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The potential for natural hybridization to occur between non-native, invasive species and closely related native species is of interest to biologists, conservationists, and land managers, particularly in regions such as the southeastern United States where numerous non-native species have become serious environmental pests. To explore this potential between the invasive plant species Ruellia simplex and the closely related, sympatric Ruellia caroliniensis, we conducted a study of reproductive crossability and hybrid viability. Results indicate that the production of interspecific hybrids is possible, but only in one direction (i.e., with R. caroliniensis as the maternal parent). Artificial hybrids were weak, slow-growing, and sterile. These data suggest that it is unlikely that R. caroliniensis 3 R. simplex hybrids could invade the gene pool of native R. caroliniensis. We also characterized hybrids at the molecular level by sequencing parents plus F1 progeny for the nuclear ribosomal internal transcribed spacer (ITS) + 5.8S region. All hybrid genotypes formed a strongly supported clade with the maternal parent, Ruellia caroliniensis. Within this clade, hybrid individuals were not differentiable from maternal genotypes. We then examined general plant morphology of hybrid individuals and the two parents. Unlike results from the molecular characterization, there was a strong signal of hybrid intermediacy from this morphological work. We conclude that morphology but not molecular sequence data (from nrITS) can be used to distinguish the two parents and their F1 hybrids. 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 Received for publication 18 Mar. 2014. Accepted for publication 6 June 2014. Research Scientist. Curator of Botany and Assistant Professor. To whom reprint requests should be addressed. e-mail rfreyre@ufl.edu. HORTSCIENCE VOL. 49(8) AUGUST 2014 991 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. Materials and Methods Plant material. One accessions and one cultivar of Ruellia simplex (as described in Wilson and Mecca, 2003) were included in this study: wild-type purple-flowered R. simplex (sim1, wild collected in Tallahassee, FL) and pink-flowered R. simplex ‘Chi Chi’ (sim2, cultivar from Boynton Botanicals, Palm Beach, FL). Additionally, three different accessions of R. caroliniensis were included: car1 (wild collected in Fort Pierce, FL), car2 (wild collected in Alachua, FL), and car3 (from Superior Trees Inc., Lee, FL). Hybridizations. All plants were propagated by cuttings from stock plants and were cared for in greenhouses at the University of Florida, Gainesville. Cuttings were grown in 128-cell cutting trays with Fafard 2P mix (60% Canadian peatmoss, 40% perlite; Concord Fafard Inc., Agawam, MA) and placed under mist in a research greenhouse for 2 weeks. After 5 to 6 weeks, rooted cuttings were transplanted individually into 15-cm Kord Traditional Standard pots with Fafard 2P mix and placed on raised benches in a polyethylene greenhouse. Plants were watered as needed with 150 ppm nitrogen using Peters liquid fertilizer (20N–10P2O5–20K2O; Everris , Charleston, SC). Hybridizations were conducted between March and May 2008 in greenhouses isolated from potential pollinators. Fully expanded flower buds (where the anthers had not dehisced pollen) on the maternal parent plants were emasculated by removing the corolla and attached anthers. Immediately afterward, the stigmas of a maternal plant were hand-pollinated using pollen from anthers of the paternal plant. The pollinated flower was then tagged with a colored plastic string. When the fruit developed, it was enclosed with an empty tea bag secured with a paper clip to prevent loss of seeds during fruit dehiscence (fruits of Acanthaceae have explosive dehiscence). Progeny analyses. For each cross in which a fruit developed, the total number of seeds per capsule was counted. Immature or damaged seeds were separated from mature, apparently viable seeds. In Sept. 2008, normal seeds were sown 1 to 2 cm deep in 20-row seeder trays (Landmark Plastics, Akron, OH) using pre-wetted Fafard 2P mix. Seed trays were placed in a polycarbonate mist house (with 30% light irradiance) and received misting from 0800 HR to 1800 HR (5 s/30 min). Temperature was maintained between 18 and 24 C. After 10 and 15 d, seedlings were transplanted into 15-cm pots and maintained in a greenhouse. Plants were grown to reproductive maturity (i.e., flowering, from which herbarium vouchers were taken for subsequent morphological and molecular study). Morphological diagnosis. Plants of artificial hybrid origin were studied using light microscopy, and standard taxonomic descriptions were prepared. Plants of car1, car2, car3, sim1, sim2, and wild-collected Ruellia caroliniensis and R. simplex of miscellaneous geographic origin were studied and described taxonomically for comparison with hybrids (Appendix 1). Analysis of the nuclear ribosomal ITS region. To determine whether Ruellia caroliniensis, R. simplex, and experimental hybrids could be differentiated genetically, data from the nrITS region were generated and analyzed. The ITS region is among the most used nuclear markers in p

hortscience

Artificial Hybridization between U.S. Native Ruellia caroliniensis and Invasive Ruellia simplex: Crossability, Morphological Diagnosis, and Molecular Characterization