2019 journal article
Variation in Genome Size, Ploidy, Stomata, and rDNA Signals in Althea
Journal of the American Society for Horticultural Science, 144(2), 130–140.
Althea ( Hibiscus syriacus ) is a shrub prized for its winterhardiness and colorful summer flowers. Altheas are tetraploids (2 n = 4 x = 80); however, breeders have developed hexaploids and octoploids. Previous studies report anatomical variation among polyploids, including stomata size. The purpose of this study was 4-fold. First, identify genome size and ploidy variation in cultivars via flow cytometry and chromosome counts. Second, create a ploidy series consisting of 4 x , 5 x , 6 x , and 8 x cytotypes. Third, investigate the ploidy series for variation in stomatal guard cell lengths, stomatal density, and copy number of fluorescent ribosomal DNA (rDNA) signals. Fourth, investigate segregation patterns of rDNA signals in a subset of pentaploid seedlings. Flow cytometry revealed most cultivars to be tetraploid with holoploid 2C genome sizes from 4.55 ± 0.02 to 4.78 ± 0.06 pg. Five taxa (‘Aphrodite’, ‘Pink Giant’, ‘Minerva’, Azurri Satin ® , and Raspberry Smoothie™) were hexaploids (6.68 ± 0.13 to 7.05 ± 0.18 pg). Peppermint Smoothie™ was a cytochimera with tetraploid cells (4.61 ± 0.06 pg) and octoploid cells (8.98 ± 0.13 pg). To create pentaploids, reciprocal combinations were made between hexaploid ‘Pink Giant’ and tetraploid cultivars. To create octoploids, seedlings were treated with agar solutions containing 0.2% colchicine or 125 μM oryzalin. Guard cell lengths were significantly different among the four cytotypes: 4 x (27.36 ± 0.04 μm), 5 x (30.35 ± 1.28 μm), 6 x (35.59 ± 0.63 μm), and 8 x (40.48 ± 1.05 μm). Measurements of stomatal density revealed a precipitous decline in average density from the 4 x cytotype (398.22 ± 15.43 stomata/mm 2 ) to 5 x cytotype (194.06 ± 38.69 stomata/mm 2 ) but no significant difference among 5 x , 6 x , and 8 x cytotypes. Fluorescent in situ hybridization (FISH) revealed an increase in 5S and 45S rDNA signals that scaled with ploidy: 4 x (two 5S + four 45S), 6 x (three 5S + six 45S), and 8 x (four 5S + eight 45S). However, pentaploid (5 x ) seedlings exhibited random segregation of rDNA signals between the 4 x and 6 x cytotypes, including all six possible combinations (two 5S, three 5S) × (four 45S, five 45S, six 45S).