Biochemical, Genetic, and Cytogenetic Studies of Stokesia laevis (Stokes Aster)
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2007-12-20
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Stokesia laevis (J. Hill) Greene is a herbaceous perennial with blue, lavender, violet, albescent, pale yellow or pale pink flowers. All cultivars are diploid (2n=2x=14) except for 'Omega Skyrocket, a tetraploid (2n=4x=28) cultivar selected from a wild population. Anthocyanidin and copigment aglycones extracted from floral tissue were characterized using high-performance liquid chromatography. Results indicated that blue, lavender, violet and albescent flowers contained petunidin, though albescent flowers contained a substantially smaller amount. Pale pink flowers were found to contain only cyanidin. Anthocyanidins and carotenoids were not present in pale yellow flowers. All flowers contained the flavone luteolin. Genetic analyses suggested that at least three genes (A, P, Y) each with two alleles control flower color: A permits normal synthesis of anthocyanins and other flavonoids, a reduces synthesis and/or prevents the accumulation of anthocyanins and other flavonoids; Y permits normal synthesis of anthocyanins, y completely blocks synthesis of anthocyanins; P produces petunidin, p produces cyanidin. All three genes are completely dominant, and yy is epistatic to A and P. We provide a model for flavonoid biosynthesis in Stokesia. Study of karyotypes and meiotic behavior of diploid cultivars and 'Omega Skyrocket' suggest that 'Omega Skyrocket' is an autotetraploid form of Stokesia. The karyotype of 'Omega Skyrocket' was almost indistinguishable from the average diploid karyotype. Meiotic pairing in diploids was normal (i.e. 100% bivalents); no meiotic irregularities such as laggards/bridges were observed and disjunction was equal (7:7). Meiotic pairing in 'Omega Skyrocket' demonstrated a high frequency (60%) of quadrivalent formation, though later stages of meiosis were regular with balanced disjunction (14:14). Meiosis in synthetic autotetraploids and triploids from crosses of diploid cultivars x 'Omega Skyrocket' included univalents, bivalents, trivalents, quadrivalents, 5-valents; abnormalities (i.e. laggards, unequal and/or premature disjunction, chromosome bridging, chromosome stickiness) were observed. Nuclear 2C DNA content for diploids and synthetic tetraploids was 20.3 pg and 39.9 pg. Nuclear 2C DNA content for 'Omega Skyrocket' was 37.3 pg (i.e. 8.2% less than twice the 2C DNA content of diploids), indicating that genomic downsizing has likely occurred in this cultivar. Differences in chromosome symmetry between the diploid and tetraploid karyotypes and the reduction in nuclear DNA content observed in 'Omega Skyrocket' both suggest that some divergence has occurred between 'Omega Skyrocket' and its diploid progenitor. A crossability study was conducted to determine the ploidy level and the frequency of progeny produced by interploid and intraploid crosses of Stokesia. A high percentage (70%) of progeny were aneuploids (i.e. 2x-1 to 6x+3) with the total percentage of aneuploids ranging from 92% to 94% in 2x x 3x, 3x x 2x, 3x x 3x, 3x x 4x and 4x x 3x crosses. Progeny (94%) from 2x x 2x crosses were diploids, and progeny (81%) from 2x x 4x and 4x x 2x crosses were triploids and 3x±1 aneuploids. Progeny from crosses of synthetic tetraploids were mostly tetraploids (16%) and tetraploid aneuploids (69%). Unreduced gamete production was estimated to be 0.7% to 1.4%. Reciprocal crosses of identical 2x and 4x parents produced viable progeny, demonstrating that a triploid block is not present in this species. Triploid fertility was higher than expected; crosses using triploids produced seed 38% of the time with an average seed set of ˜2 seeds/inflorescence. Fertility of synthetic tetraploids was reduced relative to their progenitor diploids; pollen viability was reduced by 36% and the percentage of inflorescences producing seed and average seed set/inflorescence were reduced by ˜50%. Pollen size was positively correlated with ploidy level (i.e. DNA content).
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plant breeding
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PhD
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Horticultural Science
