Size-dependent variation of gender in high density stands of the monoecious annual,Ambrosia artemisiifolia (Asteraceae)

Summary We examined the relationship between size variability and the distribution of functional gender in stands of the monoecious, wind-pollinated annual Ambrosia artemisiifolia. Populations of 60 individuals were grown in the greenhouse at a density of 372 m^–2 and at two nutrient levels. Among the surviving plants, after self-thinning, variability in above-ground biomass and gender was higher in the high nutrient treatment. Among individuals there was a significant positive correlation between maleness and both height and biomass. Fecundity was also positively correlated with both measures of size. Based on the pattern of distribution of male and female flowers within the plant, it appears that the increase in maleness in larger plants is due to increased branching and axis elongation. These results demonstrate that competitive interactions, which lead to increased variability in biomass and fecundity, can also generate variability in gender within populations.     

Phenotypic plasticity inCrepis tectorum (Asteraceae)

Two experiments were carried out using two different approaches to compare populations ofCrepis tectorum (Asteraceae). One was based on a comparison of means of various vegetative and reproductive characters and another was based on a comparison of response patterns of the same characters in a series of environments. Population divergence within two earlier recognized form series, one from weed habitats and one from alvar habitats on Baltic islands, resulted in a partially overlapping pattern in cluster analyses based on character means. However, the pattern revealed by a comparison of the direction and amount of plastic response suggested that populations within the two form series had more similar response patterns than other combinations of populations. It was concluded that patterns of plasticity may provide useful additional information on the overall similarity among taxa. An hypothesis that plants in weed populations should exhibit a greater phenotypic response to the environments than plants in alvar populations was rejected.     

Distribution map of Ambrosia artemisiifolia L. (Asteraceae) in Italy

The spread of the invasive and allergenic Ambrosia artemisiifolia L. in Italy was analysed and mapped using distribution data from a wide range of sources. Ambrosia artemisiifolia occupies 1057 floristic quadrants which are mostly distributed in the Po plain. The distribution obtained represents the basis to implement urgent management strategies.     

High outcrossing in the annual colonizing species Ambrosia artemisiifolia (Asteraceae)

Background and Aims

Variation in mating patterns may be particularly evident in colonizing species because they commonly experience wide variation in plant density. Here, the role of density for the mating system of Ambrosia artemisiifolia (common ragweed), a wind-pollinated annual colonizing species previously reported as self-compatible, is explored.

Methods

The effect of population density on the proportion of self- and cross-fertilized seeds was examined using allozyme markers and experimental arrays conducted over two seasons in the field. Also the reproductive success of isolated plants located in diverse habitats was measured. The potential occurrence of a physiological mechanism preventing self-fertilization, i.e. self-incompatibility, following controlled self- and cross-pollinations in the glasshouse was examined.

Key Results

Outcrossing rates estimated using allozyme markers were uniformly high, regardless of the spacing between plants. However, when single plants were isolated from congeners they set few seeds. Observations of pollen-tube growth and seed set following controlled pollinations demonstrated that plants of A. artemisiifolia possess a strong self-incompatibility mechanism, contrary to earlier reports and assumptions.

Conclusions

The maintenance of high outcrossing rates in colonizing populations of A. artemisiifolia is likely to be facilitated by the prodigious production of wind-borne pollen, high seed production and extended seed dormancy.Key words: Self-incompatibility, outcrossing rate, density dependence, colonization, wind-pollination, Ambrosia artemisiifolia (ragweed), Asteraceae     

Phenotypic plasticity in alpine Erigeron species (Asteraceae)

Utelli, A.-B., Huber, W. & Zopfi, H.-J. 1995. Phenotypic plasticity in alpine Erigeron species (Asteraceae). — Nord. J. Bot. 15: 483–492. Copenhagen. ISSN 0107–055X.
Twelve morphological characters were analysed in 67 E. alpinus, E. neglectus and E. uniflorus plants which had been transplanted from their natural habitat to the greenhouse. Mean values and standard deviations were compared for each species. The phenotypic plasticity of the characters resp. their genetic basis was estimated from the correlation of morphological data recorded on single individuals, each grown under both ecological conditions.
A considerable phenotypic plasticity was found in all species and for most characters. E. unijlorus was the most constant species regarding taxonomically relevant characters such as pubescence of the basal leaves, number of flowering heads and number of filiform flowers. In all three species the same highly significant directional variation in plant height and involucre length was found which suggests that these characters have some genetic basis in Erigeron .     

