There are ≈350 species of Ruellia (Acanthaceae) that are perennial herbs, subshrubs, or shrubs with mostly tropical and subtropical distributions (Tripp and McDade, 2014). A chromosome number of 2n = 34 appears to be widespread in this large and variable genus (Daniel et al., 1984, 1990; Daniel and Chuang, 1993). Twenty-four species of Ruellia have been described as found in the continental United States, Hawaii, Puerto Rico, and the Virgin Islands, five of which are native to Florida: R. caroliniensis, R. ciliosa, R. noctiflora, R. pedunculata subsp. pinetorum, and R. succulenta. Additionally, three non-native species are naturalized in the state: R. blechum, R. ciliatiflora, and R. simplex (Wunderlin and Hansen, 2014). Two of these—one native (R. caroliniensis; 2n = 34, Long, 1976) and one non-native (R. simplex; 2n = 34, Piovano and Bernadello, 1991)—are the focus of the present study.
Ruellia caroliniensis (J.F. Gmel.) Steud., also known as “Carolina Wild Petunia,” is native to 18 U.S. states, from North Carolina to Texas, reaching as far north as Illinois and Indiana. It is now considered rare in Ohio (Biota of North America Program, 2010) and extirpated in Pennsylvania (Tripp, 2004). In Florida, it occurs primarily in native woodlands, and plants are known as strong growers under adverse conditions (Gilman and Landrum, 1999). In the 1970s, Robert Long conducted detailed studies on floral polymorphisms, breeding systems (Long, 1971), and variation in natural populations (Long, 1974) of R. caroliniensis as well as artificial hybridization between this taxon and R. geminiflora (Long, 1976).
There has been no shortage of names that have been used to refer to a widespread and morphologically highly variable taxon here recognized as Ruellia simplex Wright (“Britton’s Petunia,” “Mexican Petunia,” or “Mexican Bluebell”). Scientific names for this plant that have been used throughout the botanical and horticultural literature include Ruellia brittoniana Leonard, R. coerulea Morong, R. malacosperma Greenm., and R. tweediana Griseb. The extremely complex taxonomic and nomenclatural history of these names has been discussed by several authors, most recently by Ezcurra and Daniel (2007) who reduced the aforementioned names to synonyms of the oldest name and thus that with priority, Ruellia simplex.
Ruellia simplex is found in sunny areas on periodically inundated soils in Mexico, the Antilles, and central–western South America (Ezcurra and Daniel, 2007). This species was introduced to Florida sometime before 1940 (Hupp et al., 2009) and is now a very popular landscape plant in the southeastern United States as a result of its copious flowering and low maintenance requirements (Gilman, 1999). Since its introduction to United States, it has naturalized in disturbed uplands and wetlands of seven continental U.S. states (from North Carolina west to Texas) in addition to the Virgin Islands, Puerto Rico, and Hawaii (U.S. Department of Agriculture, NRCS, 2014). In Florida, R. simplex has formed naturalized populations in 29 counties throughout the state (Wunderlin and Hansen, 2014). Of particular concern is that the species has been recorded in 21 designated conservation areas in south Florida (Institute for Regional Conservation, 2014). Since 2001, the Florida Exotic Pest Plant Council has considered Mexican petunia as a Category I invasive plant, which describes “plants that are altering native plant communities by displacing native species, changing community structures or ecological functions, or hybridizing with natives” (Florida Exotic Pest Plant Council, 2013).
Ruellia caroliniensis and R. simplex have sympatric distributions in numerous areas of the southeastern United States, and both have broad habitat affinities and are equally likely to occur in wetlands or non-wetlands. However, to date, there are no reports of populations of hybrid origin. Extensive artificial hybridizations among 25 different species of Ruellia were conducted at the University of South Florida by Long (1975). Among these, R. simplex (as R. brittoniana) and three varieties of R. caroliniensis were included in crossing studies. Only a cross between R. simplex and R. caroliniensis var. succulenta (direction unknown) was attempted; this cross produced viable seeds; however, no details on the F1 hybrids obtained were provided in this article.
The purpose of the present study was to conduct artificial crosses between Ruellia simplex and R. caroliniensis to test whether the two taxa could be artificially hybridized. Hybrids obtained were analyzed both morphologically and molecularly. Results from this investigation should be of interest to conservation biologists and botanists of the southeastern United States because they contribute new knowledge regarding potential natural hybridization between native and invasive species of Ruellia.
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