Academic journal article Genetics

Selective Sweeps Reveal Candidate Genes for Adaptation to Drought and Salt Tolerance in Common Sunflower, Helianthus Annuus

Academic journal article Genetics

Selective Sweeps Reveal Candidate Genes for Adaptation to Drought and Salt Tolerance in Common Sunflower, Helianthus Annuus

Article excerpt

ABSTRACT

Here we report the results of an analysis of variation at 128 EST-based microsatellites in wild Helianthus annuus, using populations from the species' typical plains habitat in Kansas and Colorado, as well as two arid desert and two distinct brackish marsh areas in Utah. The test statistics lnRV and lnRH were used to find regions of the genome that were significantly less variable in one population relative to the others and thus are likely to contain genes under selection. A small but detectable percentage (1.5-6%) of genes showed evidence for selection from both statistics in any particular environment, and a total of 17 loci showed evidence of selection in at least one environment. Distance-based measures provided additional evidence of selection for 15 of the 17 loci. Global F^sub ST^-values were significantly higher for candidate loci, as expected under divergent selection. However, pairwise F^sub ST^-values were lower for populations that shared a selective sweep. Moreover, while spatially separated populations undergoing similar selective pressures showed evidence of divergence at some loci, they evolved in concert at other loci. Thus, this study illustrates how selective sweeps might contribute both to the integration of conspecific populations and to the differentiation of races or species.

BECAUSE plants are sessile during a significant fraction of their life cycle, they are subject to strong selection for optimal performance under local environmental conditions. As a consequence, most plant populations are expected to exhibit significant local adaptation, an expectation that has been corroborated by a century of reciprocal transplant and common garden studies (TURESSON 1922; CLAUSEN et al. 1940, 1948; SCHEMSKE 1984; STANTON and GALEN 1997; DONOHUE et al. 2000; RAMSEY et al. 2003; VERHOEVEN et al. 2004; ANGERT and SCHEMSKE 2005). Significant progress has been made toward identifying phenotypic traits that contribute to adaptation and estimating the strength of selection on these traits (ARNQVIST 1992; DUDLEY 1996; DUDLEY and SCHMITT 1996; NAGY 1997; PETIT and THOMPSON 1998; WEINIG 2000; KINGSOLVER et al. 2001; CALLAHAN and PIGLIUCCI 2002; UNGERER et al. 2002; MCKAY et al. 2003; ETTERSON 2004; GROSS et al. 2004; GRIFFITH and WATSON 2005). However, little is known about the genetic basis of locally adapted differences, particularly outside of the model plant, Arabidopsis thaliana (MITCHELLOLDS 1996; PURUGGANAN and SUDDITH 1998; UNGERER et al. 2002; CORK and PURUGGANAN 2005; STINCHCOMBE et al. 2005). Important issues that remain to be addressed include the number, kinds, and effects of genes and mutations that contribute to local adaptation, the source of adaptive variants (new mutation, standing variation, or migration), the effects of genetic background and environment on allelic fitness, and the geographic extent of selective sweeps.

Providing answers to all of these questions, even within a singlemodel system, is a formidable task.Nonetheless, progress is being made on several fronts. For example, QTL studies have been useful for estimating aminimum number of genes underlying fitness traits, either under laboratory (SARIGORLA et al. 1992; MITCHELLOLDS 1996; BRADSHAW et al. 1998; FRY et al. 1998; KIM and RIESEBERG 1999; UNGERER et al. 2003) or under field (SCHEMSKE and BRADSHAW 1999; LEXER et al. 2003; WEINIG et al. 2003b) conditions. QTL approaches also have been employed to estimate the fitness effects of particular genotypes across environments and have shown that different QTL may contribute to fitness when the environment changes (WEINIG et al. 2003a). While these kinds of studies provide insight into the complex and conditional nature of adaptation to local environmental conditions, they rarely lead to a detailed understanding of the genetic basis of adaptation because a given QTL typically contains dozens or hundreds of genes, any one (or several) of which might be under selection. …

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