Polyploidy has been an important process in the evolution of plants that can contribute to reproductive isolation, novel gene expression, and ultimately divergence and speciation (Adams and Wendel, 2005; Comai, 2005; Hegarty and Hiscock, 2008; Soltis and Burleigh, 2009; Soltis et al., 2003). Polyploidy is also an important factor in plant breeding because it can influence reproductive compatibility, fertility, and phenotypic traits (Chen and Ni, 2006; Jones and Ranney, 2009; Ranney, 2006; Soltis et al., 2004). In some cases, the artificial induction of polyploidy in Magnolia also can enhance ornamental characteristics, including thicker leaves and larger flowers with thicker petals that persist longer (Kehr, 1985). As such, accurate and specific knowledge of ploidy levels of species and cultivars is important information for magnolia breeders.
The genus Magnolia comprises more than 250 species belonging to various sections within three subgenera (Figlar and Nooteboom, 2004). Although basic information on chromosome counts and ploidy levels of different Magnolia species have been compiled (Callaway, 1994; Chen et al., 2000), sampling has been limited and little is known about ploidy levels of specific hybrids and cultivars. The base chromosome number for Magnolia is 1n = 1x = 19. However, different subgenera contain species with a variety of ploidy levels ranging from 2n = 2x = 38 to 2n = 6x = 114. Crosses between species with varying ploidy levels may yield hybrids with nonstandard chromosome numbers that can result in reduced fertility or sterility. Because of these constraints, Magnolia breeders have attempted to induce new polyploids to overcome these limitations, yet most of these putative polyploids have never been confirmed. The range in ploidy levels within this genus also provides an opportunity to indirectly substantiate hybridity when parents differ in ploidy levels.
Because many Magnolia species are polyploids with high chromosome numbers, traditional cytology based on light microscopic examination is a difficult and time-consuming process. Flow cytometry has proved to be an efficient means of estimating genome size and associated ploidy level (Doležel et al., 2007; Jones et al., 2007). Therefore, the objectives of this study were to determine the genome sizes and relationships to ploidy levels of a diverse collection of species, hybrids, and cultivars of Magnolia to 1) develop an extensive database of ploidy levels for use by magnolia breeders; 2) determine the ploidy levels of plants that were chemically treated to artificially induce polyploidy; 3) confirm hybridity of interploid and interspecific (when parents vary substantially in genome size) crosses; and 4) compare estimates of genome size using DAPI (AT preferential) or propidium iodide (PI) (intercalating) fluorochrome stains and estimate bp composition for representative taxa from 10 taxonomic sections.
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