The genus Rudbeckia consists of ≈30 species endemic to North America (Armitage, 1997; Palmer et al., 2009). The genus includes annuals, biennials, and perennial species (Perdue, 1957) and is divided into two subgenera, Rudbeckia subg. Macrocline and Rudbeckia subg. Rudbeckia (Urbatsch et al., 2000). These two subgenera can be distinguished cytogenetically with R. subg. Macrocline having a base chromosome number of 18 and R. subg. Rudbeckia having a base chromosome number of 19 (Urbatsch et al., 2000). Many of the commercially important species of Rudbeckia are from R. subg. Rudbeckia. Two of these species, R. subtomentosa and R. hirta, are closely allied based on phylogeny and are commonly cultivated wildflowers (Urbatsch et al., 2000).
Rudbeckia subtomentosa is a durable, diploid (2n = 2x = 38) perennial, hardy to USDA zone 4. It is well adapted to many environments and has showy yellow ray florets. The tall stature of R. subtomentosa (2 to 3 m) limits its use within many cultivated landscapes. A reduction in height and an increase in the range of flower colors in R. subtomentosa would be highly desirable. Rudbeckia subtomentosa ‘Henry Eilers’ is a cultivar with showy tubular ray florets providing additional ornamental interest.
The annual species, Rudbeckia hirta, includes diploid (2n = 2x = 38) and tetraploid (2n = 4x = 76) cultivars with a diverse range of flower colors and forms (Palmer et al., 2009). Cultivars of R. hirta range in mature height from 0.5 to 1.0 m with tetraploid cultivars typically having larger flowers and greater height (Hansen and Stahl, 1993; Palmer et al., 2009). However, R. hirta is short-lived and susceptible to certain diseases including cercospora leaf spot (Cercospora sp.) and rhizoctonia blight (Rhizoctonia sp.) (Fulcher et al., 2003; Harkess and Lyons, 1994).
Interspecific hybrids between R. subtomentosa and R. hirta were developed at the Mountain Crop Improvement Laboratory (Palmer et al., 2008) to potentially combine desirable traits from both species including moderate plant height, a range of flower colors and forms, disease resistance, and perennialness. However, like many wide hybrids, these appeared to be infertile. Hybrid sterility may occur when chromosomes of different taxa are sufficiently different that pairing of chromosomes during meiosis fails (asynapsis) (Martin and Jouve, 1992). The different chromosome sizes of R. subtomentosa and R. hirta may be creating a barrier to fertility in hybrids. Palmer et al. (2009) found the 1Cx DNA content (DNA content of one complete set of chromosomes) to vary among Rudbeckia species by 320%. Rudbeckia subtomentosa had a 1Cx DNA content of 11.0 ± 0.1 pg, whereas that of R. hirta ranged from 3.4 ± 0.2 pg to 4.0 ± 0.2 pg.
In many cases, fertility can be restored in wide hybrids by doubling the genomes to create allopolyploids. Allopolyploids have duplicate homologous chromosome sets from each original parent that allows for disomic pairing and the formation of balanced gametes (Contreras et al., 2007; Lu and Bridgen, 1997; Olsen et al., 2006; Ranney, 2006). In addition to restored fertility, allopolyploids may display improved ornamental characteristics that are often intermediate between the two parents (Horn, 2002). Similarly, autopolyploids may also display improved characteristics such as an increase in flower size and vegetative tissues along with a decrease in internode lengths, although morphological responses vary (Horn, 2002). Furthermore, autotetraploids may also have a slower growth rate compared with their diploid cytotypes (Chahal and Gosal, 2002).
A greater understanding of the influence of induced polyploidy on fertility and morphology of Rudbeckia will better enable the development of improved varieties. The objectives of this study were to evaluate fertility, morphology, phenology of flowering, and perennialness for lines of diploid and induced allotetraploids of R. subtomentosa × hirta and diploid and induced autotetraploids lines of R. subtomentosa ‘Henry Eilers’.
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