Production of Antioxidant Molecules in Polygonum aviculare (L.) and Senecio vulgaris (L.) under Metal Stress: A Possible Tool in the Evaluation of Plant Metal Tolerance

Plants growing on heavy metal (HM)-polluted soils show toxicity symptoms, such as chlorosis and growth reduction, and undergo oxidative stress due to the formation of reactive oxygen species (ROS). Plants overcome oxidative stress by producing a wide range of antioxidant molecules, such as polyphenols and flavonoids. The aim of the present work was to study the accumulation of these molecules in response to increasing concentrations of Cd, Cr, Cu, Ni, Pb and Zn and to assess whether they can be used as a tool in assessing metal-related stress in Polygonum aviculare and Senecio vulgaris. On average, P. aviculare shoots accumulated lower amounts of metals than S. vulgaris shoots. The uptake of all six elements was correlated and proportional to their concentration in the nutrient solution (ρ > 0.9), with the bioaccumulation factor (BAF) being >1 for most of them. The present research demonstrated that 82% of the samples showed a good correlation (|ρ| > 0.5) between the level of polyphenols, flavonoids and antioxidant activity and the metal concentration in plant shoots, confirming that the metal stress level and production of phenolic compounds having antioxidant activity were strictly connected. Nonetheless, the mere quantification of these molecules cannot identify the type of metal that caused the oxidative stress, neither determine the concentration of the stressors. The five tested populations of each species did not show any specific adaptation to the environment of origin.

Arbuscular Mycorrhizal Fungi and Plant Chemical Defence: Effects of Colonisation on Aboveground and Belowground Metabolomes

Arbuscular mycorrhizal fungal (AMF) colonisation of plant roots is one of the most ancient and widespread interactions in ecology, yet the systemic consequences for plant secondary chemistry remain unclear. We performed the first metabolomic investigation into the impact of AMF colonisation by Rhizophagus irregularis on the chemical defences, spanning above- and below-ground tissues, in its host-plant ragwort (Senecio jacobaea). We used a non-targeted metabolomics approach to profile, and where possible identify, compounds induced by AMF colonisation in both roots and shoots. Metabolomics analyses revealed that 33 compounds were significantly increased in the root tissue of AMF colonised plants, including seven blumenols, plant-derived compounds known to be associated with AMF colonisation. One of these was a novel structure conjugated with a malonyl-sugar and uronic acid moiety, hitherto an unreported combination. Such structural modifications of blumenols could be significant for their previously reported functional roles associated with the establishment and maintenance of AM colonisation. Pyrrolizidine alkaloids (PAs), key anti-herbivore defence compounds in ragwort, dominated the metabolomic profiles of root and shoot extracts. Analyses of the metabolomic profiles revealed an increase in four PAs in roots (but not shoots) of AMF colonised plants, with the potential to protect colonised plants from below-ground organisms.

Control of Floret Symmetry by RAY3, SvDIV1B, and SvRAD in the Capitulum of Senecio vulgaris

All members of Asteraceae, the largest flowering family, have a unique compressed inflorescence known as a capitulum, which resembles a solitary flower. The capitulum often consists of bilateral (zygomorphic) ray florets and radial (actinomorphic) disc florets. In Antirrhinum majus, floral zygomorphy is established by the interplay between dorsal petal identity genes, CYCLOIDEA (CYC) and RADIALIS (RAD), and a ventral gene DIVARICATA (DIV). To investigate the role of CYC, RAD, and DIV in the development of ray and disc florets within a capitulum, we isolated homologs of these genes from an Asteraceae species, Senecio vulgaris (common groundsel). After initial uniform expression of RAY3 (CYC), SvRAD, and SvDIV1B in ray florets only, RAY3 and SvRAD were exclusively expressed in the ventral petals of the ray florets. Our functional analysis further showed that RAY3 promotes and SvDIV1B represses petal growth, confirming their roles in floral zygomorphy. Our results highlight that while floral symmetry genes such as RAY3 and SvDIV1B appear to have a conserved role in petal growth in both Senecio and Antirrhinum, the regulatory relationships and expression domains are divergent, allowing ventral petal elongation in Senecio versus dorsal petal elongation in Antirrhinum In S vulgaris, diversification of CYC genes has led to novel interactions; SvDIV1B inhibits RAY3 and SvRAD, and may activate RAY2 This highlights how recruitment of floral symmetry regulators into dynamic networks was crucial for creating a complex and elaborate structure such as the capitulum.

