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10 results for “white clover (Trifolium repens L.)”
Beyond cyanogenesis: Temperature gradients drive environmental adaptation in North American white clover (Trifolium repens L.)
<p>Species that repeatedly evolve phenotypic clines across environmental gradients have been highlighted as ideal systems for characterizing the genomic basis of local environmental adaptation. However, few studies have assessed the importance of observed phenotypic clines for local adaptation: conspicuous traits that vary clinally may not necessarily be the most critical in determining local fitness. The present study was designed to fill this gap, using a plant species characterized by repeatedly-evolved adaptive phenotypic clines. White clover is naturally polymorphic for its chemical defense cyanogenesis (HCN release with tissue damage); climate-associated cyanogenesis clines have evolved throughout its native and introduced range worldwide. We performed landscape genomic analyses on 415 wild genotypes from 43 locations spanning much of the North American species range to assess the relative importance of cyanogenesis loci vs. other genomic factors in local climatic adaptation. We find clear evidence of local adaptation, with temperature-related climatic variables best describing genome-wide differentiation between sampling locations. The same climatic variables are also strongly correlated with cyanogenesis frequencies and gene copy number variations (CNVs) at cyanogenesis loci. However, landscape genomic analyses indicate no significant contribution of cyanogenesis loci to local adaptation. Instead, several genomic regions containing promising candidate genes for plant response to seasonal cues are identified — some of which are shared with previously-identified QTLs for locally-adaptive fitness traits in North American white clover. Our findings suggest that local adaptation in white clover is likely determined primarily by genes controlling the timing of growth and flowering in response to local seasonal cues. More generally, this work suggests that caution is warranted when considering the importance of conspicuous phenotypic clines as primary determinants of local adaptation.</p>
Variable expression of cyanide detoxification and tolerance genes in cyanogenic and acyanogenic white clover (Trifolium repens L.)
<p><strong>Premise of the study:</strong> β-cyanoalanine synthase (β-CAS) and alternative oxidase (AOX) play important roles in the ability of plants to detoxify and tolerate hydrogen cyanide (HCN) stress. These functions are critical for all plants, as HCN is produced at low levels during basic metabolic processes, but are likely to be especially important in cyanogenic species, which release high levels of HCN following tissue damage. However, their expression has not been examined in cyanogenic species, nor has it been compared between cyanogenic and acyanogenic genotypes within a species.</p> <p><strong>Methods:</strong> We used a natural polymorphism for cyanogenesis in white clover to examine β-CAS and Aox gene expression in relation to cyanogenesis-associated HCN exposure. We identified all β-CAS and Aox gene copies present in the genome, including members of the <em>Aox1, Aox2a</em> and <em>Aox2d </em>subfamilies previously reported in legumes. Expression levels were compared between cyanogenic and acyanogenic genotypes, and under conditions of leaf tissue damage compared to undamaged tissue. </p> <p><strong>Key results:</strong> Results indicate that β-CAS and Aox2a expression are differentially elevated in cyanogenic genotypes, and that tissue damage is not required to induce this increased expression. <em>Aox2d</em>, in contrast, appears to be upregulated as a generalized wounding response.</p> <p><strong>Conclusions:</strong> These findings suggest a heightened constitutive role for both HCN detoxification (via elevated β-CAS expression) and HCN-toxicity mitigation (via elevated <em>Aox2a </em>expression) in plants that are capable of cyanogenesis. As such, freezing-induced cyanide autotoxicity is unlikely to be the primary selective factor in the evolution of climate-associated cyanogenesis clines. </p>
Variable expression of cyanide detoxification and tolerance genes in cyanogenic and acyanogenic white clover (Trifolium repens L.)
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Beyond cyanogenesis: Temperature gradients drive environmental adaptation in North American white clover (Trifolium repens L.)
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Data from: Searching for the bull's-eye: agents and targets of selection vary among geographically disparate cyanogenesis clines in white clover (Trifolium repens L.)
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Data from: Evolution of drought resistance strategies following the introduction of white clover (Trifolium repens L.)
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Data from: Aridity shapes cyanogenesis cline evolution in white clover (Trifolium repens L.)
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Data from: Recurrent gene deletions and the evolution of adaptive cyanogenesis polymorphisms in white clover (Trifolium repens L.)
Understanding the molecular evolution of genes that underlie intraspecific polymorphisms can provide insights into the process of adaptive evolution. For adaptive polymorphisms characterized by gene presence/absence (P/A) variation, underlying loci commonly show signatures of long-term balancing selection, with gene-presence and gene-absence alleles maintained as two divergent lineages. We examined the molecular evolution of two unlinked P/A polymorphisms that underlie a well-documented adaptive polymorphism for cyanogenesis (hydrogen cyanide release with tissue damage) in white clover. Both cyanogenic and acyanogenic plants occur in this species, and the ecological forces that maintain this chemical defense polymorphism have been studied for several decades. Using a sample of 65 plants, we investigated the molecular evolution of sequences flanking the two underlying cyanogenesis genes: Ac/ac (controlling the presence/absence of cyanogenic glucosides), and Li/li (controlling the presence/absence of their hydrolyzing enzyme, linamarase). A combination of genome-walking, PCR assays, DNA sequence analysis, and Southern blotting was used to test whether these adaptive P/A polymorphisms show evidence of long-term balancing selection, or whether gene-absence alleles have evolved repeatedly through independent deletion events. For both loci, we detect no signatures of balancing selection in closest flanking genomic sequences. Instead, we find evidence for variation in the size of the deletions characterizing gene-absence alleles. These observations strongly suggest that both of these polymorphisms have been evolving through recurrent gene deletions over time. We discuss the genetic mechanisms that could account for this surprising pattern and the implications of these findings for mechanisms of rapid adaptive evolution in white clover.
Data from: Rapid evolution of an adaptive cyanogenesis cline in introduced North American white clover (Trifolium repens L.)
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Data from: Recurrent gene deletions and the evolution of adaptive cyanogenesis polymorphisms in white clover (Trifolium repens L.)
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