1
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Gianella M, Balestrazzi A, Pagano A, Müller JV, Kyratzis AC, Kikodze D, Canella M, Mondoni A, Rossi G, Guzzon F. Heteromorphic seeds of wheat wild relatives show germination niche differentiation. PLANT BIOLOGY (STUTTGART, GERMANY) 2020; 22:191-202. [PMID: 31639249 DOI: 10.1111/plb.13060] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/06/2019] [Accepted: 10/12/2019] [Indexed: 05/12/2023]
Abstract
Crop wild relatives are fundamental genetic resources for crop improvement. Wheat wild relatives often produce heteromorphic seeds that differ in morphological and physiological traits. Several Aegilops and Triticum species possess, within the same spikelet, a dimorphic seed pair, with one seed being larger than the other. A comprehensive analysis is needed to understand which traits are involved in seed dimorphism and if these aspects of variation in dimorphic pairs are functionally related. To this end, dispersal units of Triticum urartu and five Aegilops species were X-rayed and the different seed morphs weighed. Germination tests were carried out on seeds, both dehulled and left in their dispersal units. Controlled ageing tests were performed to detect differences in seed longevity among seed morphs, and the antioxidant profile was assessed in terms of antioxidant compounds equipment and expression of selected antioxidant genes. We used PCA to group seed morphs sharing similar patterns of germination traits, longevity estimates and antioxidant profile. Different seed morphs differed significantly in terms of mass, final germination, germination timing, longevity estimates and antioxidant profile in most of the tested species. Small seeds germinated slower, had lower germination when left in their dispersal units, a higher antioxidant potential and were longer-lived than large seeds. The antioxidant gene expression varied between morphs, with different patterns across species but not clearly reflecting the phenotypic observations. The results highlight different trait trade-offs in dimorphic seeds of Aegilops and T. urartu, affecting their germination phenology and longevity, thereby resulting in recruitment niche differentiation.
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Affiliation(s)
- M Gianella
- Department of Biology and Biotechnology "L. Spallanzani", University of Pavia, Pavia, Italy
| | - A Balestrazzi
- Department of Biology and Biotechnology "L. Spallanzani", University of Pavia, Pavia, Italy
| | - A Pagano
- Department of Biology and Biotechnology "L. Spallanzani", University of Pavia, Pavia, Italy
| | - J V Müller
- Millennium Seed Bank, Conservation Science Department, Royal Botanic Gardens Kew, Wakehurst Place, UK
| | - A C Kyratzis
- Vegetable Crops Sector, Agricultural Research Institute of Cyprus, Nicosia, Cyprus
| | - D Kikodze
- Institute of Botany, Ilia State University, Tbilisi, Georgia
| | - M Canella
- Department of Earth and Environmental Sciences, University of Pavia, Pavia, Italy
| | - A Mondoni
- Department of Earth and Environmental Sciences, University of Pavia, Pavia, Italy
| | - G Rossi
- Department of Earth and Environmental Sciences, University of Pavia, Pavia, Italy
| | - F Guzzon
- International Maize and Wheat Improvement Center (CIMMYT), Texcoco, Estado de Mexico, Mexico
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2
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Saastamoinen M, Bocedi G, Cote J, Legrand D, Guillaume F, Wheat CW, Fronhofer EA, Garcia C, Henry R, Husby A, Baguette M, Bonte D, Coulon A, Kokko H, Matthysen E, Niitepõld K, Nonaka E, Stevens VM, Travis JMJ, Donohue K, Bullock JM, Del Mar Delgado M. Genetics of dispersal. Biol Rev Camb Philos Soc 2017; 93:574-599. [PMID: 28776950 PMCID: PMC5811798 DOI: 10.1111/brv.12356] [Citation(s) in RCA: 127] [Impact Index Per Article: 18.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2017] [Revised: 07/03/2017] [Accepted: 07/05/2017] [Indexed: 12/12/2022]
Abstract
