced its mating behavior. S allele number, frequency, and dominance interactions in populations were assessed using full diallels of controlled pollinations. A mean of 5.1 S alleles per population was observed, and no population contained more than six S alleles. Numbers of S alleles within populations of S. squalidus declined with increasing distance from the center of its introduction (Oxford). Cross-classification of S alleles allowed an estimate of approximately seven and no more than 11 S alleles for the entire British S. squalidus population. The low number of S alleles observed in British S. squalidus compared to other SI species is consistent with the population bottleneck associated with S. squalidus' introduction to the Oxford Botanic Garden and subsequent colonization of Britain. Extensive S allele dominance interactions were observed to be a feature of the S. squalidus SSI system and may represent an adaptive response to improve limited mate availability imposed by the presence of so few S alleles. Multilocus allozyme genotypes were also identified for individuals in all populations and geographic patterns of S locus and allozyme loci variation investigated. Less interpopulation structure was observed for the S locus than for allozyme diversity--a finding indicative of the effects of negative frequency-dependent selection at the S locus maintaining equal S phenotypes within populations and enhancing effective migration between populations.     

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

Twenty-six individuals of the sporophytic self-incompatible (SSI) weed, Senecio squalidus were crossed in a full diallel to determine the number and frequency of S alleles in an Oxford population. Incompatibility phenotypes were determined by fruit-set results and the mating patterns observed fitted a SSI model that allowed us to identify six S alleles. Standard population S allele number estimators were modified to deal with S allele data from a species with SSI. These modified estimators predicted a total number of approximately six S alleles for the entire Oxford population of S. squalidus. This estimate of S allele number is low compared to other estimates of S allele diversity in species with SSI. Low S allele diversity in S. squalidus is expected to have arisen as a consequence of a disturbed population history since its introduction and subsequent colonisation of the British Isles. Other features of the SSI system in S. squalidus were also investigated: (a) the strength of self-incompatibility response; (b) the nature of S allele dominance interactions; and (c) the relative frequencies of S phenotypes. These are discussed in view of the low S allele diversity estimates and the known population history of S. squalidus.     

The role of stigma peroxidases in flowering plants: insights from further characterization of a stigma-specific peroxidase (SSP) from Senecio squalidus (Asteraceae)

Angiosperm stigmas have long been known to exhibit high levels of peroxidase activity when they are mature and most receptive to pollen but the biological function of stigma peroxidases is not known. A novel stigma-specific class III peroxidase gene, SSP (stigma-specific peroxidase) expressed exclusively in the stigmas of Senecio squalidus L. (Asteraceae) has recently been identified. Expression of SSP is confined to the specialized secretory cells (papillae) that compose the stigma epidermis. The literature on stigma peroxidases and hypotheses on their function(s) is reviewed here before further characterization of SSP and an attempt to determine its function are described. It is shown that SSP is localized to cytoplasmic regions of stigmatic papillae and also to the surface of these cells, possibly as a component of the pellicle, a thin layer of condensed protein typical of "dry" stigmas. Enzyme assays on recombinant SSP showed it to be a peroxidase with a preference for diphenolic substrates (ABTS and TMB) and a pH optimum of approximately 4.5. In such assays the peroxidase activity of SSP was low when compared with horseradish peroxidase. To explore the function of SSP and other stigmatic peroxidases, levels of reactive oxygen species (ROS) in stigmas of S. squalidus were investigated. Relatively large amounts of ROS, principally H(2)O(2), were detected in S. squalidus stigmas where most ROS/H(2)O(2) was localized to the stigmatic papillae, the location of SSP. These observations are discussed in the context of possible functions for SSP, other peroxidases, and ROS in the stigmas of angiosperms.     

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.     

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.     

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.     

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 <Emphasis Type="Italic">Senecio squalidus</Emphasis> 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.     

