Genome‐size Variation in Switchgrass ( Panicum virgatum ): Flow Cytometry and Cytology Reveal Rampant Aneuploidy

Abstract

Switchgrass (Panicum virgatum L.), a native perennial dominant of the prairies of North America, has been targeted as a model herbaceous species for biofeedstock development. A fl owcytometric survey of a core set of 11 primarily upland polyploid switchgrass accessions indicated that there was considerable variation in genome size within each accession, particularly at the octoploid (2n = 8X = 72 chromosome) ploidy level. Highly variable chromosome counts in mitotic cell preparations indicated that aneuploidy was more common in octoploids (86.3%) than tetraploids (23.2%). Furthermore, the incidence of hyper-versus hypoaneuploidy is equivalent in tetraploids. This is clearly not the case in octoploids, where close to 90% of the aneuploid counts are lower than the euploid number. Cytogenetic investigation using fl uorescent in situ hybridization (FISH) revealed an unexpected degree of variation in chromosome structure underlying the apparent genomic instability of this species. These results indicate that rapid advances in the breeding of polyploid biofuel feedstocks, based on the molecular-genetic dissection of biomass characteristics and yield, will be predicated on the continual improvement of our understanding of the cytogenetics of these species. T HE GENETIC STRUCTURE of an organism has profound eff ects on all aspects of its basic biology. Th e manner in which its genes are distributed, whether in few or many chromosomes, the numbers of whole sets of chromosomes (ploidy levels) present, and the presence or absence of individual copies of chromosomes, all contribute to its phenotype. Ploidy diff erences among individuals in a population will determine mating patterns and gene fl ow, aff ecting the distribution of genetic variation both within and among individuals. In natural populations, the resulting phenotypic variation is the foundation for ecotype formation, and, ultimately, over time, can lead to speciation. In a breeding program, it is the raw material for cultivar development and improvement of the crop. Th us, a thorough knowledge of the extent of any form of genome-size variation in a study organism is a critical fi rst step for any type of genetics or genomics research program.Switchgrass (Panicum virgatum L.) has been the subject of intensive and extensive agronomic and breeding research for biofeedstock production since it was

Production of Autopolyploid Lowland Switchgrass Lines Through In Vitro Chromosome Doubling

“…FISH, using pTa71 as a probe, detected two pairs of telomeric hybridization sites (Figure 4). A similar pTa71 FISH pattern was previously observed in all tetraploid accessions of switchgrass analyzed [39]. This indicates that the number and location of nucleolus organizer regions (NORs) in tetraploid accessions is highly conserved.…”

“…However, this pattern is unique and very different from the octoploid accessions of switchgrass that were previously analyzed [39]. Those octoploid accessions were highly variable in NOR size and number and also differed in the chromosomal location of the probe.…”

“…The results of this study suggests that aneuploidy is rampant in natural switchgrass populations [39]. In this study, we employed both flow cytometry and a root tip chromosome counting method to assay the genome size of the autooctoploids and their derived hybrid plants (Figure 2 and 4).…”

Linkage Maps of Lowland and Upland Tetraploid Switchgrass Ecotypes

“…Aneuploidy has been observed in tetraploid as well as octoploid switchgrass, but the frequency is much higher in the latter [27]. The same authors also demonstrated through in situ hybridization of a ribosomal probe to mitotic metaphase chromosomes that many octoploids showed chromosomal rearrangements.…”

“…The same authors also demonstrated through in situ hybridization of a ribosomal probe to mitotic metaphase chromosomes that many octoploids showed chromosomal rearrangements. No such rearrangements were observed in the tetraploids examined, but the resolution of this study was low since only a single probe was used [27].…”