The Experts below are selected from a list of 315 Experts worldwide ranked by ideXlab platform
G H Jones - One of the best experts on this subject based on the ideXlab platform.
-
meiosis in autopolyploid Crepis capillaris iii comparison of triploids and tetraploids evidence for non independence of autonomous pairing sites
Heredity, 1994Co-Authors: G H JonesAbstract:Meiosis in autopolyploid Crepis capillaris . III. Comparison of triploids and tetraploids; evidence for non independence of autonomous pairing sites
-
meiosis in allopolyploid Crepis capillaris ii autotetraploids
Genome, 1994Co-Authors: G H Jones, J E VincentAbstract:Meiotic chromosome pairing of autotetraploid Crepis capillaris was analysed by electron microscopy of surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I chromosome configurations. Prophase I quadrivalent frequencies are high in all three tetrasomes. (A, D, and C) and partially dependent on chromosome size. At metaphase I quadrivalent frequencies are much lower and strongly dependent on chromosome size. There is no evidence for multivalent elimination during prophase I in this system, and the reduction in multivalent frequency at metaphase I can be explained by an insufficiency of appropriately placed chiasmata. The high frequencies of prophase I quadrivalents far exceed the two-thirds expected on a simple model with two terminal independent pairing initiation sites per tetrasome, suggesting that multiple pairing initiation occurs. Direct observations reveal relatively high frequencies of pairing partner switches (PPSs) at prophase I, which confirms this sugges...
-
meiosis in autopolyploid Crepis capillaris
Chromosoma, 1993Co-Authors: J E Vincent, G H JonesAbstract:Meiotic chromosome pairing of triploid and trisomic Crepis capillaris was analysed by electron microscopy in surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I. This system allows identification and separate analysis of each chromosome of the C. capillaris genome. Prophase I trivalent frequencies are very high in all three trisomes and only slightly dependent on chromosome size. At metaphase I, on the other hand, trivalent frequencies are much lower and strongly dependent on chromosome size. There is no evidence for trivalent elimination during prophase I in this system, and the reduction in trivalent frequency at metaphase I can be explained by an insufficiency of appropriately placed chiasmata. The high prophase I trivalent frequencies far exceed the two-thirds expected on a simple model with two terminal independent pairing initiation sites per trisome, suggesting that multiple pairing initiation occurs. Direct observations reveal high frequencies of pairing partner switches (PPSs) in prophase I trisomes, which confirms this supposition. The numbers of PPSs per trisome shows a better fit to the Poisson than to the binomial distribution and their positional distribution along trisomes is random and non-localized. All these observations favour a model of pairing initiation in trisomes based on a large number of evenly distributed autonomous pairing sites each with a uniform and low probability of generating a PPS.
-
the population cytogenetics of Crepis capillaris iv the distribution of b chromosomes in british populations
Heredity, 1991Co-Authors: J S Parker, G H Jones, L A Edgar, C WhitehouseAbstract:The population cytogenetics of Crepis capillaris IV. The distribution of B-chromosomes in British populations
Pierreolivier Cheptou - One of the best experts on this subject based on the ideXlab platform.
-
competition colonization syndrome mediated by early germination in non dispersing achenes in the heteromorphic species Crepis sancta
Annals of Botany, 2012Co-Authors: Jonathan Dubois, Pierreolivier CheptouAbstract:† Background and Aims The competition –colonization trade-off theory postulates that the competitive and colonizing abilities of organisms are negatively related; this trade-off has been proposed as a major force in the maintenance of diversity. In plants, the competition –colonization trade-off is often considered to result from variation in resource partitioning, thus generating heavy competitive (non-dispersing) seeds and light (dispersing) non-competitive seeds. Here, the possibility is explored that early germination provides a competitive advantage, thus mediating competitive interactions. † Methods Using eight populations of the heterocarpic species Crepis sancta (Asteraceae), the possibility was tested that dispersing and non-dispersing achenes differ in germination timing, and the impact of early germination on individual fitness components was analysed in the context of intraspecific competition. To evaluate whether seed reserve varies among achene types, endosperm size was also measured by analysing photographs of cross-sections taken under a binocular microscope. † Key Results and Conclusions The results show that non-dispersing achenes germinated 4 d earlier (on average) than dispersing achenes. It is also shown that early germination provides a positive advantage for the survival and final biomass of individuals, a pattern that was consistent over the eight populations and independent of achene type. Dispersing and non-dispersing achenes did not differ in terms of seed reserve (endosperm size). It is proposed that germination phenology may mediate the competition–colonization trade-off in Crepis sancta and the evolutionary significance of this phenomenon is discussed.