Recent hybrid origin and invasion of the British Isles by a self-incompatible species, Oxford ragwort (Senecio squalidus L., Asteraceae)

Senecio squalidus is a diploid hybrid species which originated in the British Isles following the introduction of material collected from a hybrid zone on Mount Etna, Sicily, approximately 300 years ago. Introduced hybrid material was cultivated in the Oxford Botanic Garden and gave rise to the stabilized diploid hybrid species, which later spread throughout much of the UK and into some parts of Ireland. Unusually for an invasive species, S. squalidus has a strong system of sporophytic self-incompatibility (SSI) that may have become modified as a result of its recent hybrid origin and spread. First, S. squalidus contains relatively few S alleles (between 2 and 6 S alleles within individual UK populations) compared to other species with SSI (estimates average ~17 S alleles per population). This most probably reflects the population bottleneck experienced by introduced hybrid material. Second, dominance relationships among S. squalidus S alleles are more extensive than those reported in other species with SSI. Third, although pseudo-self-compatibility occurs sporadically in S. squalidus, it is not widespread, indicating that SSI is maintained in the species despite potential mate availability restrictions imposed by low numbers of S alleles. Surveys of other forms of genetic diversity in S. squalidus show that allozyme variation is reduced relative to that within the progenitor species, but Randomly Amplified Polymorphic DNA variation is relatively high. Both types of genetic variation show little or no pattern of isolation-by-distance between populations in keeping with the recent range expansion of the species. During its spread in the British Isles, S. squalidus has hybridized with the native self-compatible (SC) tetraploid species, S. vulgaris, which has led to the origin of three new SC hybrid taxa: a radiate form of S. vulgaris (var. hibernicus), a tetrapoid hybrid species (S. eboracensis) and an allohexaploid (S. cambrensis).     

Production of reactive oxygen species and reactive nitrogen species by angiosperm stigmas and pollen: potential signalling crosstalk?

Angiosperm stigmas exhibit high levels of peroxidase activity when receptive to pollen. To explore possible function(s) of this peroxidase activity we investigated amounts of reactive oxygen species (ROS), particularly hydrogen peroxide, in stigmas and pollen. Because nitric oxide (NO) was recently implicated in pollen tube growth, we also investigated amounts of NO in pollen and stigmas. Reactive oxygen species accumulation was assessed with confocal microscopy and light microscopy using ROS probes DCFH2-DA and TMB, respectively. NO was assayed using the NO probe DAF-2DA and confocal microscopy. Stigmas from various different angiosperms were found to accumulate ROS, predominantly H2O2, constitutively. In Senecio squalidus and Arabidopsis thaliana high amounts of ROS/H2O2 were localized to stigmatic papillae. ROS/H2O2 amounts appeared reduced in stigmatic papillae to which pollen grains had adhered. S. squalidus and A. thaliana pollen produced relatively high amounts of NO compared with stigmas; treating stigmas with NO resulted in reduced amounts of stigmatic ROS/H2O2. Constitutive accumulation of ROS/H2O2 appears to be a feature of angiosperm stigmas. This novel finding is discussed in terms of a possible role for stigmatic ROS/H2O2 and pollen-derived NO in pollen-stigma interactions and defence.     

Fewer S‐alleles are maintained in plant populations with sporophytic as opposed to gametophytic self‐incompatibility

Plants use self‐incompatibility to reject pollen bearing alleles in common at the S‐locus. These systems are classified as gametophytic (GSI) if recognition involves haploid pollen or sporophytic (SSI) if recognition involves diploid paternal genotypes. Dominance in SSI systems reduces the number of S‐alleles, but it has not been clear which system should maintain greater diversity when all else is equal. We simulated finite populations to compare the equilibrium number of S‐alleles in populations with either GSI or a co‐dominant SSI system. When population size was constant, SSI systems maintained more S‐alleles than GSI systems. When populations fluctuated in response to an S‐Allee effect, fewer S‐alleles were observed in SSI systems when S‐allele diversity was low, and SSI populations were vulnerable to extinction over a broader range of parameters. Turnover rates at the S‐locus were also faster in SSI populations experiencing strong S‐Allee effects. Given the variable expectations concerning S‐allele diversity in these systems, we reviewed published estimates of S‐allele diversity. GSI populations have significantly more S‐alleles on average than SSI populations (GSI = 25.70 and SSI = 16.80). Dominance likely contributes to this pattern, although the demographic consequences of the S‐Allee effect may be important in populations with fewer than 10 S‐alleles.     

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.     