Dispersal is a process of central importance for the ecological and evolutionary dynamics of populations and communities, because of its diverse consequences for gene flow and demography. It is subject to evolutionary change, which begs the question, what is the genetic basis of this potentially complex trait? To address this question, we (i) review the empirical literature on the genetic basis of dispersal, (ii) explore how theoretical investigations of the evolution of dispersal have represented the genetics of dispersal, and (iii) discuss how the genetic basis of dispersal influences theoretical predictions of the evolution of dispersal and potential consequences. Dispersal has a detectable genetic basis in many organisms, from bacteria to plants and animals. Generally, there is evidence for significant genetic variation for dispersal or dispersal‐related phenotypes or evidence for the micro‐evolution of dispersal in natural populations. Dispersal is typically the outcome of several interacting traits, and this complexity is reflected in its genetic architecture: while some genes of moderate to large effect can influence certain aspects of dispersal, dispersal traits are typically polygenic. Correlations among dispersal traits as well as between dispersal traits and other traits under selection are common, and the genetic basis of dispersal can be highly environment‐dependent. By contrast, models have historically considered a highly simplified genetic architecture of dispersal. It is only recently that models have started to consider multiple loci influencing dispersal, as well as non‐additive effects such as dominance and epistasis, showing that the genetic basis of dispersal can influence evolutionary rates and outcomes, especially under non‐equilibrium conditions. For example, the number of loci controlling dispersal can influence projected rates of dispersal evolution during range shifts and corresponding demographic impacts. Incorporating more realism in the genetic architecture of dispersal is thus necessary to enable models to move beyond the purely theoretical towards making more useful predictions of evolutionary and ecological dynamics under current and future environmental conditions. To inform these advances, empirical studies need to answer outstanding questions concerning whether specific genes underlie dispersal variation, the genetic architecture of context‐dependent dispersal phenotypes and behaviours, and correlations among dispersal and other traits.
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Affiliation(s)
- Marjo Saastamoinen
- Department of Biosciences, Metapopulation Research Centre, University of Helsinki, P.O. Box 65, 00014 Helsinki, Finland
| | - Greta Bocedi
- School of Biological Sciences, University of Aberdeen, Aberdeen AB24 2TZ, U.K
| | - Julien Cote
- Laboratoire Évolution & Diversité Biologique UMR5174, CNRS, Université Toulouse III Paul Sabatier, 31062 Toulouse, France
| | - Delphine Legrand
- Centre National de la Recherche Scientifique and Université Paul Sabatier Toulouse III, SETE Station d'Ecologie Théorique et Expérimentale, UMR 5321, 09200 Moulis, France
| | - Frédéric Guillaume
- Department of Evolutionary Biology and Environmental Studies, University of Zurich, CH-8057 Zurich, Switzerland
| | - Christopher W Wheat
- Population Genetics, Department of Zoology, Stockholm University, S-10691 Stockholm, Sweden
| | - Emanuel A Fronhofer
- Department of Evolutionary Biology and Environmental Studies, University of Zurich, CH-8057 Zurich, Switzerland.,Department of Aquatic Ecology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH-8600 Dubendorf, Switzerland
| | - Cristina Garcia
- CIBIO-InBIO, Universidade do Porto, 4485-661 Vairão, Portugal
| | - Roslyn Henry
- School of Biological Sciences, University of Aberdeen, Aberdeen AB24 2TZ, U.K.,School of GeoSciences, University of Edinburgh, Edinburgh EH89XP, U.K
| | - Arild Husby
- Department of Biosciences, Metapopulation Research Centre, University of Helsinki, P.O. Box 65, 00014 Helsinki, Finland
| | - Michel Baguette
- Centre National de la Recherche Scientifique and Université Paul Sabatier Toulouse III, SETE Station d'Ecologie Théorique et Expérimentale, UMR 5321, 09200 Moulis, France.,Museum National d'Histoire Naturelle, Institut Systématique, Evolution, Biodiversité, UMR 7205, F-75005 Paris, France
| | - Dries Bonte
- Department of Biology, Ghent University, B-9000 Ghent, Belgium
| | - Aurélie Coulon
- PSL Research University, CEFE UMR 5175, CNRS, Université de Montpellier, Université Paul-Valéry Montpellier, EPHE, Biogéographie et Ecologie des Vertébrés, 34293 Montpellier, France.,CESCO UMR 7204, Bases écologiques de la conservation, Muséum national d'Histoire naturelle, 75005 Paris, France
| | - Hanna Kokko
- Department of Evolutionary Biology and Environmental Studies, University of Zurich, CH-8057 Zurich, Switzerland
| | - Erik Matthysen
- Evolutionary Ecology Group, Department of Biology, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium
| | - Kristjan Niitepõld
- Department of Biosciences, Metapopulation Research Centre, University of Helsinki, P.O. Box 65, 00014 Helsinki, Finland
| | - Etsuko Nonaka