Ecological differentiation of diploid and polyploid cytotypes of Senecio carniolicus sensu lato (Asteraceae) is stronger in areas of sympatry

BACKGROUND AND AIMS

Ecological differentiation is recognized as an important factor for polyploid speciation, but little is known regarding whether the ecological niches of cytotypes differ between areas of sympatry and areas where single cytotypes occur (i.e. niche displacement).

METHODS

Ecological niches of four groups of Senecio carniolicus sensu lato (s.l.) (western and eastern diploid lineages, tetraploids and hexaploids) were characterized via Landolt indicator values of the accompanying vascular plant species and tested using multivariate and univariate statistics.

KEY RESULTS

The four groups of S. carniolicus s.l. were ecologically differentiated mainly with respect to temperature, light and soil (humus content, nutrients, moisture variability). Niche breadths did not differ significantly. In areas of sympatry hexaploids shifted towards sites with higher temperature, less light and higher soil humus content as compared with homoploid sites, whereas diploids and tetraploids shifted in the opposite direction. In heteroploid sites of tetraploids and the western diploid lineage the latter shifted towards sites with lower humus content but higher aeration.

CONCLUSIONS

Niche displacement can facilitate the formation of stable contact zones upon secondary contact of polyploids and their lower-ploid ancestors and/or lead to convergence of the cytotypes' niches after they have attained non-overlapping ranges. Niche displacement is essential for understanding ecological consequences of polyploidy.

Application of heat stress in situ demonstrates a protective role of irradiation on photosynthetic performance in alpine plants

The impact of sublethal heat on photosynthetic performance, photosynthetic pigments and free radical scavenging activity was examined in three high mountain species, Rhododendron ferrugineum, Senecio incanus and Ranunculus glacialis using controlled in situ applications of heat stress, both in darkness and under natural solar irradiation. Heat treatments applied in the dark reversibly reduced photosynthetic performance and the maximum quantum efficiency of photosystem II (Fv /Fm), which remained impeded for several days when plants were exposed to natural light conditions subsequently to the heat treatment. In contrast, plants exposed to heat stress under natural irradiation were able to tolerate and recover from heat stress more readily. The critical temperature threshold for chlorophyll fluorescence was higher under illumination (Tc (')) than in the dark (Tc). Heat stress caused a significant de-epoxidation of the xanthophyll cycle pigments both in the light and in the dark conditions. Total free radical scavenging activity was highest when heat stress was applied in the dark. This study demonstrates that, in the European Alps, heat waves can temporarily have a negative impact on photosynthesis and, importantly, that results obtained from experiments performed in darkness and/or on detached plant material may not reliably predict the impact of heat stress under field conditions.

Testing the generalist-specialist dilemma: the role of pyrrolizidine alkaloids in resistance to invertebrate herbivores in Jacobaea species