中国维吾尔族人群MSY1(DYF155S1)基因座多态性及其结构特点

应用荧光标记MVR-PCR、Amp-FLP与DNA序列分析技术等检测106例中国维吾尔族人群无关男性个体血纱样品,揭示了中国维吾尔族人群Y特异的小卫星MSY1 (DYF155S1)基因座5′和3′端多态性及其基因结构特点。DYF155S1基因座的多态性表现为3个方面：（1）长度多态性;（2）5′端多态性;（3）3′端多态性。106例无关个体共检出37个不同长度的片段,5′端检出68个类型,3′端检出23个类型。综合这3方面多态性,106例个体间没有相同,其基因多样性（h）超过0.9999。DNA序列分析发现该基因座5′端表现有7种模块结构,3′端有2种模块结构。DYF155S2片段缺失率约为4.7%。MVR-PCR、Amp-FLP与DNA序列分析技术结合起来可以更充分地揭示人群Y染色体特异的小卫星MSY1（DYF155S1）基因座多态性,并提出命名方式,从而为人类遗传学及法医学研究提供了有用的方法和基础资料。 Abstract:The study is to reveal the diversity and gene structure of 5′ and 3′ end of DYF155S1 locus in Y-chromosome minisatellite among Chinese Uygur population.Fluorescent MVR-PCR（minisatellite variant repeat by PCR）,Amp-FLP(Amplified fragment length polymorphism) and DNA sequencing methods were used repectively to detect 106 unrelated males among Chinese Uygur population.The polymorphisms of DYF155S1 locus could be revealed in three aspects:(1) polymorphic length:the sizes of amplified fragments ranged from 1405 to 2505bp.There are 37 types found among the 106 unrelated males.(2) polymorphism at 5′ end of DYF155S1 locus,68 types found among the 106 unrelated males.(3) polymorphism at 3′ end of DYF155S1 locus,23 types found among the 106 unrelated males.In combination of these three aspects of polymorphism,none of the 106 unrelated males tested had the same allele,and the gene diversity(h) was over 0.9999.Seven and two types of modular structure were founded in the 5′ and 3′ end of DYF155S1 locus,respectively,by DNA sequencing.The alleles at DYF155S2 locus showed yes/no dimorphism and the rate of deletion was 4.7%.The polymorphisms of DYF155S1 locus were fully revealed by using combination of MVR-PCR, Amp-FLP and DNA sequencing methods, and we suggested the nomenclature for alleles of MVR loci.These methods are useful tools and provide basic data for the study of human genetics and forensic medicine.     

Population decline despite high genetic diversity in the new allopolyploid species Senecio cambrensis (Asteraceae)

Senecio cambrensis (Welsh groundsel) is a new allohexaploid species, which originated in Wales, UK, in the early part of the 20th century following hybridization between the native tetraploid groundsel (Senecio vulgaris) and the introduced diploid Oxford ragwort (Senecio squalidus). A survey of the number of populations and flowering individuals per population of S. cambrensis in Wales was conducted at peak flowering time in June 2002, 2003 and 2004. The results show a dramatic decrease in both population number and population size of the species since the 1980s when the last population census was conducted. A survey of amplified fragment length polymorphism (AFLP) variation showed that this decline has occurred despite the fact that S. cambrensis contains a high level of genetic diversity with each individual screened possessing a unique multilocus phenotype. The level of variance within the species was similar to that found in one parent (S. vulgaris) and slightly greater than that among samples of the other parent (S. squalidus). Only a small proportion (5%) of AFLP diversity was partitioned among populations indicating a lack of population structure and possibly high levels of gene flow via seed dispersal in what is predominantly a selfing species. Senecio cambrensis showed closer similarity in AFLP phenotype to S. vulgaris than to S. squalidus. Possible causes of this and also the high level of AFLP diversity found in S. cambrensis are discussed. It is suggested that intergenomic recombination following occasional multivalent formation during meiosis in S. cambrensis is likely to be an important cause of both phenomena, although other causes are not ruled out.     

Molecular genetic and quantitative trait divergence associated with recent homoploid hybrid speciation: a study of Senecio squalidus (Asteraceae)

Hybridization is increasingly seen as a trigger for rapid evolution and speciation. To quantify and qualify divergence associated with recent homoploid hybrid speciation, we compared quantitative trait (QT) and molecular genetic variation between the homoploid hybrid species Senecio squalidus and its parental species, S. aethnensis and S. chrysanthemifolius, and also their naturally occurring Sicilian hybrids. S. squalidus originated and became invasive in the United Kingdom following the introduction of hybrid plants from Mount Etna, Sicily, about 300 years ago. We recorded considerable molecular genetic differentiation between S. squalidus and its parents and their Sicilian hybrids in terms of both reduced genetic diversity and altered allele frequencies, potentially due to the genetic bottleneck associated with introduction to the United Kingdom. S. squalidus is also distinct from its parents and Sicilian hybrids for QTs, but less so than for molecular genetic markers. We suggest that this is due to resilience of polygenic QTs to changes in allele frequency or lack of selection for hybrid niche divergence in geographic isolation. While S. squalidus is intermediate or parental-like for most QTs, some trangressively distinct traits were observed, which might indicate emerging local adaptation in its invasive range. This study emphasizes the important contribution of founder events and geographic isolation to successful homoploid hybrid speciation.     