- Department of Biosciences, Metapopulation Research Centre, University of Helsinki, P.O. Box 65, 00014 Helsinki, Finland
| | - Virginie M Stevens
- Centre National de la Recherche Scientifique and Université Paul Sabatier Toulouse III, SETE Station d'Ecologie Théorique et Expérimentale, UMR 5321, 09200 Moulis, France
| | - Justin M J Travis
- School of Biological Sciences, University of Aberdeen, Aberdeen AB24 2TZ, U.K
| | | | - James M Bullock
- NERC Centre for Ecology & Hydrology, Wallingford OX10 8BB, U.K
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3
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Ågren J, Schemske DW. SEX ALLOCATION IN THE MONOECIOUS HERB
BEGONIA SEMIOVATA. Evolution 2017; 49:121-130. [DOI: 10.1111/j.1558-5646.1995.tb05964.x] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/1993] [Accepted: 02/24/1994] [Indexed: 11/29/2022]
Affiliation(s)
- Jon Ågren
- Department of Botany, KB‐15 University of Washington Seattle Washington 98195
| | - Douglas W. Schemske
- Department of Botany, KB‐15 University of Washington Seattle Washington 98195
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4
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Waser NM, Shaw RG, Price MV. SEED SET AND SEED MASS IN
IPOMOPSIS AGGREGATA
: VARIANCE PARTITIONING AND INFERENCES ABOUT POSTPOLLINATION SELECTION. Evolution 2017; 49:80-88. [DOI: 10.1111/j.1558-5646.1995.tb05960.x] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/1993] [Accepted: 03/30/1994] [Indexed: 11/27/2022]
Affiliation(s)
- Nickolas M. Waser
- Department of Biology University of California Riverside California 92521
- Rocky Mountain Biological Laboratory Crested Butte Colorado 81224
| | - Ruth G. Shaw
- Rocky Mountain Biological Laboratory Crested Butte Colorado 81224
- Department of Botany and Plant Sciences University of California Riverside California 92521
| | - Mary V. Price
- Department of Biology University of California Riverside California 92521
- Rocky Mountain Biological Laboratory Crested Butte Colorado 81224
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5
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O'Neil P, Schmitt J. GENETIC CONSTRAINTS ON THE INDEPENDENT EVOLUTION OF MALE AND FEMALE REPRODUCTIVE CHARACTERS IN THE TRISTYLOUS PLANT
LYTHRUM SALICARIA. Evolution 2017; 47:1457-1471. [DOI: 10.1111/j.1558-5646.1993.tb02168.x] [Citation(s) in RCA: 40] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/1992] [Accepted: 03/30/1993] [Indexed: 11/28/2022]
Affiliation(s)
- Pamela O'Neil
- Graduate Program in Ecology and Evolutionary Biology Brown University Providence Rhode Island 02912
| | - Johanna Schmitt
- Graduate Program in Ecology and Evolutionary Biology Brown University Providence Rhode Island 02912
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6
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Venable DL, Búrquez MA. QUANTITATIVE GENETICS OF SIZE, SHAPE, LIFE-HISTORY, AND FRUIT CHARACTERISTICS OF THE SEED HETEROMORPHIC COMPOSITE HETEROSPERMA PINNATUM. II. CORRELATION STRUCTURE. Evolution 2017; 44:1748-1763. [PMID: 28567804 DOI: 10.1111/j.1558-5646.1990.tb05246.x] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/1989] [Accepted: 02/09/1990] [Indexed: 11/27/2022]
Abstract
We have investigated phenotypic, environmental, within-population broad-sense genetic correlations and among-population genetic correlations for 17 traits in six populations of Heterosperma pinnatum Cav. (Compositae) grown in the greenhouse. The within-population genetic, environmental, and phenotypic correlations were somewhat similar while the among-population genetic correlations showed little correspondence to these. The different correlation matrices were compared to a hypothesis matrix, which predicted higher correlations for groups of functionally and developmentally related traits. The groups were seed and head traits, size and shape traits, and life history traits, with subgroups predicted to have still higher correlations. The phenotypic and environmental matrices corresponded well to the hypothesis matrix, the within-population broad-sense genetic matrix showed weaker, though still significant, correspondence, and the among-population genetic correlations showed no correspondence. Genetic correlations did not differ significantly among populations, though the power of these comparisons was low. Some particular genetic correlations are discussed as possible examples of adaptive correlations (e.g., a negative correlation between dispersal and dormancy) and as examples of developmental or physiological constraints including life-history tradeoffs.
