Grabowski Paul P, Morris Geoffrey P, Casler Michael D, Borevitz Justin O
U.S. Dairy Forage Research Center, USDA-ARS, 1925 Linden Drive, Madison, WI 53706, USA; Department of Ecology and Evolution, University of Chicago, 1101 East 57th Street, Chicago, IL 60637, USA.
Mol Ecol. 2014 Aug;23(16):4059-73. doi: 10.1111/mec.12845. Epub 2014 Jul 21.
Geographic patterns of genetic variation are shaped by multiple evolutionary processes, including genetic drift, migration and natural selection. Switchgrass (Panicum virgatum L.) has strong genetic and adaptive differentiation despite life history characteristics that promote high levels of gene flow and can homogenize intraspecific differences, such as wind-pollination and self-incompatibility. To better understand how historical and contemporary factors shape variation in switchgrass, we use genotyping-by-sequencing to characterize switchgrass from across its range at 98 042 SNPs. Population structuring reflects biogeographic and ploidy differences within and between switchgrass ecotypes and indicates that biogeographic history, ploidy incompatibilities and differential adaptation each have important roles in shaping ecotypic differentiation in switchgrass. At one extreme, we determine that two Panicum taxa are not separate species but are actually conspecific, ecologically divergent types of switchgrass adapted to the extreme conditions of coastal sand dune habitats. Conversely, we identify natural hybrids among lowland and upland ecotypes and visualize their genome-wide patterns of admixture. Furthermore, we determine that genetic differentiation between primarily tetraploid and octoploid lineages is not caused solely by ploidy differences. Rather, genetic diversity in primarily octoploid lineages is consistent with a history of admixture. This suggests that polyploidy in switchgrass is promoted by admixture of diverged lineages, which may be important for maintaining genetic differentiation between switchgrass ecotypes where they are sympatric. These results provide new insights into the mechanisms shaping variation in widespread species and provide a foundation for dissecting the genetic basis of adaptation in switchgrass.
遗传变异的地理模式是由多种进化过程塑造的，包括遗传漂变、迁移和自然选择。柳枝稷（Panicum virgatum L.）尽管具有促进高水平基因流动并能使种内差异同质化的生活史特征，如风力授粉和自交不亲和，但仍具有强烈的遗传和适应性分化。为了更好地理解历史和当代因素如何塑造柳枝稷的变异，我们使用简化基因组测序对来自其分布范围内的柳枝稷进行了98042个单核苷酸多态性（SNP）的特征分析。种群结构反映了柳枝稷生态型内部和之间的生物地理和倍性差异，并表明生物地理历史、倍性不相容性和差异适应在塑造柳枝稷的生态型分化中各自发挥着重要作用。在一个极端情况下，我们确定两个黍属分类群不是不同的物种，而是实际上属于同一物种，是适应沿海沙丘栖息地极端条件的生态上不同的柳枝稷类型。相反，我们在低地和高地生态型中鉴定出天然杂种，并可视化了它们全基因组的混合模式。此外，我们确定主要四倍体和八倍体谱系之间的遗传分化不仅仅是由倍性差异引起的。相反，主要八倍体谱系中的遗传多样性与混合历史一致。这表明柳枝稷中的多倍体是由分化谱系的混合促进的，这对于维持柳枝稷生态型在同域分布时的遗传分化可能很重要。这些结果为塑造广泛分布物种变异的机制提供了新的见解，并为剖析柳枝稷适应的遗传基础提供了基础。