-
the town Crepis and the country Crepis how does fragmentation affect a plant pollinator interaction
Acta Oecologica-international Journal of Ecology, 2009Co-Authors: Emilie Andrieu, Antoine Dornier, Soraya Rouifed, Bertrand Schatz, Pierreolivier CheptouAbstract:In fragmented habitats, one cause of the decrease of plant diversity and abundance is the disruption of plant–animal interactions, and in particular plant–pollinator interactions. Since habitat fragmentation acts both on pollinator behaviour and plant reproduction, its consequences for the stability of such interactions are complex. An extreme case of habitat fragmentation occurs in urbanised areas where suitable habitat (in the present study small patches around ornamental trees) is embedded in a highly unsuitable environment (concrete matrix). Based on simple experiments, we ask whether pollinators can adapt their foraging behaviour in response to the amount of available resources (flowers) in the fragments and their isolation, as predicted by the optimal foraging theory. To do so we analysed the effect of fragmentation on the behaviour of pollinators visiting Crepis sancta (L.) Bornm. (Asteraceae), which forms large populations in the countryside and patchy populations in urban environments. More precisely we studied pollinator visitation rates, capitulum visit durations, capitulum search durations and capitulum size choice. Pollinators chose larger capitula in both types of populations and their foraging behaviour differed between the two population types in three ways: (1) pollinator visits were lower in urban fragmented populations, perhaps due to the lower accessibility of urban patches; (2) capitulum visit durations were longer in urban fragmented populations, a possible compensation of energy lost during flights among patches; and (3) capitulum search durations where longer in urban fragmented populations, which may represent an increase in capitulum prospecting effort. We discuss the possible impacts of such differences for plant population functioning in the two types of populations.
-
nine polymorphic microsatellite markers in Crepis sancta asteraceae
Molecular Ecology Notes, 2007Co-Authors: Mariepierre Dubois, Antoine Dornier, Philippe Jarne, Pierreolivier CheptouAbstract:We describe nine new variable di- and trinucleotide microsatellites in the plant Crepis sancta, as well as conditions for multiplexing and simultaneous genotyping sets of loci. Their variability was assessed in four populations. Four to 33 alleles were detected per locus; two to 23 alleles per population. Significant deviations from Hardy–Weinberg equilibrium could be attributed either to a mating system involving partial selfing or to null alleles. Genetic differentiation over all loci was significant among the four populations studied. These markers will constitute an efficient tool for investigating dispersal patterns in fragmented landscapes. Cross-species amplification provided few loci variability in three other Crepis species (C. pulcra, C. foetida and C. rubra).
-
the effect of drought stress on inbreeding depression in four populations of the mediterranean outcrossing plant Crepis sancta asteraceae
Heredity, 2000Co-Authors: Pierreolivier Cheptou, Andre Berger, A Blanchard, Christian Collin, Jose EscarreAbstract:The effect of drought stress on inbreeding depression in four populations of the Mediterranean outcrossing plant Crepis sancta (Asteraceae)
Eric Imbert - One of the best experts on this subject based on the ideXlab platform.
-
Capitulum characters in a seed heteromorphic plant, Crepis sancta (Asteraceae): variance partitioning and inference for the evolution of dispersal rate
Heredity, 2001Co-Authors: Eric ImbertAbstract:In Crepis sancta (Asteraceae), achenes produced in the periphery of the flower head have reduced dispersal ability and are larger than achenes produced in the centre of the head, which disperse farther. The proportion of central achenes produced by a single individual represents the potential dispersal rate of its progeny. Seed variation in dispersal ability may be important where there is spatio-temporal variability of habitats, but its evolutionary significance mainly depends on the heritability of the relative proportions of each achene morph. However, the number of peripheral achenes in a capitulum, and that of involucral bracts are suggested to depend on the number of parastichies, a canalized character. From a diallel cross design, phenotypic variance for several capitulum traits was partitioned among six variance components, including the additive variance. The phenotypic values of some head traits reflected the expected frequency due to ontogeny, in particular the number of involucral bracts. Yet, this character also had a significant heritability, suggesting that variation around the mode of the distribution was not only due to developmental noise. The additive variance for number of peripheral and central achenes was not significantly different from zero. In contrast, their respective proportion had a narrow sense heritability greater than 0.20. The present results suggest that the percentage of central achenes per individual, and thus the potential dispersal rate in Crepis sancta, is under quantitative genetic control, and could undergo microevolutionary changes in natural populations.
