Plasticity and evolution in drought avoidance and escape in the annual plant Brassica rapa

Department of Biology, Fordham University, 160 Larkin Hall, 441 E. Fordham Road, Bronx, NY 10458, USA.
New Phytologist (Impact Factor: 7.67). 04/2011; 190(1). DOI: 10.1111/j.1469-8137.2010.03603.x
Source: PubMed


• A key question in ecological genetics is to what extent do plants adapt to changes in climatic conditions, such as drought, through plasticity or evolution. • To address this question, seeds of 140 maternal families of Brassica rapa were generated from collections made before (1997) and after (2004) a natural drought. These seeds were planted in the glasshouse and grown under low-water and high-water conditions. • Post-drought lines flowered earlier than pre-drought lines, showing an evolutionary shift to earlier flowering. There was significant genetic variation and genotype by environment (G × E) interactions in flowering time, indicating genetic variation in plasticity in this trait. Plants that flowered earlier had fewer leaf nodes and lower instantaneous (A/g) and integrated (δ(13) C) water use efficiency than late-flowering plants. • These results suggest that B. rapa plants escape drought through early flowering rather than avoid drought through increased water use efficiency. The mechanism of this response appears to be high transpiration and inefficient water use, leading to rapid development. These findings demonstrate a trade-off between drought avoidance and escape, and indicate that, in this system, where drought acts to shorten the growing season, selection for drought escape through earlier flowering is more important than phenotypic plasticity.

