There are ≈350 species of Ruellia L. (Acanthaceae) that are perennial herbs, subshrubs, or shrubs with mostly tropical and subtropical distributions (Tripp and McDade, 2014). A chromosome number of 2n = 2X = 34 appears to be widespread in this large genus (Daniel et al., 1984, 1990; Daniel and Chuang, 1993). The most commonly found Ruellia species in southeastern U.S. gardens is Ruellia simplex (britton’s petunia, mexican petunia, or mexican bluebell). Scientific names for this plant that have been used throughout the botanical and horticultural literature include R. brittoniana Leonard, R. coerulea Morong, R. malacosperma Greenm., and R. tweediana Griseb. These names have been reduced to synonyms of the oldest name and thus that with priority, R. simplex (Ezcurra and Daniel, 2007). The wild form of this species is found in sunny areas on periodically inundated soils in Mexico, the Antilles, and central western South America (Ezcurra and Daniel, 2007). It was introduced to Florida sometime before 1940 (Hupp et al., 2009) and is now a very popular landscape plant as a result of its low maintenance requirements and copious flowering (Gilman, 1999).
Flower colors are attributable primarily to flavonoid and carotenoid pigments and are inferred to serve to attract pollinators (Davies, 2004). Flavonoids are the most common flower color pigment, and the predominant flavonoid pigments are the anthocyanins. Anthocyanins are composed of an anthocyanidin and sugar moieties. They are the basis for most orange, pink, red, magenta, purple, blue, and blue–black floral colors. The common anthocyanidins are pelargonidin, cyanidin, peonidin, delphinidin, petunidin, and malvinidin, named for the genera from which they were first isolated. Most anthocyanins are derived from just the following three basic anthocyanidin types: pelargonidin, cyanidin, and delphinidin (Schwinn and Davies, 2004). Cyanidin and delphinidin can be further modified by glycosylation, methylation, and acylation to form peonidin and malvidin, respectively (Holton and Cornish, 1995). Orange and pink colors tend to be based on pelargonidin derivatives, magenta colors on cyanidin derivatives, and purple and blue colors on delphinidin derivatives (Harborne, 1976). Flowers can accumulate mixtures of anthocyanin types, providing further variation in color. Other factors such as vacuolar pH and petal cell shape can also affect the flower color (Mol et al., 1998). The genetics of flower color are best known in Petunia ×hybrida Vilm, where nine major genes involved in the inheritance of flower color were initially identified (Paris and Haney, 1958). Based on biochemical data, these genes were assigned specific functions in the biosynthetic pathway (Forkmann, 1991; Holton and Cornish, 1995; Mol et al., 1998; Wiering and deVlaming, 1984). Additionally, genetic differences in flower color resulting from modifications in the pH were explained by genes Ph1, Ph2, and Ph6 (Griesbach, 1996, 1998).
Flower morphology, flower size, and flower color in Ruellia are varied. Flower color ranges from white, cream, yellow, lavender, purple, pink, magenta, and red (Tripp, 2014). However, the range of flower colors in R. simplex is more limited. The wild-type and cultivars Purple Showers and the dwarf Purple Katie have purple flowers [Royal Horticultural Society (RHS) 87A (Royal Horticultural Society, 1995)]. Over the years, cultivars with the following other colors have become available: pink-flowered Chi Chi and Pink Katie (RHS 66D) and white-flowered Snow White and White Katie (RHS 155D). All purple and pink-flowered forms have a darker throat (either dark purple RHS 86B or dark pink RHS 74A, respectively). Additionally, in 2010, a mutant with white corolla and purple throat was found in cultivation in Vero Beach, FL (S.B. Wilson, personal communication), although not commercially available. Breeding efforts at the University of Florida have resulted in the recent release of non-invasive, sterile cultivars Mayan Purple, Mayan White (Freyre et al., 2012), and Mayan Pink (Freyre and Wilson, 2014).
In this study, we developed an F2 population segregating for flower color in R. simplex from a cross of an individual with white corollas with purple throat and an individual with pink corollas. We were then able to elucidate the genetic control for flower color in this species. We also performed high-performance liquid chromatography mass spectrometry analyses to determine the anthocyanins responsible for four flower colors in R. simplex.
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