Plants produce a diversity of secondary metabolites (SMs) to protect them from generalist herbivores. On the other hand, specialist herbivores use SMs for host plant recognition, feeding and oviposition cues, and even sequester SMs for their own defense. Therefore, plants are assumed to face an evolutionary dilemma stemming from the contrasting effects of generalist and specialist herbivores on SMs. To test this hypothesis, bioassays were performed with F2 hybrids from Jacobaea species segregating for their pyrrolizidine alkaloids (PAs), using a specialist flea beetle (Longitarsus jacobaeae) and a generalist slug (Deroceras invadens). Our study demonstrated that while slug feeding damage was negatively correlated with the concentration of total PAs and that of senecionine-like PAs, flea beetle feeding damage was not affected by PAs. It was positively correlated though, with leaf fresh weight. The generalist slug was deterred by senecionine-like PAs but the specialist flea beetle was adapted to PAs in its host plant. Testing other herbivores in the same plant system, it was observed that the egg number of the specialist cinnabar moth was positively correlated with jacobine-like PAs, while the silver damage of generalist thrips was negatively correlated with senecionine- and jacobine-like PAs, and the pupae number of generalist leaf miner was negatively correlated with otosenine-like PAs. Therefore, while the specialist herbivores showed no correlation whatsoever with PA concentration, the generalist herbivores all showed a negative correlation with at least one type of PA. We concluded that the generalist herbivores were deterred by different structural groups of PAs while the specialist herbivores were attracted or adapted to PAs in its host plants.

Exogenous selection shapes germination behaviour and seedling traits of populations at different altitudes in a Senecio hybrid zone

BACKGROUND AND AIMS

The Senecio hybrid zone on Mt Etna, Sicily, is characterized by steep altitudinal clines in quantitative traits and genetic variation. Such clines are thought to be maintained by a combination of 'endogenous' selection arising from genetic incompatibilities and environment-dependent 'exogenous' selection leading to local adaptation. Here, the hypothesis was tested that local adaptation to the altitudinal temperature gradient contributes to maintaining divergence between the parental species, S. chrysanthemifolius and S. aethnensis.

METHODS

Intra- and inter-population crosses were performed between five populations from across the hybrid zone and the germination and early seedling growth of the progeny were assessed.

KEY RESULTS

Seedlings from higher-altitude populations germinated better under low temperatures (9-13 °C) than those from lower altitude populations. Seedlings from higher-altitude populations had lower survival rates under warm conditions (25/15 °C) than those from lower altitude populations, but also attained greater biomass. There was no altitudinal variation in growth or survival under cold conditions (15/5 °C). Population-level plasticity increased with altitude. Germination, growth and survival of natural hybrids and experimentally generated F(1)s generally exceeded the worse-performing parent.

CONCLUSIONS

Limited evidence was found for endogenous selection against hybrids but relatively clear evidence was found for divergence in seed and seedling traits, which is probably adaptive. The combination of low-temperature germination and faster growth in warm conditions might enable high-altitude S. aethnensis to maximize its growth during a shorter growing season, while the slower growth of S. chrysanthemifolius may be an adaptation to drought stress at low altitudes. This study indicates that temperature gradients are likely to be an important environmental factor generating and maintaining adaptive divergence across the Senecio hybrid zone on Mt Etna.

Pollen-pistil interactions and self-incompatibility in the Asteraceae: new insights from studies of Senecio squalidus (Oxford ragwort)

BACKGROUND

Pollen-pistil interactions are an essential prelude to fertilization in angiosperms and determine compatibility/incompatibility. Pollen-pistil interactions have been studied at a molecular and cellular level in relatively few families. Self-incompatibility (SI) is the best understood pollen-pistil interaction at a molecular level where three different molecular mechanisms have been identified in just five families. Here we review studies of pollen-pistil interactions and SI in the Asteraceae, an important family that has been relatively understudied in these areas of reproductive biology.

