Abstract
The process of plant speciation often involves the evolution of divergent ecotypes in response to differences in soil water availability between habitats. While the same set of traits is frequently associated with xeric/mesic ecotype divergence, it is unknown whether those traits evolve independently or if they evolve in tandem as a result of genetic colocalization either by pleiotropy or genetic linkage.The self-fertilizing C4 grass species Panicum hallii includes two major ecotypes found in xeric (var. hallii) or mesic (var. filipes) habitats. We constructed the first linkage map for P. hallii by genotyping a reduced representation genomic library of an F2 population derived from an intercross of var. hallii and filipes. We then evaluated the genetic architecture of divergence between these ecotypes through quantitative trait locus (QTL) mapping.Overall, we mapped QTLs for nine morphological traits that are involved in the divergence between the ecotypes. QTLs for five key ecotype-differentiating traits all colocalized to the same region of linkage group five. Leaf physiological traits were less divergent between ecotypes, but we still mapped five physiological QTLs. We also discovered a two-locus Dobzhansky–Muller hybrid incompatibility.Our study suggests that ecotype-differentiating traits may evolve in tandem as a result of genetic colocalization.
Highlights
The process of speciation often occurs as a continuum over time through the accumulation of reproductive isolating barriers that prevent gene flow between new species (Dobzhansky, 1937; Clausen, 1951; Wu, 2001; Schemske, 2010; Nosil, 2012)
We addressed the following four major questions: what genomic regions contribute to the morphological and leaf physiological trait variation/divergence within and among var. hallii and var. filipes? Do quantitative trait locus (QTL) for multiple ecotype-differentiating traits cluster in particular chromosomal regions? What are the patterns of synteny between P. hallii and S. italica and is there evidence for colocalization of flowering time QTLs between those two species? What reproductive isolating barriers might account for maintenance of divergence between var. hallii and var. filipes?
Our study suggests that some ecotypedifferentiating traits evolve independently while others may evolve in tandem due to genetic colocalization resulting from linkage or pleiotropy
Summary
The process of speciation often occurs as a continuum over time through the accumulation of reproductive isolating barriers that prevent gene flow between new species (Dobzhansky, 1937; Clausen, 1951; Wu, 2001; Schemske, 2010; Nosil, 2012). Adaptation to soil water availability can drive the evolution of divergence in physiology, development, morphology, phenology, life-history, and reproductive allocation strategies between plant ecotypes (Stebbins, 1952; Clausen & Heisey, 1958; Baker, 1972; Roux et al, 2006; Juenger, 2013). Ecotypes that are adapted to mesic habitats typically have leaves with greater area, more axillary branching, and are larger overall than ecotypes adapted to drier habitats (Clausen & Heisey, 1958; Kruckeberg, 1986; Rajakaruna, 2004; Lowry, 2012) These classic differences in morphology suggest a trade-off between aboveground growth, of leaves, and the demand for water imposed by transpiration, which results from a greater transpiring surface area (Geber & Dawson, 1990; Dudley, 1996; Ackerly et al, 2000). Ecotypes occurring in dry habitats frequently evolve to develop fast and flower early, a life history that avoids periods
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