Figure 1.
Figure 1.

Nuclear DNA content (picograms of DNA/nucleus) measured by flow cytometry of 182 switchgrass plants. For tetraploids (4X), n = 59, mean = 2.35, SE = 0.027, range = 1.81–2.77. For octoploids (8X), n = 123, mean = 4.88, SE = 0.033, range = 3.89–5.85.

 


Figure 2.
Figure 2.

Variation among switchgrass accessions in survey (Experiment 1): Best linear unbiased estimates of genome sizes of cultivars within ploidy levels, with the significant block effect removed, as calculated by the statistical model in Table 3. Tetraploid (4X) cultivar estimates compared to Kanlow, octoploid (8X) cultivar estimates compared to Pathfinder (* = 0.05; ** = 0.01).

 


Figure 3.
Figure 3.

Mitotic chromosome counts in metaphase meristematic cells in root tips of switchgrass plants. Sample sizes vary for individual plants. Euploid numbers are indicated in the x-axes: 4X = 36 chromosomes; 6X = 54; 8X = 72. (A) Counts from tetraploid plants: mean ± se = 36.0 ± 0.1; n = 56. (B) Counts from octoploid plants: mean ± se = 69.2 ± 0.4; n = 80.

 


Figure 4.
Figure 4.

Variation in chromosome number counts within a single root tip of ‘Cave-In-Rock’. (A) Raw images of acetocarmine-stained root tip chromosome spreads from three cells. Final chromosome counting traces have been overlaid (red). (B) Threshold images generated from raw images (in A) were used to automate counting with the Analyze Particle function of ImageJ (Rasband, 2009).

 


Figure 5.
Figure 5.

Mean genome size measured by flow cytometry versus mean mitotic chromosome number counts from root tips for individual plants (tetraploid N = 10; octoploid N = 15).

 


Figure 6.
Figure 6.

A representative fluorescent in situ hybridization (FISH) detection of nuclear organizing regions (NORs) in a tetraploid switchgrass cultivar. (A) Root tip spread prepared from ‘Dacotah’ (2n = 4X = 36) and stained with acetocarmine. (B) The same cell as in A with chromosomes stained with DAPI (pseudocolored in red) and subjected to FISH with a NOR probe (pseudocolored in green). There are two pairs of pTa71 FISH sites in tetraploids, and NORs are located at the telomeres (D) (see also Supplemental Fig. 1; Table 5). For comparison, (C) shows the acetocarmine-stained chromosomes from D.

 


Figure 7.
Figure 7.

A representative fluorescent in situ hybridization (FISH) detection of nuclear organizing regions (NORs) in an octoploid switchgrass cultivar. (A) A root tip spread prepared from ‘Blackwell’ (2n = 8X = 72) and stained with acetocarmine. (B) The same cell as in (A) with chromosomes stained with DAPI (pseudocolored in red) and subjected to FISH with a NOR probe (pseudocolored in green). There are three types of pTa71 FISH sites in octoploids (see also Supplemental Fig. 2; Table 5): (F) interstitial (located between the centromere and telomere), (G) at both telomeres on an isochromosome, and (H) telomeric. For comparison, (B), (C), and (D) show the acetocarmine-stained chromosomes from (F), (G), and (H), respectively.