SCOPE

We begin by describing the historical literature which first identified sporophytic SI (SSI) in species of Asteraceae, the SI system later identified and characterized at a molecular level in the Brassicaceae. Early structural and cytological studies in these two families suggested that pollen-pistil interactions and SSI were similar, if not the same. Recent cellular and molecular studies in Senecio squalidus (Oxford ragwort) have challenged this belief by revealing that despite sharing the same genetic system of SSI, the Brassicaceae and Asteraceae molecular mechanisms are different. Key cellular differences have also been highlighted in pollen-stigma interactions, which may arise as a consequence of the Asteraceae possessing a 'semi-dry' stigma, rather than the 'dry' stigma typical of the Brassicaceae. The review concludes with a summary of recent transcriptomic analyses aimed at identifying proteins regulating pollen-pistil interactions and SI in S. squalidus, and by implication the Asteraceae. The Senecio pistil transcriptome contains many novel pistil-specific genes, but also pistil-specific genes previously shown to play a role in pollen-pistil interactions in other species.

CONCLUSIONS

Studies in S. squalidus have shown that stigma structure and the molecular mechanism of SSI in the Asteraceae and Brassicaceae are different. The availability of a pool of pistil-specific genes for S. squalidus offers an opportunity to elucidate the molecular mechanisms of pollen-pistil interactions and SI in the Asteraceae.

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 effectiveness of management interventions used to control ragwort species

Ragwort (Senecio jacobaea and S. aquaticus) causes major losses to agricultural revenue and induces livestock morbidity throughout parts of Europe, North America, and Australasia. The introduction of legislation in the U.K. and Australia has meant that landowners can be prosecuted if the plant spreads to adjacent land, which has led to an increase in activities attempting to control these species. Commonly used interventions include natural enemies, herbicide applications, manual and mechanical removal. Through the use of explicit systematic methodology involving comprehensive searches and detailed inclusion criteria, data from primary research are collated for each type of intervention. Meta-analyses show that 2,4-D, Asulam, Clopyralid, and MCPA are effective at reducing ragwort densities. However, when the datasets were analysed for their effectiveness against individual species, 2,4-D and MCPA were only effective against S. jacobaea, while Asulam was only effective against S. aquaticus. Natural enemies Longitarsus jacobaeae and a combination of L. jacobaeae and Tyria jacobaeae appear to have the potential to reduce S. jacobaea densities. Only applying T. jacobaeae does not appear to significantly reduce S. jacobaea densities, but does reduce the number of capitula per plant, seeds per capitula, viability of seeds, and dry weight of the plants. There is insufficient experimental evidence available to assess other interventions such as manual or mechanical removal. Further research into these types of interventions is recommended, as well as more detailed reporting of site characteristics and experimental design to allow full investigation of each intervention to explain possible reasons for variations in their effectiveness.

Can hybridization cause local extinction: a case for demographic swamping of the Australian native Senecio pinnatifolius by the invasive Senecio madagascariensis?

Hybridization between native and invasive species can have several outcomes, including enhanced weediness in hybrid progeny, evolution of new hybrid lineages and decline of hybridizing species. Whether there is a decline of hybridizing species largely depends on the relative frequencies of parental taxa and the viability of hybrid progeny. Here, the individual- and population-level consequences of hybridization between the Australian native Senecio pinnatifolius and the exotic Senecio madagascariensis were investigated with amplified fragment length polymorphism (AFLP) markers, and this information was used to estimate the annual loss of viable seeds to hybridization. A high frequency (range 8.3-75.6%) of hybrids was detected in open pollinated seeds of both species, but mature hybrids were absent from sympatric populations. A hybridization advantage was observed for S. madagascariensis, where significantly more progeny than expected were sired based on proportional representation of the two species in sympatric populations. Calculations indicated that S. pinnatifolius would produce less viable seed than S. madagascariensis, if hybridization was frequency dependent and S. madagascariensis reached a frequency of between 10 and 60%. For this native-exotic species pair, prezygotic isolating barriers are weak, but low hybrid viability maintains a strong postzygotic barrier to introgression. As a result of asymmetric hybridization, S. pinnatifolius would appear to be under threat if S. madagascariensis increases numerically in areas of contact.

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.

