. "12 Flower Color Variation: A Model for the Experimental Study of Evolution." Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years after Stebbins. Washington, DC: The National Academies Press, 2000.
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Variation and Evolution in Plants and Microorganisms: TOWARD A NEW SYNTHESIS 50 YEARS AFTER STEBBINS
NATURAL HISTORY OFI. PURPUREA
The genus Ipomoea includes approximately 600 species distributed on a worldwide scale (Austin and Huaman, 1996) that are characterized by a diversity of floral morphologies and pigmentation patterns. In addition, a wide variety of growth habits, ranging from annual species to perennial vines to longer-lived arborescent forms, are represented in the genus. The common morning glory is an annual bee-pollinated self-compatible vine with showy flowers that is a native of the highlands of central Mexico. As the name morning glory suggests, the flowers open early in the morning and are available for fertilization for a few hours, after which the flower wilts and abscises from the vine. The plant is also a common weed in the southeastern U.S., where it is found in association with field corn and soybean plantings, as well as in roadside and disturbed habitats. The common morning glory is characterized by a series of flower color polymorphisms that include white, pink, and blue (or dark blue) phenotypes. A primary pollinator in the southeastern U.S. is the bumblebee (Bombuspennsylvanicus and Bombus impatiens), but occasionally plants are visited by honey bees and some lepidopterans (Epperson and Clegg, 1987a). The flower color phenotypes are thought to have been selected by pre-Columbian peoples, perhaps in association with maize culture (Glover etal., 1996). At some point, the plant was introduced into the southeastern U.S., although the routes and times of introduction remain uncertain. Early floras of the southeastern U.S. indicate the presence of I. purpurea populations by the late 1600s, providing a minimum estimate of the residence time in this geographical region.
GENETICS OF FLOWER COLOR VARIANTS IN I. PURPUREA
At least 21 floral phenotypes are determined by five genetic loci (Barker, 1917; Ennos and Clegg, 1983; Epperson and Clegg, 1988). Most of these phenotypes have analogous forms in the Japanese morning glory (Ipomoea nil), and the genetics of the floral variants in both species appear to be similar, but not identical (Imai, 1927). A widespread polymorphism determines pink versus blue flowers (P/p locus), but the genotype at two other loci modifies the intensity of expression of the P/p locus. One modifier is an intensifier locus (I/i) that doubles the anthocyanin pigmentation in the recessive ii genotype (Schoen et al., 1984). The second modifier locus is a regulatory locus that determines the patterning and degree of floral pigmentation (W/w locus). A fourth locus, the A/a locus, is epistatic to the P/p, I/i, and W/w loci in that the recessive albino phenotype yields a white floral limb independent of genotypic state at the other loci. The A/a locus is also characterized by unstable alleles (denoted a* or af)