Intrapopulation gender variation in common ragweed (Asteraceae: Ambrosia artemisiifolia L.), a monoecious,annual herb

Summary We measured variation in gender among individuals within populations of ragweed (Ambrosia artemisiifolia) in an abandoned old-field and in the greenhouse. There was great variability in sex expression, from all-female to approximately 78% male. Plants differed significantly in gender in different locations within the field. Plants in an area abandoned from agriculture one year previously were more male than plants in a nearby area abandoned four years previously. In the greenhouse, soil moisture treatments and levels of attack by spittlebugs (Philaenus spumarius) did not affect gender. Height was positively correlated with relative maleness in both populations. Plants with greater shoot weight were relatively more male in the greenhouse, but not in the field. The gender variation we observed either has a genetic basis or is controlled by environmental variables other than those we investigated.     

Allometric plasticity in a polyphenic beetle

Abstract 1. Environmental conditions, such as variation in nutrition, commonly contribute to morphological variation among individuals by affecting body size and the expression of certain morphological traits; however the scaling relationship between a morphological trait and body size over a range of body sizes is generally assumed not to change in response to environmental fluctuation (allometric plasticity), but instead to be constant and diagnostic for a particular trait and species or population. The work reported here examined diet‐induced allometric plasticity in the polyphenic beetle Onthophagus taurus Schreber (1759) (Coleoptera: Scarabaeidae). 2. Male O. taurus vary in body size depending on larval nutrition. Only males above a critical body size threshold express fully developed horns; males smaller than this threshold develop only rudimentary horns or no horns at all. 3. Field populations that naturally utilise two different resources for feeding larvae (horse dung vs. cow manure) exhibited significant differences in the average scaling relationship between body size and male horn length over the same range of body sizes. Males collected from cow manure populations expressed consistently longer horns for a given body size than males collected from horse dung populations. 4. Males reared in the laboratory on horse dung or cow manure also exhibited significant differences in the average scaling relationship between body size and horn length. Differences between laboratory populations reared on horse dung or cow manure were of the same kind and magnitude as differences between field populations that utilise these different resources naturally. 5. These findings suggest that between‐population differences in scaling relationships between horn length and body size can be the product of differences in the quality of resources available to developing larvae. Results are discussed in the context of onthophagine mating systems and recent insights in the developmental and endocrine control of horn polyphenisms.     

Phenotypic plasticity for dispersal ability in the seed heteromorphic Crepis sancta (Asteraceae)

In several species of Asteraceae with seed heteromorphism, the proportion of seeds equipped with wind-dispersal structure decreases plastically under stressful conditions. This contradicts the adaptive expectation that the dispersal rate should increase with stress. Alternatively, developmental constraints related to the ontogeny of the capitulum may explain these observations, as they may explain the effect of senescence on seed morph proportions. In the present paper, we report on an experiment investigating the effect of stress and senescence on the seed heteromorphic Crepis sancta (Asteraceae). Plants were subjected to one of four different treatments: control, simulated herbivory, nutrient depletion, and simulated herbivory plus nutrient depletion. Plants experiencing herbivory produced more fecund seed heads immediately after the application of stress, which was later compensated by a decrease in fecundity per seed head. Environmental stress increased the proportion of seeds with dispersal structures. Though senescence had large effects on fecundity, it had no effect on seed morph proportions in nonstressed plants. These findings agree with adaptive expectations about the effect of stress on dispersal ability, and contrast with results obtained in other seed heteromorphic Asteraceae. Our results cast doubts on the existence of strong developmental constraints acting on the seed morph proportions in Crepis sancta .     

The effects of feeding damage in ragweed Ambrosia artemisiifolia (Asteraceae) on populations of Zygogramma suturalis (Coleoptera,Chrysomelidae)

Summary Field sampling indicated that the number of eggs laid by a Zygogramma suturalis female within a sampling plot correlated inversely with the mean degree of ragweed damage. Feeding on extensively damaged ragweed in the laboratory caused a drop in oviposition intensity, and a considerable proportion of females completely stopped oviposition. Feeding on slightly damaged ragweed had no significant effect on oviposition intensity. Ovipositing females preferred to feed on the intact ragweed and lay their eggs close to it. The locomotory activity of ovipositing females was significantly higher on highly damaged ragweed whereas non-ovipositing (diapausing) females and males were behaviourally indifferent to the extent of ragweed damage. Under natural conditions, ovipositing females more frequently left damaged host plants for less damaged one. If the degree of ragweed damage is high over a large area, the insects that were unable to find undamaged plants for several days oviposited less and some females entered diapause. The adaptive effect of these reactions is a decrease of population density in advance, before it might drop as a result of starvation. These results are in agreement with the second model of insect reaction to the damage-induced changes in a host plant (Edwards and Wratten 1987).     

