It is well established that switchgrass has a base chromosome number of x= nine (Gould 1975). Diploid (2n=2x=18) was reported once by Nielsen (1944), but not confirmed for its existence in recent extensive investigations (Riley and Vogel 1982; Hopkins et al. 1996; Hultquist et al. 1996; Lu et al. 1998; Costich et al. 2010). Tetraploid (2n=4x=36) is the sole ploidy level in lowland ecotype and also one of the two major ploidy levels (2n=4x=36 and 2n=8x=72) in upland ecotype (Church 1940; Nielsen 1944; McMillan and Weiler 1959; Porter 1966; Riley and Vogel 1982; Hopkins et al. 1996; Hultquist et al. 1996; Lu et al. 1998; Costich et al. 2010). Hexaploid

(2n=6x=54) was reported for upland plants in several studies, but not confirmed in the recent extensive investigation by Costich et al. (2010). This may be interpreted by Martinez-Reyna and Vogel (2002) reporting a post-fertilization incompatibility system that exists in crosses of tetraploid

by octoploid in preventing the production of hexaploid progeny. Octoploid occurs at a higher frequency than tetraploid in upland plants (Church 1940; Nielsen 1944; McMillan and Weiler 1959; Porter 1966; Riley and Vogel 1982; Hopkins et al. 1996; Hultquist et al. 1996; Lu et al. 1998; Costich et al. 2010). Higher ploidy levels (2n=10x=90 and 2n=12x=108) was only reported in an early study (Nielsen 1944), but not confirmed in recent studies (Riley and Vogel 1982; Hopkins et al. 1996; Hultquist et al. 1996; Lu et al. 1998; Costich et al. 2010). Using flow cytometry, chromosome counting and florescent in situ hybridization, Costich et al. (2010) reported extensive aneuploidy chromosome numbers in both upland and lowland switchgrass. They reported observations of aneuploidy at 86% of octoploid chromosome counts, but only 23% for tetraploid counts, suggesting a less stable genome in the octoploid upland switchgrass.

Homologous chromosome pairing and segregation behavior provide critical information with respect to sexual reproduction, chromosome homology and species evolution, and breeding procedures. Chromosome pairing in meiosis of tetraploid plants in both lowland and upland types is regular (Barnett and Carver 1967; Brunken and Estes 1975; Lu et al. 1998). Various studies all reported consistent bivalent pairing in tetraploid switchgrass plants. Martinez-Reyna et al. (2001) reported that chromosome pairing of hybrids between lowland and upland tetraploid parents was primarily bivalent. The regularity of tetraploid chromosomes in meiosis was substantiated by high vigor of pollen grains and good set of seeds (Barnett and Carver 1967; Martinez-Reyna et al. 2001). One important result of Martinez-Reyna et al. (2001) revealed that the genomes of tetraploid upland and tetraploid lowland are highly similar. More recently, two independent groups reported nuclear genome inheritance of lowland tetraploid is disomic (Okada et al. 2010b; Liu and Wu 2012; Liu et al. 2012). Using a population of 279 first-generation selfed progeny of a lowland tetraploid plant genotyped by 12 simple sequence repeat markers, Liu and Wu (2012) demonstrated segregation of the polymorphic codominant markers was consistent with a typical diploid "1:2:1" Mendelian segregation ratio (Fig. 4). More recently experimental results revealed that tetraploid switchgrass is an allotetraploid containing two distinct ("A" and "B") genomes (Young et al. 2010; Liu et al. 2012; Triplett et al. 2012).

Working with octoploids and its aneuploids, and a hexaploid, Barnett and Carver (1967) observed more univalents than with tetraploids. Trivalents and quadrivalents occur at a low number of cells in octoploids and its aneuploids, but were not observed in tetraploid cells (Barnett and Carver 1967). They observed much higher frequencies of abnormal pollen grains in octoploids and aneuploids than in tetraploids and a hexaploid. More studies indicated homologous chromosome pairing of octoploids is primarily of bivalent associations although univalent and multivalents

also occurred at a lower rate (Brunken and Estes 1975; Lu et al. 1998). The prevalence of bivalent associations in meioses of octoploid switchgrass may suggest that some mechanism(s) have evolved to insure the pairing behavior if octoploids are autopolyploid (Lu et al. 1998). Inheritance of octoploid is not known yet.

In addition to the pairing behavior and inheritance information of chromosomes in switchgrass, the mode of inheritance of chloroplast DNA in tetraploids was determined. Using upland x lowland reciprocal hybrids and their DNA samples hybridized with a special chloroplast DNA probe, Martinez-Reyna et al. (2001) indicated the maternal inheritance of chloroplast DNA in tetraploid switchgrass, which is consistent with those of most angiosperm species.