Genome-Size Variation In Switchgrass (Panicum virgatum): Examination of the Origin and Consequences of Aneuploidy.

Switchgrass (Panicum virgatum L.) has been targeted as a model herbaceous species for biofeedstock development, with little attention paid to the complexity of its extreme ploidy-level variation.  A flow-cytometric 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 root tip cell preparations indicated that aneuploidy was more common in octoploids (86.3%) than tetraploids (23.2%).  The incidence of hyper- versus hypoaneuploidy is equivalent in tetraploids, unlike octoploids, where close to 90% of the aneuploid counts are lower than the euploid number.  Cytogenetic investigation using FISH revealed an unexpected degree of variation in chromosome structure underlying this apparent genomic instability in the upland ecotype.  Future studies will involve counts of meiotic (pollen mother cell) chromosomes and flow-cytometric surveys of mature pollen from known aneuploid and euploid plants.