Effects of regional origin and genotype on intraspecific root communication in the desert shrub Ambrosia dumosa (Asteraceae)

Previous work has shown that the contact inhibition that occurs among roots of Ambrosia dumosa shrubs has a self/nonself recognition capability. In the current study, we investigated some of the geographic and genotypic dimensions of this recognition capability by using root observation chambers to observe the effects of encounters of individual roots on root elongation rates. We measured such effects in encounters between roots of plants from the same region and compared these to effects in encounters between roots of plants from two different regions. We also measured effects of encounters between roots of plants from the same clones and compared these to effects of encounters of roots of plants from different clones. Roots of plants from the same region (population) showed the usual “nonself” precipitous decline in elongation rates following contact, but when roots of plants from different regions contacted each other, elongation rates continued unchanged. When roots of separate plants from the same clone contacted each other, the same “nonself” precipitous decline in elongation rates as seen in encounters between roots of plants of different clones from the same region occurred. Meanwhile, in these same experiments “self” contacts between sister roots connected to the same plants resulted in no changes in elongation rates. Thus, differences between individuals from two geographically separate populations of Ambrosia dumosa may be sufficient to thwart the “nonself,” population-level recognition of similarity apparently necessary for contact inhibition. Furthermore, the “self” recognition mechanism, which precludes contact inhibition between two roots on the same plant, appears to be physiological rather than genetic in nature.     

Isolation of five polymorphic microsatellite loci in the invasive weed Ambrosia artemisiifolia (Asteraceae) using an enrichment protocol

We report the development of five microsatellite markers for the invasive North American native plant Ambrosia artemisiifolia (Asteraceae). An enrichment protocol was used to isolate microsatellite loci and polymorphism was explored with samples from 16 natural populations collected in the native range, across North America, and in an invaded area along the Rhône Valley in France. In these areas, we found a high level of polymorphism within population and significant heterozygote deficiencies.     

Ambrosia johnstoniorum (Asteraceae), a new species of ragweed from the Chihuahuan Desert

Ambrosia johnstoniorum is described from two collections of I. M. Johnston from central Coahuila. The taxon has a distinctive assortment of characteristics and appears most closely related toAmbrosia nivea (Robins. & Fern.) Payne of northern Chihuahua.     

No evolutionary shift in the mating system of north American Ambrosia artemisiifolia (Asteraceae) following its introduction to China

The mating system plays a key role during the process of plant invasion. Contemporary evolution of uniparental reproduction (selfing or asexuality) can relieve the challenges of mate limitation in colonizing populations by providing reproductive assurance. Here we examined aspects of the genetics of colonization in Ambrosia artemisiifolia, a North American native that is invasive in China. This species has been found to possess a strong self-incompatibility system and have high outcrossing rates in North America and we examined whether there has been an evolutionary shift towards the dependence on selfing in the introduced range. Specifically, we estimated outcrossing rates in one native and five invasive populations and compared levels of genetic diversity between North America and China. Based on six microsatellite loci we found that, like the native North American population, all five Chinese populations possessed a completely outcrossing mating system. The estimates of paternity correlations were low, ranging from 0.028-0.122, which suggests that populations possessed ~8-36 pollen donor parents contributing to each maternal plant in the invasive populations. High levels of genetic diversity for both native and invasive populations were found with the unbiased estimate of gene diversity ranging from 0.262-0.289 for both geographic ranges based on AFLP markers. Our results demonstrate that there has been no evolutionary shift from outcrossing to selfing during A. artemisiifolia's invasion of China. Furthermore, high levels of genetic variation in North America and China indicate that there has been no erosion of genetic variance due to a bottleneck during the introduction process. We suggest that the successful invasion of A. artemisiifolia into Asia was facilitated by repeated introductions from multiple source populations in the native range creating a diverse gene pool within Chinese populations.     