EVOLUTION OF DOMINANCE IN SPOROPHYTIC SELF-INCOMPATIBILITY SYSTEMS: I. GENETIC LOAD AND COEVOLUTION OF LEVELS OF DOMINANCE IN POLLEN AND PISTIL

Recent theoretical advances have suggested that various forms of balancing selection may promote the evolution of dominance through an increase of the proportion of heterozygote genotypes. We test whether dominance can evolve in the sporophytic self-incompatibility (SSI) system in plants. SSI prevents mating between individuals expressing identical SI phenotypes by recognition of pollen by pistils, which avoids selfing and inbreeding depression. SI phenotypes depend on a complex network of dominance relationships between alleles at the self-incompatibility locus ( S -locus). Empirical studies suggest that these relationships are not random, but the exact evolutionary processes shaping these relationships remain unclear. We investigate the expected patterns of dominance under the hypothesis that dominance is a direct target of natural selection. We follow the fate of a mutant allele at the S -locus whose dominance relationships are changed but whose specificity remains unaltered. We show that strict codominance is not evolutionarily stable in SSI, and that inbreeding depression due to deleterious mutations linked or unlinked to the S -locus exerts strong constraints on changes in relative levels of dominance in pollen and pistil. Our results provide a general adaptive explanation for most patterns of dominance relationships empirically observed in natural plant populations.     

Extreme changes to gene expression associated with homoploid hybrid speciation

Hybridization is an important cause of abrupt speciation. Hybrid speciation without a change in ploidy (homoploid hybrid speciation) is well-established in plants but has also been reported in animals and fungi. A notable example of recent homoploid hybrid speciation is Senecio squalidus (Oxford ragwort), which originated in the UK in the 18th Century following introduction of hybrid material from a hybrid zone between S. chrysanthemifolius and S. aethnensis on Mount Etna, Sicily. To investigate genetic divergence between these taxa, we used complementary DNA microarrays to compare patterns of floral gene expression. These analyses revealed major differences in gene expression between the parent species and wild and resynthesized S. squalidus . Comparisons of gene expression between S. aethnensis , S. chrysanthemifolius and natural S. squalidus identified genes potentially involved in local environmental adaptation. The analysis also revealed non-additive patterns of gene expression in the hybrid relative to its progenitors. These expression changes were more dramatic and widespread in resynthesized hybrids than in natural S. squalidus , suggesting that a unique expression pattern may have been fixed during the allopatric divergence of British S. squalidus . We speculate that hybridization-induced gene-expression change may provide an immediate source of novel phenotypic variation upon which selection can act to facilitate homoploid hybrid speciation in plants.     

Origin of allelic diversity in antirrhinum S locus RNases.

In many plant species, self-incompatibility (SI) is genetically controlled by a single multiallelic S locus. Previous analysis of S alleles in the Solanaceae, in which S locus ribonucleases (S RNases) are responsible for stylar expression of SI, has demonstrated that allelic diversity predated speciation within this family. To understand how allelic diversity has evolved, we investigated the molecular basis of gametophytic SI in Antirrhinum, a member of the Scrophulariaceae, which is closely related to the Solanaceae. We have characterized three Antirrhinum cDNAs encoding polypeptides homologous to S RNases and shown that they are encoded by genes at the S locus. RNA in situ hybridization revealed that the Antirrhinum S RNase are primarily expressed in the stylar transmitting tissue. This expression is consistent with their proposed role in arresting the growth of self-pollen tubes. S alleles from the Scrophulariaceae form a separate group from those of the Solanaceae, indicating that new S alleles have been generated since these families separated (approximately 40 million years). We propose that the recruitment of an ancestral RNase gene into SI occurred during an early stage of angiosperm evolution and that, since that time, new alleles subsequently have arisen at a low rate.