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Affiliation(s)
- D. Lawrence Venable
- Department of Ecology and Evolutionary Biology University of Arizona Tucson AZ 85721 USA
| | - M. Alberto Búrquez
- Centro de Ecología Universidad Nacional Autónoma de México Mexico City 04510 MÉXICO
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7
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Venable DL, Dyreson E, Piñero D, Becerra JX. SEED MORPHOMETRICS AND ADAPTIVE GEOGRAPHIC DIFFERENTIATION. Evolution 2017; 52:344-354. [DOI: 10.1111/j.1558-5646.1998.tb01636.x] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/1997] [Accepted: 12/19/1997] [Indexed: 11/28/2022]
Affiliation(s)
- D. Lawrence Venable
- Department of Ecology and Evolutionary Biology, BSW 310 University of Arizona Tucson Arizona 85721
| | - Eric Dyreson
- Department of Ecology and Evolutionary Biology, BSW 310 University of Arizona Tucson Arizona 85721
| | - Daniel Piñero
- Instituto de Ecología Universidad Nacional Autónoma de México, Apdo. Postal 70‐275, Ciudad Universitaria México D.F. 04510 México
| | - Judith X. Becerra
- Department of Ecology and Evolutionary Biology, BSW 310 University of Arizona Tucson Arizona 85721
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8
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Charlesworth D, Charlesworth B. QUANTITATIVE GENETICS IN PLANTS: THE EFFECT OF THE BREEDING SYSTEM ON GENETIC VARIABILITY. Evolution 2017; 49:911-920. [DOI: 10.1111/j.1558-5646.1995.tb02326.x] [Citation(s) in RCA: 109] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/1993] [Accepted: 08/12/1994] [Indexed: 11/28/2022]
Affiliation(s)
- D. Charlesworth
- Department of Ecology and Evolution University of Chicago 1101 East 57th Street Chicago Illinois 60637‐1573
| | - B. Charlesworth
- Department of Ecology and Evolution University of Chicago 1101 East 57th Street Chicago Illinois 60637‐1573
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9
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Willis JH. MEASURES OF PHENOTYPIC SELECTION ARE BIASED BY PARTIAL INBREEDING. Evolution 2017; 50:1501-1511. [DOI: 10.1111/j.1558-5646.1996.tb03923.x] [Citation(s) in RCA: 54] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/1995] [Accepted: 08/24/1995] [Indexed: 11/30/2022]
Affiliation(s)
- John H. Willis
- Department of Biology University of Oregon Eugene Oregon 97403
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10
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Zakas C, Hall DW. Asymmetric dispersal can maintain larval polymorphism: a model motivated by Streblospio benedicti. Integr Comp Biol 2012; 52:197-212. [PMID: 22576818 DOI: 10.1093/icb/ics055] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Polymorphism in traits affecting dispersal occurs in a diverse variety of taxa. Typically, the maintenance of a dispersal polymorphism is attributed to environmental heterogeneity where parental bet-hedging can be favored. There are, however, examples of dispersal polymorphisms that occur across similar environments. For example, the estuarine polychaete Streblospio benedicti has a highly heritable offspring dimorphism that affects larval dispersal potential. We use analytical models of dispersal to determine the conditions necessary for a stable dispersal polymorphism to exist. We show that in asexual haploids, sexual haploids, and in sexual diploids in the absence of overdominance, asymmetric dispersal is required in order to maintain a dispersal polymorphism when patches do not vary in intrinsic quality. Our study adds an additional factor, dispersal asymmetry, to the short list of mechanisms that can maintain polymorphism in nature. The region of the parameter space in which polymorphism is possible is limited, suggesting why dispersal polymorphisms within species are rare.
