Flow Cytometry for Estimating Plant Genome Size: Revisiting Assumptions, Sources of Variation, Reference Standards, and Best Practices

Abstract

Flow cytometry has been widely used to estimate relative and absolute genome sizes (DNA contents) of plants for more than 50 years. However, the accuracy of these estimates can vary widely because of many factors, including errors in the genome size estimates of reference standards and various experimental methods. The objectives of this study were to reassess genome sizes of commonly used reference standards and quantify sources of variation and error in estimating plant genome sizes that arise from buffers, confounding plant tissues, tissue types, and plant reference standards using both 40,6-diamidino-2-phenylindole (DAPI) and propidium iodide (PI) fluorochromes. Five separate studies were performed to elucidate these objectives. Revised estimates of genome sizes of commonly used plant reference standards were determined using human male leukocytes as a primary standard with an updated genomesize (6.15 pg; 12.14% lower than that of earlier studies) using both DAPI and PI fluorochromes. Comparison of six different buffers (Galbraith’s, LB01, MB01, MgSO4, Otto’s, and Sysmex) resulted in variations in genome size estimatesby as much as 18.1% for a given taxon, depending on the buffer–fluoro chrome combination. The addition of different confounding plant tissues (representing 10 diverse taxa and associated secondary metabolites) resulted in variations in genome size estimates by as much as 10.3%, depending on the tissue–fluoro chrome combination. Different plant tissue types (leaf color/exposure and roots) resulted in a variation in genome size estimates of 10.7%, independent of the fluorochrome. The selection of different internal reference standards introduced an additional variation in genome size estimates of 5.9%, depending on the standard–fluorochrome combination. The choice of fluoro chrome (DAPI vs. PI) hadone of the largest impacts on genome size estimates and differed by as much as 32.9% for Glycine max ‘Polanka when using human male leukocytes as an internal standard. A portion of this variation (~10.0%) can be attributed to the base pair (bp) bias of DAPI and variations in Guanine-Cytosine (GC): Adenine-Thymine (AT) ratios between the sample and standard. However, as much as 22.9% of the variation in genome size estimates may result from how effectively these fluorochromes stain and report the genome. The combined variation/error from all these factors (excluding variation from bp bias for different fluorochromes, and assuming variations from confounding tissues and tissue types to both result from secondary metabolites) to taled 57.6%. Additional details of how selected factors impact accuracy, precision, and the interaction of these factors are presented. Overall, flow cytometry can be precise, repeatable, and extremely valuable for determining the relative genome size and ploidy of closely related plants when using consistent methods, regardless of fluorochrome. However, accurate determination of the absolute genome size by flow cytometry remains elusive, and estimates of genome size using flow cytometry should be considered gross approximations thatmay vary by ±29% or more as a function of experimental methods and plant environments. Additional recommendations of best practices are provided.