Multiple genomes give switchgrass an advantage.

Abstract

Admixed Unknown Switchgrass (Panicum virgatum) is a keystone species of the North American tallgrass prairie. The habitats of switchgrass have undergone many cycles of contraction and expansion over time, owing to the advances and retreats of glaciers. This makes it a good species in which to study how plants adapt to varied and variable environments — a timely subject, given that Earth is experiencing what is projected to be the most rapid change in global temperature for at least the past 65 million years. More over, given its high biomass yield and tolerance to drought, switchgrass has long been bred as a forage crop for livestock, and has been studied as a model bioenergy feedstock crop since 1992 (ref. 1). As such, the high-quality genome sequence for switchgrass now reported by Lovell et al. in Nature is of great value. Polyploidy — having more than two complete sets of chromosomes — is a common theme in plant evolution, and has been hypothesized to promote the generation and preservation of gene variants that can confer adaptation to new ecological niches. Like most polyploid plants, switchgrass arose from a cross between two species and a whole-genome duplication, thereby giving it four sets of chromosomes (a state called tetraploidy, involving two subgenomes, one from each parental species). Switchgrass plants have many different versions of each gene because progeny are typically produced from the mating of two genetically different individuals (they are outbred), rather than by self-pollination. Such complexity makes the genomes of polyploid outbred plants difficult to sequence, but Lovell et al. took advantage of advances in long-read DNA sequencing technology to generate a complete and highly accurate reference genome for tetraploid switchgrass (varieties that have eight sets of chromosomes also exist, but were not included in the current study). The authors used this genome to estimate that the two parental species of switchgrass diverged from a common ancestor about 6.7 million years ago, and that the two genomes came back together in a whole-genome duplication at least 4.6 million years ago. This places the evolutionary origins of switchgrass in the latter, and, on average, colder and more variable, part of the ongoing Late Cenozoic ice age, which began 33.9 million years ago. The parental species of switchgrass were probably from the tropics of Central or South America; how a polyploid genome might have contributed to the northward migration of switchgrass to temperate latitudes during a time of increased climate change and low temperatures is a key question. Currently, switchgrass has a vast natural geographical and environmental range, from the tropics of central Mexico in the south to Plant biology