Ploidy Levels, Relative Genome Sizes, and Base Pair Composition in Magnolia

ThegenusMagnoliaincludesover 250species thatrangeinploidylevelfromdiploidtohexaploid.Although there is basic information on ploidy levels of various species, sampling has been limited and little information on specific cultivars and hybrids is available. The objective of this research was to determine relative genome sizes and relationships to ploidy levels among a diverse collection of species, hybrids, and cultivars using flow cytometry. Nuclei were extracted, stained with 4#, 6-diamidino-2-phenylindole (DAPI), and analyzed using a flow cytometer. Relative genome sizes were determined using Pisum sativum as the reference genome. Genome size was calibrated with ploidy level for species with documented chromosome numbers. Relative genome size for a given ploidy level varied significantly among most taxonomic sections indicating these groups have undergone considerable genomic divergence. These data also indicate it is desirable to calibrate ploidy level with relative genome size for each section separately. Within a section, relative 2C genome sizes, for a given ploidy level, had narrow ranges and could be used to clearly distinguish between euploid levels. Genome size estimates, determined with DAPI or propidium iodide fluorochromes, varied (by 0% to 14%) as a function of species and base pair (bp) composition. Both methods were suitable for determining euploid level. Base pair composition of representative Magnolia species ranged from 61.6% to 63.91% AT. Genome sizes and ploidy levels are presented for a broad range of species and hybrids within genus Magnolia. This information also provides further insight into reproductive biology, substantiation of numerous hybrids and induced polyploids, and comparison of methods for determining genome size that will help facilitate the development of improved hybrids in the future. 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 =2 x =3 8 to 2n =6 x = 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.