Establishment of trophic associations of native leaf beetle species (Coleoptera,Chrysomelidae) with Ambrosia artemisiifolia (Asteraceae) in Primorskii Territory of Russia

The establishment of trophic associations between the native leaf beetle species and the adventive ragweed Ambrosia artemisiifolia in the territory of southern Primorskii Territory of Russia is considered. The native leaf beetles Neocrepidodera obscuritarsis (Motschulsky, 1859) and Chrysolina aurichalcea (Gebler in Mannerheim, 1825) feed on A. artemisiifolia despite their trophic preferences for native plants.     

Duplication and adaptive evolution of the chalcone synthase genes of Dendranthema (Asteraceae)

Chalcone synthase (CHS) is a key enzyme in the biosynthesis of flavonoids, which are important for the pigmentation of flowers and act as attractants to the pollinators. Genes encoding CHS constitute a multigene family in which the copy number varies among plant species and functional divergence appears to have occurred repeatedly. Plants of the Dendranthema genus have white, yellow, and pink flowers, exhibiting considerable variation in flower color. In this article, 18 CHS genes from six Dendranthema species were sequenced. Two of them were found to be pseudogenes. The functional Dendranthema CHS genes formed three well-supported subfamilies: SF1, SF2, and SF3. The inferred phylogeny of the CHS genes of Dendranthema and Gerbera suggests that those genes originated as a result of duplications before divergence of these two genera, and the function of Dendranthema CHS genes have diverged in a similar fashion to the Gerbera CHS genes; i.e., the genes of SF1 and SF3 code for typical CHS enzymes expressed during different stages of development, whereas the genes of SF2 code for another enzyme that is different from CHS in substrate specificity and reaction. Relative rate tests revealed that the Dendranthema CHS genes significantly deviated from clocklike evolution at nonsynonymous sites. Maximum likelihood analysis showed that the nonsynonymous-synonymous (omega = d(N)/d(S)) rate ratio for the lineage ancestral to SF2 was much higher than for other lineages, with some sites having a ratio well above one. Positive selective pressure appears to have driven the divergence of SF2 from SF1 and SF3.     

Floret specialization, seed production and gender in Artemisia vulgaris L. (Asteraceae, Anthemideae)

Flowering and fruiting behaviour of female and hermaphrodite florets is described and assessed in samples from three populations from Denmark, England and Sweden. Between 25 and 50% of the florets in capitula are female, and flowering gender varies little among plants in each population. Fruiting gender of individuals, G (femaleness), varies from 0 to 0.85, because of variation in fruit set and fruit abortion. Variation in fruiting gender was correlated with plant size parameters in two populations, but not in the third. The data suggest that post-anthesis regulation of maternal investment may be operating. Florets of A. vulgaris are either totally specialized for pollen receipt (female florets) or largely specialized for pollen donation (hermaphrodite florets), and show adaptations for avoiding interference with each other in these functions. Movement of capitula from a pendent position at flowering to an erect position at fruiting optimizes positions for dissemination of pollen and of seeds respectively.     

Effects of mating system on adaptive potential for leaf morphology in Crepis tectorum (Asteraceae)

Background and Aims

A shift from outcrossing to selfing is thought to reduce the long-term survival of populations by decreasing the genetic variation necessary for adaptation to novel ecological conditions. However, theory also predicts an increase in adaptive potential as more of the existing variation becomes expressed as homozygous genotypes. So far, relatively few studies have examined how a transition to selfing simultaneously affects means, variances and covariances for characters that might be under stabilizing selection for a spatially varying optimum, e.g. characters describing leaf morphology.

Methods

Experimental crosses within an initially self-sterile population of Crepis tectorum were performed to produce an outbred and inbred progeny population to assess how a shift to selfing affects the adaptive potential for measures of leaf morphology, with special emphasis on the degree of leaf dissection, a major target of diversifying selection within the study species.

Key Results

Three consecutive generations of selfing had a minor impact on survival, the total number of heads produced and the mean leaf phenotype, but caused a proportional increase in the genetic (co)variance matrix for foliar characters. For the degree of leaf dissection, the lowest 50th percentile of the inbred progeny population showed a disproportionate increase in the genetic variance, consistent with the recessive nature of the weakly lobed phenotype observed in interpopulation crosses. Comparison of inbreeding response with large-scale patterns of variation indicates a potential for selection in a (recently) inbred population to drive a large evolutionary reduction in degree of leaf dissection by increasing the frequency of particular sibling lines.