Reciprocal hybridization at different times between Senecio flavus and Senecio glaucus gave rise to two polyploid species in north Africa and south-west Asia

The analysis of hybrid plant taxa using molecular methods has considerably extended understanding of possible pathways of hybrid evolution. Here, we investigated the origin of the tetraploid Senecio mohavensis ssp. breviflorus and the hexaploid Senecio hoggariensis by sequencing of nuclear and chloroplast DNA, and by analysis of the distribution of taxon-specific amplified fragment length polymorphism (AFLP) fragments. Both taxa originated from hybridization between the diploid Senecio flavus and Senecio glaucus. Whereas S. glaucus was the female parent in the origin of S. mohavensis ssp. breviflorus, S. flavus was the female parent in the origin of S. hoggariensis. The distribution of AFLP fragments suggests that S. hoggariensis is an allohexaploid species with two diploid genomes of S. glaucus and one diploid genome of S. flavus. The high frequency of S. flavus-specific fragments in S. mohavensis ssp. breviflorus is explained either as the result of introgression between a primary hybrid and S. flavus or as the result of intergenomic recombination in a primary hybrid. These two alternative processes cannot easily be distinguished.

Maternal effects and heterosis influence the fitness of plant hybrids

Here we tested two possible nonexclusive explanations for the maintenance of a hybrid swarm between Senecio jacobaea and Senecio aquaticus; first, that genotype-by-environment interactions involving water and nutrient clines are involved in hybrid fitness, and second, heterosis in early hybrid generations may provide an initial hybrid advantage that contributes to hybrid persistence. In three climate chamber studies, fitness and root growth were measured for parental species and natural and artificial F1 hybrids, in order to determine whether hybrids occur in habitats where they are more fit than parental species. Natural hybrids, which are generally back-crossed to S. jacobaea, always equaled S. jacobaea in growth characteristics. Maternal effects played a role in the fitness of F1 hybrids, with offspring from S. jacobaea mothers exhibiting higher fitness than those from S. aquaticus mothers, and compared with parental species and natural hybrids. Natural hybrids are not distributed in zones where they are most fit with respect to nutrient and water regimes. Superior fitness of early generation hybrids may contribute to hybrid swarm stability.

Disentangling food quality from resistance against parasitoids: diet choice by a generalist caterpillar

The relative importance of food quality and enemy-reduced space is a central but unresolved issue in the evolutionary ecology of host use by phytophagous insects. Indeed, a practical obstacle to experimentally disentangling the functional roles of these factors is the host specificity of insect herbivores, particularly toxic plant specialists. In this study, we employ a toxic plant generalist to uniquely disentangle these alternative explanations. We experimentally demonstrate that the value of enemy-reduced space supersedes that of food quality in determining the diet and host preference of the polyphagous woolly bear caterpillar Grammia geneura (Lepidoptera: Arctiidae). Caterpillars sacrificed superior growth efficiency in choosing a mixed diet that included toxic host plants and provided resistance against parasitoids. The resistance of individual caterpillars was associated with the relative amount of defensive plants eaten as well as with the sequestration of pyrrolizidine alkaloids from one such plant (Senecio longilobus).

Stomatal conductance in a tropical xerophilous shrubland at a lava substratum

Diurnal variation in leaf stomatal conductance (gs) of three xerophilous species (Buddleia cordata, Senecio praecox and Dodonaea viscosa) was measured over a 10-month period during the dry and wet seasons in a shrubland that is developing in a lava substratum in Mexico. Averaged stomatal conductances were 147 and 60.2 (B. cordata), 145 and 24.8 (D. viscosa) and 142.8 and 14.1 mmol m(-2) s(-1) (S. praecox) during the wet and dry season respectively. Leaf water potential (Psi) varied in a range of -0.6 to -1.2 (S. praecox), -0.6 to -1.8 (B. cordata) and -0.9 to -3.4 MPa (D. viscosa) during the same measurement periods. Stomata were more sensitive to changes in irradiance, air temperature and leaf-air vapour pressure difference in the rainy season than the dry season. Although stomatal responses to Psi were difficult to distinguish in any season (dry or rainy), data for the entire period of measurement showed a positive correlation, stomata tending to open as Psi increased, but there is strong evidence of isohydric behaviour in S. praecox and B. cordata. A multiplicative model relating gs to environmental variables and to Psi accounted for 79%-83% of the variation of gs in three sites (pooled data); however, the performance of the model was poorer (60%-76%) for individual species from other sites not included in the pooled data.