-
Phenotypic plasticity for dispersal ability in the seed heteromorphic Crepis sancta (Asteraceae)
Oikos, 2001Co-Authors: Eric Imbert, Ophélie RonceAbstract: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.
-
local adaptation and non genetic maternal effects among three populations of Crepis sancta asteraceae
Ecoscience, 1999Co-Authors: Eric Imbert, Jose Escarre, Jacques LepartAbstract:To test whether local adaptation occurs in the Mediterranean species Crepis sancta (Asteraceae), we performed reciprocal transplants among three populations inhabiting recently abandoned old fields...
-
seed heteromorphism in Crepis sancta asteraceae performance of two morphs in different environments
Oikos, 1997Co-Authors: Eric Imbert, Jose Escarre, Jacques LepartAbstract:Crepis sancta is an annual composite that produces heteromorphic seeds. Peripheral achenes, named from the position they occupy within the fruiting heads, are heavier and without a pappus whereas central achenes are lighter and have a pappus. The morphological differentiation is associated with a difference in embryos size: embryos from peripheral achenes are twice as heavy as those from central achenes. As seedling size has consequences for growth, we tested the hypothesis that the difference in weight between peripheral and central achenes leads to a differentiation in resistance to biotic and abiotic stress. We studied the reproductive biomass produced by plants from each seed morph, grown under different environmental conditions: at high and low nutrient levels, and at various densities of intraspecific and interspecific competition$In the absence of competition, there was no difference in reproductive biomass between the two morphs at high and low nutrient levels. In intraspecific competition, the plants derived from peripheral achenes dominated those derived from central achenes. In interspecific competition, the greater size of the embryo of peripheral achenes was advantageous at high density. The difference between morphs was more important in competition with an annual species of high RGR, Bromus madritensis, than with a perennial species of low RGR, Bromus erectus when competing with The low weight of central achenes represents a disadvantage when competing with other annual species. Theory suggests that each seed morph must differ ecologically in order to maintain heteromorphism. The differentiation in competitive ability represents one of the ecological differences between the two types of achenes and may therefore contribute to the success of the mixed strategy in Crepis sancta.
-
achene dimorphism and among population variation in Crepis sancta asteraceae
International Journal of Plant Sciences, 1996Co-Authors: Eric Imbert, Jose Escarre, Jacques LepartAbstract:Crepis sancta is an annual composite that colonizes abandoned agricultural land and shows achene dimorphism. The achenes on the periphery of the capitulum are heavy (0.27 mg) and are without a pappus, whereas those in the center are light (0.10 mg), more numerous, and bear a pappus. Several studies of other species of Asteraceae have shown that these morphological differentiations are accompanied by biological differences, but few studies have tested whether these differences are constant between populations. We compared the germination, growth, phenology, and reproductive effort of plants derived from these two types of achenes from three populations that had been established for 3, 13, and 36 yr. The peripheral achenes from the young population germinated fastest and gave rise to the most vigorous individuals. The differences in the two other populations were slight or nonexistent. By comparing the morphs between populations, pregermination dormancy and plant vigor were found to be different among popul...
Jose Escarre - One of the best experts on this subject based on the ideXlab platform.
-
the effect of drought stress on inbreeding depression in four populations of the mediterranean outcrossing plant Crepis sancta asteraceae
Heredity, 2000Co-Authors: Pierreolivier Cheptou, Andre Berger, A Blanchard, Christian Collin, Jose EscarreAbstract:The effect of drought stress on inbreeding depression in four populations of the Mediterranean outcrossing plant Crepis sancta (Asteraceae)
-
local adaptation and non genetic maternal effects among three populations of Crepis sancta asteraceae
Ecoscience, 1999Co-Authors: Eric Imbert, Jose Escarre, Jacques LepartAbstract:To test whether local adaptation occurs in the Mediterranean species Crepis sancta (Asteraceae), we performed reciprocal transplants among three populations inhabiting recently abandoned old fields...