Conclusions

The results point to a positive role for inbreeding in phenotypic evolution, at least during or immediately after a rapid shift in mating system.     

Phenotypic plasticity and similarity among gall morphotypes on a superhost,Baccharis reticularia (Asteraceae)

Understanding factors that modulate plant development is still a challenging task in plant biology. Although research has highlighted the role of abiotic and biotic factors in determining final plant structure, we know little of how these factors combine to produce specific developmental patterns. Here, we studied patterns of cell and tissue organisation in galled and non‐galled organs of Baccharis reticularia, a Neotropical shrub that hosts over ten species of galling insects. We employed qualitative and quantitative approaches to understand patterns of growth and differentiation in its four most abundant gall morphotypes. We compared two leaf galls induced by sap‐sucking Hemiptera and stem galls induced by a Lepidopteran and a Dipteran, Cecidomyiidae. The hypotheses tested were: (i) the more complex the galls, the more distinct they are from their non‐galled host; (ii) galls induced on less plastic host organs, e.g. stems, develop under more morphogenetic constraints and, therefore, should be more similar among themselves than galls induced on more plastic organs. We also evaluated the plant sex preference of gall‐inducing insects for oviposition. Simple galls were qualitative and quantitatively more similar to non‐galled organs than complex galls, thereby supporting the first hypothesis. Unexpectedly, stem galls had more similarities between them than to their host organ, hence only partially supporting the second hypothesis. Similarity among stem galls may be caused by the restrictive pattern of host stems. The opposite trend was observed for host leaves, which generate either similar or distinct gall morphotypes due to their higher phenotypic plasticity. The Relative Distance of Plasticity Index for non‐galled stems and stem galls ranged from 0.02 to 0.42. Our results strongly suggest that both tissue plasticity and gall inducer identity interact to determine plant developmental patterns, and therefore, final gall structure.     

Allometric growth and adaptive value of exaggerated body parts in the larvae of Gratiana spadicea (Klug) (Coleoptera: Chrysomelidae)

1. Larvae of tortoise beetles present exaggerated body parts in association with an abdominal shield, which is made of faeces and exuviae that are deposited on the urogomphi throughout ontogeny. Growth trajectories and scaling relationships of these functional structures associated with the shield, if any, are unknown. 2. This study of Gratiana spadicea first tested, under field conditions, whether there is adaptive value associated with the shield regarding protection against predation and sunlight. Then, under laboratory conditions, the growth trajectory and allometric relationships among body parts were investigated, including scoli, individual and apparent furcae, and shield. The influence of food deprivation on the development of these structures was also determined. 3. Findings from previous studies were confirmed, suggesting that the adaptive value assigned to the shield is related to protection against predators. The present study demonstrated for the first time that the shield acts as a parasol in cassidines, decreasing the exposure of their larval body to sunlight. The scoli and apparent furca are exaggerated structures of G. spadicea, the development of which involves allometric growth and greater energetic investment (positive allometry) during ontogeny. There was proportionally less energetic investment for somatic construction of individual furca (negative allometry) due to the accumulation of the exuviae. 4. The possible consequences, in terms of developmental costs and survivorship benefits associated with the evolution of such exaggerated structures, are discussed.     

Polyploidy in Ambrosia dumosa (Compositae: Ambrosieae)

Ambrosia dumosa (A. Gray ex Torr.) Payne is one of the most abundant shrubs of the Sonoran Desert. Allopatric and sympatric populations of this species frequently exhibit distinctive morphological features, particularly with regard to leaf-segment width, but also in blooming time and ecology. Chromosome numbers ofn = 18, 36, and 54 were frequently encountered in our preliminary investigations ofA. dumosa. Fifty-six chromosome number determinations have now accumulated for this taxon, and these additional counts confirm the existence of three major chromosome races withinA. dumosa. Diploids generally occur throughout the range of the species; hexaploids tend to be confined to the deserts of California; and tetraploids occur through most of the range of the species, with the exception of southern Baja California and Owens Valley, California. External morphology was found to be correlated with chromosome number only locally, but even in many local areas, morphological intergradation occurs between different chromosome races. It is therefore considered impractical to attempt a formal taxonomy which would recognize these chromosomal races.