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Affiliation(s)
- Christina Zakas
- Department of Genetics, University of Georgia, Athens, GA 30606, USA.
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11
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Riba M, Mayol M, Giles BE, Ronce O, Imbert E, Van Der Velde M, Chauvet S, Ericson L, Bijlsma R, Vosman B, Smulders MJM, Olivieri I. Darwin's wind hypothesis: does it work for plant dispersal in fragmented habitats? THE NEW PHYTOLOGIST 2009; 183:667-677. [PMID: 19659587 DOI: 10.1111/j.1469-8137.2009.02948.x] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/10/2023]
Abstract
Using the wind-dispersed plant Mycelis muralis, we examined how landscape fragmentation affects variation in seed traits contributing to dispersal. Inverse terminal velocity (Vt(-1)) of field-collected achenes was used as a proxy for individual seed dispersal ability. We related this measure to different metrics of landscape connectivity, at two spatial scales: in a detailed analysis of eight landscapes in Spain and along a latitudinal gradient using 29 landscapes across three European regions. In the highly patchy Spanish landscapes, seed Vt(-1)increased significantly with increasing connectivity. A common garden experiment suggested that differences in Vt(-1) may be in part genetically based. The Vt(-1) was also found to increase with landscape occupancy, a coarser measure of connectivity, on a much broader (European) scale. Finally, Vt(-1)was found to increase along a south-north latitudinal gradient. Our results for M. muralis are consistent with 'Darwin's wind dispersal hypothesis' that high cost of dispersal may select for lower dispersal ability in fragmented landscapes, as well as with the 'leading edge hypothesis' that most recently colonized populations harbour more dispersive phenotypes.
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Affiliation(s)
- Miquel Riba
- CREAF (Center for Ecological Research and Forestry Applications), Autonomous University of Barcelona, ES-08193 Bellaterra, Spain
| | - Maria Mayol
- CREAF (Center for Ecological Research and Forestry Applications), Autonomous University of Barcelona, ES-08193 Bellaterra, Spain
| | - Barbara E Giles
- Department of Ecology and Environmental Science, Umeå University, SE-901 87 Umeå, Sweden
| | - Ophélie Ronce
- Université Montpellier 2, Institut des Sciences de l'Evolution, UMR CNRS 5554, Place Eugène Bataillon, F-34095 Montpellier cedex 05, France
| | - Eric Imbert
- Université Montpellier 2, Institut des Sciences de l'Evolution, UMR CNRS 5554, Place Eugène Bataillon, F-34095 Montpellier cedex 05, France
| | - Marco Van Der Velde
- Animal Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, PO Box 14, NL-9750 AA Haren, The Netherlands
| | - Stéphanie Chauvet
- Université Montpellier 2, Institut des Sciences de l'Evolution, UMR CNRS 5554, Place Eugène Bataillon, F-34095 Montpellier cedex 05, France
- Association Tela Botanica, Institut de Botanique, 163 Rue Auguste Broussonnet, F-34090 Montpellier, France
| | - Lars Ericson
- Department of Ecology and Environmental Science, Umeå University, SE-901 87 Umeå, Sweden
| | - R Bijlsma
- Population and Conservation Genetics, Theoretical Biology, University of Groningen, PO Box 14, NL-9750 AA Haren, The Netherlands
| | - Ben Vosman
- Plant Research International, Wageningen UR, PO Box 16, NL-6700 AA Wageningen, The Netherlands
| | - M J M Smulders
- Plant Research International, Wageningen UR, PO Box 16, NL-6700 AA Wageningen, The Netherlands
| | - Isabelle Olivieri
- Université Montpellier 2, Institut des Sciences de l'Evolution, UMR CNRS 5554, Place Eugène Bataillon, F-34095 Montpellier cedex 05, France
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12
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Ronsheim ML, Bever JD. Genetic variation and evolutionary trade-offs for sexual and asexual reproductive modes in Allium vineale (Liliaceae). AMERICAN JOURNAL OF BOTANY 2000. [PMID: 11118412 DOI: 10.2307/2656827] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/13/2023]
Abstract