-
seed heteromorphism in Crepis sancta asteraceae performance of two morphs in different environments
Oikos, 1997Co-Authors: Eric Imbert, Jose Escarre, Jacques LepartAbstract:Crepis sancta is an annual composite that produces heteromorphic seeds. Peripheral achenes, named from the position they occupy within the fruiting heads, are heavier and without a pappus whereas central achenes are lighter and have a pappus. The morphological differentiation is associated with a difference in embryos size: embryos from peripheral achenes are twice as heavy as those from central achenes. As seedling size has consequences for growth, we tested the hypothesis that the difference in weight between peripheral and central achenes leads to a differentiation in resistance to biotic and abiotic stress. We studied the reproductive biomass produced by plants from each seed morph, grown under different environmental conditions: at high and low nutrient levels, and at various densities of intraspecific and interspecific competition$In the absence of competition, there was no difference in reproductive biomass between the two morphs at high and low nutrient levels. In intraspecific competition, the plants derived from peripheral achenes dominated those derived from central achenes. In interspecific competition, the greater size of the embryo of peripheral achenes was advantageous at high density. The difference between morphs was more important in competition with an annual species of high RGR, Bromus madritensis, than with a perennial species of low RGR, Bromus erectus when competing with The low weight of central achenes represents a disadvantage when competing with other annual species. Theory suggests that each seed morph must differ ecologically in order to maintain heteromorphism. The differentiation in competitive ability represents one of the ecological differences between the two types of achenes and may therefore contribute to the success of the mixed strategy in Crepis sancta.
-
achene dimorphism and among population variation in Crepis sancta asteraceae
International Journal of Plant Sciences, 1996Co-Authors: Eric Imbert, Jose Escarre, Jacques LepartAbstract:Crepis sancta is an annual composite that colonizes abandoned agricultural land and shows achene dimorphism. The achenes on the periphery of the capitulum are heavy (0.27 mg) and are without a pappus, whereas those in the center are light (0.10 mg), more numerous, and bear a pappus. Several studies of other species of Asteraceae have shown that these morphological differentiations are accompanied by biological differences, but few studies have tested whether these differences are constant between populations. We compared the germination, growth, phenology, and reproductive effort of plants derived from these two types of achenes from three populations that had been established for 3, 13, and 36 yr. The peripheral achenes from the young population germinated fastest and gave rise to the most vigorous individuals. The differences in the two other populations were slight or nonexistent. By comparing the morphs between populations, pregermination dormancy and plant vigor were found to be different among popul...
Stefan Andersson - One of the best experts on this subject based on the ideXlab platform.
-
experimental demonstration of floral allocation costs in Crepis tectorum
Botany, 2006Co-Authors: Stefan AnderssonAbstract:Information on floral resource costs is fundamental for understanding how selection operates on floral morphology. In this study, I explored the cost of maturing flowers in a self-incompatible population of the ligulate composite Crepis tectorum L. by experimentally manipulating floral investment and then monitoring the response in reproductive effort. Plants on which the heads were removed during the initial stage of ligule expansion had a higher reproductive effort than plants whose heads were removed immediately after flower maturation, and the latter plants had a higher reproductive effort than plants on which all flowers were permitted to set fruit. Judging from biomass estimates and the magnitude of the observed tradeoffs, the amount of resources allocated to maturing flowers was about half as great as the amount of resources devoted to fruit maturation. These and other results suggest that floral tradeoffs may exert negative selection on floral size variables.
-
floral reduction in Crepis tectorum asteraceae tradeoffs and dominance relationships
Biological Journal of The Linnean Society, 1996Co-Authors: Stefan AnderssonAbstract:The present investigation of Crepis tectorum examines the extent to which tradeoffs and patterns of dominance contribute to the association between flower size (head width) and degree of self-fertility among populations adapted to rock outcrops. Partial correlation analyses of phenotypic data from a F2 generation derived from a cross between two outcrop plants and of family means representing one of the parent populations indicate that small-flowered plants fail to reallocate resources to flower and fruit production, that small flower size offers little or no advantage in terms of autofertility, and that floral reduction has little influence on the rate of flower development. Hence, it may be necessary to invoke factors other than tradeoffs to explain the decline in flower size associated with the evolution of autogamy in C. tectorum. Comparison of parent and Fl hybrid means in the crossing experiment suggests incomplete dominance in the alleles for large flower size. Under the assumption that Fl hybrid means reflect the average degree of dominance across loci, I propose inbreeding depression as the basis for some of the reduction in floral morphology.
-
phenotypic plasticity in Crepis tectorum asteraceae genetic correlations across light regimens
Heredity, 1994Co-Authors: Stefan Andersson, Ruth G ShawAbstract:Phenotypic plasticity in Crepis tectorum (Asteraceae): genetic correlations across light regimens