The genus Rudbeckia contains ≈30 species of annuals, biennials, and perennials known for their colorful, golden ray corollas and prominent disk-shaped receptacles that can bloom from midsummer through October. Rudbeckia are popular crops and are valued for their ornamental diversity, low-maintenance requirements, and heat and drought resistance.
Rudbeckia include two genetically and morphologically distinct subgenera: Macrocline and Rudbeckia (Urbatsch et al., 2000). Species in the subgenus Rudbeckia include many desirable ornamentals and range from opportunistic annuals to persistent perennials. These species generally flourish in full sun and well-drained soils and are commonly found in recently disturbed habitats such as roadsides or old fields and thrive during primary and secondary succession. The annual species R. hirta includes cultivars with a diverse range of flower colors and forms, including bicoloration and double inflorescences (greater number of ray florets). Petal colors range from a lemon yellow and gold exhibited by ‘Prairie Sun’ to the deep red and mahogany expressed by ‘Cherokee Sunset’. However, R. hirta is short-lived and susceptible to diseases, including rhizoctonia blight (Rhizoctonia sp.) and cercospora leaf spot (Cercospora sp.) (Fulcher et al., 2003; Harkess and Lyons, 1994). Other Rudbeckia species, including R. fulgida, R. missouriensis, and R. subtomentosa, are reliable perennials with superior disease resistance and good performance in southern climates (Armitage, 1997). Interspecific hybridization between durable perennial species and showier annual species could lead to development of valuable new cultivars.
There has been little published on the genetics and breeding of Rudbeckia. Species in Rudbeckia subgenus Rudbeckia have a base chromosome number of x = 19 and polyploids have been reported to exist in R. hirta, R. fulgida var. speciosa, and R. triloba L. (McCrea, 1981; Urbatsch et al., 2000). However, there is little information available on ploidy levels of specific cultivars. Pseudogamy, a form of apomixis, has also been implicated as the sole means of reproduction in triploid R. triloba (McCrea, 1981). A greater understanding of reproductive pathways and occurrence of apomixis would provide valuable information for both breeding and commercial propagation of Rudbeckia.
The objectives of this research were to determine ploidy levels and relative DNA contents of selected species and cultivars, to evaluate self-compatibility and crossability between species, to determine interploid crossability among R. hirta cultivars, and to assess reproductive pathways in triploid R. hirta to better facilitate future Rudbeckia breeding projects.
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