Populations of Allium vineale commonly include individuals with very different allocation patterns to three modes of reproduction: sexual flowers, aerially produced asexual bulbils, and belowground asexual offsets. If selection is currently acting to maintain these different allocation patterns there must be a genetic basis for variation in allocation to these three reproductive modes. In addition, negative genetic correlations between reproductive traits would imply evolutionary trade-offs among reproductive strategies. We evaluated the heritability of these allocation patterns by growing 16 clones from a single population in the greenhouse at two levels of fertilization. Bulb mass and the mass and number of bulbils, offsets, and flowers were used as response variables, in addition to the proportion allocated to each reproductive mode. We found evidence of substantial heritable variation in allocation to sexual reproduction and in allocation within the two modes of asexual reproduction, indicating high sensitivity of these allocation patterns to natural selection. We also found evidence of strong negative genetic correlations between bulbil and flower traits, as well as between bulbil and offset traits, with one group of genotypes allocating greater resources to aerial asexual bulbils and the second group allocating more resources to belowground asexual offsets and aerial flowers. Phenotypic plasticity in allocation to above- vs. belowground asexual reproduction and sexual vs. asexual aerial reproduction was limited, indicating that plants are unlikely to change reproductive mode in response to nutrient availability. Together, then, we have demonstrated strong heritability for, and trade-offs in, the reproductive allocation patterns within this plant population.
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Affiliation(s)
- M L Ronsheim
- Department of Biology, Box 513, Vassar College, 124 Raymond Avenue, Poughkeepsie, New York 12604-0513 USA; and
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13
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The interactive effects of temperature, food level and maternal phenotype on offspring size in Daphnia magna. Oecologia 1996; 107:189-196. [PMID: 28307304 DOI: 10.1007/bf00327902] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/1995] [Accepted: 01/29/1996] [Indexed: 10/26/2022]
Abstract
Invertebrate offspring are usually larger in colder environments. To test for possible effects of covariates (e.g. maternal phenotype and feeding conditions) on this pattern, we performed a laboratory experiment to look at the effect of temperature on newborn weight in the planktonic crustacean Daphnia magna. Three tempèratures (12°C, 16°C and 22°C) and two food levels (10,000 cells ml-1 and 150,000 cells ml-1) were used, and offspring were examined from the first five clutches of mothers that had been maintained under the constant experimental conditions for three generations. Preliminary analysis suggested that newborn weight was significantly affected by temperature although patterns in the data were not clear cut. In addition, the covariates mother weight and clutch size were positively and negatively correlated with newborn weight, respectively; and later clutches tended to contain heavier offspring. Therefore, in an effort to control for the effects of the covariates, repeated-measures analysis of covariance was performed using ratio values of newborn weight/mother weight (relative newborn weight) as the dependent variable, clutch size as the covariate and clutch number as the repeated measures term. Now, temperature as a main effect in an ANCOVA model did not significantly influence relative newborn weight. The repeatedmeasure term clutch number also became nonsignificant, indicating that when differences in mother weight due to age were accounted for there were no overall differences in relative newborn weight between clutches from a particular mother. Temperature effects on relative newborn weight were only significant as part of interaction terms with food concentration and with clutch number. Thus there were different weight responses to temperature within food levels, and between clutch numbers within food levels. Under the low-food conditions newborn were heaviest at 16°C, lightest at 12°C and intermediate at 22°C. Conversely, under the high-food condition newborn were lightest at 16°C, heaviest at 12°C and again intermediate at 22°C. However, newborn tended to be heavier under the low food condition, and food concentration was highly significant as a main effect. Mother growth rate showed no significant relationship with newborn weight. It is concluded that direct temperature effects on relative newborn weight are marginal and nonsignificant. Temperature effects through interactions with food concentration and clutch number are important determinants of newborn weight, but relatively speaking account for only a small proportion of observed variance in newborn weight (25%), compared with the direct effect of food concentration (67%).
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