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Available from: Steven Joseph Franks, Mar 17, 2014
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    • "plant and organ growth rate and phenology (Eckhart et al. 2004). For some annual species, arid environments favor early maturity as a drought escape mechanism (e.g., Donovan et al. 2007; Franks 2011). In our system, the subspecies in the more arid part of the range (parviflora) flowers earlier and has shorter flower development time, as might be expected, yet the subspecies in the wetter portion of the range (xantiana) tends to exhibit faster rates of leaf growth (Eckhart et al. 2004). "
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    ABSTRACT: Sister taxa with distinct phenotypes often occupy contrasting environments in parapatric ranges, yet we generally do not know whether trait divergence reflects spatially-varying selection. We conducted a reciprocal transplant experiment to test whether selection favors "native phenotypes" in two subspecies of Clarkia xantiana (Onagraceae), an annual plant in California. For four quantitative traits that differ between subspecies, we estimated phenotypic selection in subspecies' exclusive ranges and their contact zone in two consecutive years. We predicted that in the arid, pollinator-scarce eastern region, selection favors phenotypes of the native subspecies parviflora: small leaves, slow leaf growth, early flowering, and diminutive flowers. In the wetter, pollinator-rich, western range of subspecies xantiana, we expected selection for opposite phenotypes. We investigated pollinator contributions to selection by comparing naturally-pollinated and pollen-supplemented individuals. For reproductive traits and for subspecies xantiana, selection generally matched expectations. The contact zone sometimes showed distinctive selection, and in ssp. parviflora selection sometimes favored non-native phenotypes. Pollinators influenced selection on flowering time but not on flower size. Little temporal variation in selection occurred, possibly because of plastic trait responses across years. Though there were exceptions and some causes of selection remain obscure, phenotypic differentiation between subspecies appears to reflect spatially variable selection. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
    Evolution 08/2015; 69(9). DOI:10.1111/evo.12745 · 4.61 Impact Factor
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    • "Smaller and thicker leaves could be an adaptation to reduce water loss during warm and dry spells (Scheepens et al., 2010b) at the continental sites. Early flowering could be an adaptation to avoid heat or drought periods (Latta and Gardner, 2009; Franks, 2011), which may be exacerbated by the dark rock faces or screes in S. sponhemica habitats that strongly absorb solar radiation. The earlier timing of reproduction and the harsher climate may have led to the observed lower reproduction and reduced petal size (Obeso, 2002) at the more "
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    ABSTRACT: The effects of habitat fragmentation on quantitative genetic variation in plant populations are still poorly known. Saxifraga sponhemica is a rare endemic of Central Europe with a disjunct distribution, and a stable and specialized habitat of treeless screes and cliffs. This study therefore used S. sponhemica as a model species to compare quantitative and molecular variation in order to explore (1) the relative importance of drift and selection in shaping the distribution of quantitative genetic variation along climatic gradients; (2) the relationship between plant fitness, quantitative genetic variation, molecular genetic variation and population size; and (3) the relationship between the differentiation of a trait among populations and its evolvability. Genetic variation within and among 22 populations from the whole distribution area of S. sponhemica was studied using RAPD (random amplified polymorphic DNA) markers, and climatic variables were obtained for each site. Seeds were collected from each population and germinated, and seedlings were transplanted into a common garden for determination of variation in plant traits. In contrast to previous results from rare plant species, strong evidence was found for divergent selection. Most population trait means of S. sponhemica were significantly related to climate gradients, indicating adaptation. Quantitative genetic differentiation increased with geographical distance, even when neutral molecular divergence was controlled for, and QST exceeded FST for some traits. The evolvability of traits was negatively correlated with the degree of differentiation among populations (QST), i.e. traits under strong selection showed little genetic variation within populations. The evolutionary potential of a population was not related to its size, the performance of the population or its neutral genetic diversity. However, performance in the common garden was lower for plants from populations with reduced molecular genetic variation, suggesting inbreeding depression due to genetic erosion. The findings suggest that studies of molecular and quantitative genetic variation may provide complementary insights important for the conservation of rare species. The strong differentiation of quantitative traits among populations shows that selection can be an important force for structuring variation in evolutionarily important traits even for rare endemic species restricted to very specific habitats. © The Author 2015. Published by Oxford University Press on behalf of the Annals of Botany Company.
    Annals of Botany 04/2015; 115(7). DOI:10.1093/aob/mcv040 · 3.65 Impact Factor
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    • "Consistent with drought escape, water-limited conditions often favor earlier flowering (e.g., Bennington and McGraw 1995; Heschel and Riginos 2005; Franks et al. 2007; Franks 2011; Ivey and Carr 2012; but see Sherrard and Maherali 2006). Plant species and populations often show genetically based phenotypic differentiation consistent with patterns of differential selection and/or predictions of climatic adaptation (e.g., Dudley 1996a,b; McKay et al. 2001; Heschel et al. 2002; Franks et al. 2007; Lowry et al. 2008; Wu et al. 2010; Franks 2011). Additionally, WUE and flowering time are often positively correlated (e.g., McKay et al. 2003; Sherrard and Maherali 2006); however, this is not always the case (Ivey and Carr 2012) and may depend on the scale of comparison (Wu et al. 2010) or developmental stage (Sherrard and Maherali 2006). "
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    ABSTRACT: Flowering time and water-use efficiency (WUE) are two ecological traits that are important for plant drought response. To understand the evolutionary significance of natural genetic variation in flowering time, WUE, and WUE plasticity to drought in Arabidopsis thaliana, we addressed the following questions: (1) How are ecophysiological traits genetically correlated within and between different soil moisture environments? (2) Does terminal drought select for early flowering and drought escape? (3) Is WUE plasticity to drought adaptive and/or costly? We measured a suite of ecophysiological and reproductive traits on 234 spring flowering accessions of A. thaliana grown in well-watered and season-ending soil drying treatments, and quantified patterns of genetic variation, correlation, and selection within each treatment. WUE and flowering time were consistently positively genetically correlated. WUE was correlated with WUE plasticity, but the direction changed between treatments. Selection generally favored early flowering and low WUE, with drought favoring earlier flowering significantly more than well-watered conditions. Selection for lower WUE was marginally stronger under drought. There were no net fitness costs of WUE plasticity. WUE plasticity (per se) was globally neutral, but locally favored under drought. Strong genetic correlation between WUE and flowering time may facilitate the evolution of drought escape, or constrain independent evolution of these traits. Terminal drought favored drought escape in these spring flowering accessions of A. thaliana. WUE plasticity may be favored over completely fixed development in environments with periodic drought.
    Ecology and Evolution 11/2014; 4(23). DOI:10.1002/ece3.1270 · 2.32 Impact Factor
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