Contrasting dynamics in the same plant-herbivore interaction

Long-term studies of two-species interactions under field conditions are unusual; most long-term field studies are of single species dynamics (1-6). Concurrent long-term studies on the dynamics of the same two interacting species in different locations are very rare. This result has led to the tacit assumption that different cases of the same two-species interaction would involve essentially quantitative differences (e.g., context-specific differences in the numeric values of demographic parameters like fecundity or death rates). Here, we show that for one of the best-known two-species systems (ragwort and cinnabar moth), this finding does not hold. The interaction between the plant and its herbivore is fundamentally different in coastal dunes in The Netherlands and in grasslands in Southeast England. In the first case, the dynamics are cyclic and the interaction involves both direct and delayed density dependence; in the second case, the insect has little impact on plant dynamics and there are no time lags in density dependence. The difference is caused by differences in the importance of seed-limitation in plant recruitment in the two locations.

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.

Plant introductions, hybridization and gene flow

Many regional floras contain a high proportion of recently introduced plant species. Occasionally, hybridization between an introduced species and another species (introduced or native) can result in interspecific gene flow. This may occur even in instances where the F(1) hybrid shows very high sterility, but occasionally produces a few viable gametes. We provide examples of gene flow occurring between some rhododendrons recently introduced to the British flora, and between an introduced and native Senecio species. Neutral molecular markers have normally been employed to obtain evidence of interspecific gene flow, but the challenge now is to isolate and characterize functional introgressed genes and to determine how they affect the fitness of introgressants and whether they improve adaptation to novel habitats allowing introgressants to expand the range of a species. We outline a candidate gene approach for isolating and characterizing an allele of the RAY gene in Senecio vulgaris, which is believed to have introgressed from S. squalidus, and which causes the production of ray florets in flower heads. We discuss the effects of this introgressed allele on individual fitness, including those that originate directly from the production of ray florets plus those that may arise from pleiotropy and/or linkage.

Possible causes of morphological variation in an endemic Moroccan groundsel (Senecio leucanthemifolius var. casablancae): evidence from chloroplast DNA and random amplified polymorphic DNA markers

Genetic variation was assessed in Senecio leucanthemifolius var. casablancae (Compositae), a Moroccan Atlantic coast endemic, in order to examine possible causes of atypical leaf morphology in three populations south of the known range. Evidence for introgression from S. glaucus ssp. coronopifolius and/or divergence was investigated with molecular markers. Both random amplified polymorphic DNA (RAPD) and chloroplast (cp) DNA restriction fragment length polymorphism (RFLP) differentiated the species well. Some evidence that hybridization may have occurred between the two species was provided by cpDNA markers. However, biparentally inherited RAPD markers failed to provide any support for the hypothesis that intermediate leaf morphologies in atypical populations arose through hybridization. Consequently, they are most likely to have arisen via divergence caused by drift and/or selection. Genetic distances among populations of S. leucanthemifolius were significant in all but one case. Isolation by distance was indicated by a significant positive correlation between genetic and geographical distances (r = 0.68, P = 0.01, Mantel test). These results suggest that long-distance achene dispersal is rare, despite the presence of a well-developed pappus. The observed loss of pappus at achene maturity may explain this unexpected result. Due to the morphological distinction of var. casablancae from other varieties of S. leucanthemifolius, we suggest elevation to species rank and treatment of the atypical material at infraspecific rank.