The Journal of Heredity 2002:93(1)
© 2002 The American Genetic Association 93:55-57
Brief Communication |
Inheritance of Flower Color in Periwinkle: Orange-Red Corolla and White Eye
From the Central Institute of Medicinal and Aromatic Plants, Field Station, Allalasandra, Bangalore 560 065, India. This study and Y. Sreevalli were supported by financial assistance from the Karnataka State Council for Science and Technology, Bangalore, India.
Address correspondence to R. N. Kulkarni at the address above.
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Abstract |
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The commonly found flower colors in periwinkle (Catharanthus roseus)—pink, white, red-eyed, and pale pink center—are reported to be governed by the epistatic interaction between four genes—A, R, W, and I. The mode of inheritance of an uncommon flower color, orange-red corolla and white eye, was studied by crossing an accession possessing this corolla color with a white flowered variety (Nirmal). The phenotype of the F1 plants and segregation data of F2 and backcross generations suggested the involvement of two more interacting and independently inherited genes, one (proposed symbol E) determining the presence or absence of red eye and another (proposed symbol O) determining orange-red corolla.
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Introduction |
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Periwinkle [Catharanthus roseus (L.) G. Don], a perennial semishrub native to Madagascar, is now found in many tropical and subtropical regions. It is grown as an ornamental plant in gardens and parks for its colored flowers and ever-blooming nature. It is also cultivated as a medicinal plant for its leaves and roots, which contain anticancer (vincristine and vinblastine) and antihypertension (ajmalicine) alkaloids, respectively. The most commonly observed flower colors in periwinkle are pink (pink corolla and red eye), red-eyed (white corolla and red eye), and white. Flory (1944) attributed these three corolla colors to the epistatic interaction of two genes, R and W, with the R-W-genotype being pink, R-ww being red-eyed, and rrW- and rrww being white flowered. Simmonds (1960) implicated two additional genes, A and B, in the determination of flower color, with A being complementary to R, without which flowers would be white, and B being a copigmentation gene which blues the pink pigment in the A-R-W- and A-R-ww genotypes. In addition to the above three flower colors, Milo et al. (1985) identified another flower color, pale pink center. They attributed this flower color to another gene, I, which like gene W is also epistatic to the gene R. The genotypes and phenotypes of different flower colors according to above three models are shown in Table 1.
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Recently we procured another flower color type from a local dealer in horticultural plants. This accession, designated as OR, had flowers with an orange-red corolla and white eye. The mode of inheritance of orange-red corolla and white eye has not been reported earlier. Also, this flower color could not be explained by earlier models of Flory (1944), Milo et al. (1985), or Simmonds (1960). We therefore studied the mode of inheritance of these two flower color traits, namely, orange-red corolla color and white eye, using accession OR, as a part of our inheritance studies in periwinkle (Kulkarni et al. 1999a,b, 2001).
Extensive chemogenetic, biosynthetic, and, in recent years, molecular genetic studies on the biosynthesis of pigments in plants, particularly flavonoids, have greatly facilitated our understanding of the pathway of flavonoid biosynthesis, evolution of species, and evolution of genes involved in flavonoid biosynthesis, as well as their cloning from a number of plant species, particularly maize, petunia, and snapdragon (Chandler et al. 1989; Coe 1985; Forkmann 1991; Gottlieb and Ford 1988; Quattrocchio et al. 1993). In ornamental plants, where novelty in flower color is an important breeding objective, molecular breeding, which can overcome crossing barriers between species, has opened up the possibility of molecular engineering for flower color. The generation of transgenic petunia plants with "brick red" flowers using the A1 gene from maize is the first example of the creation of a novel flower color in a highly directed manner (Meyer et al. 1987). However, a detailed knowledge of the function and interaction of genes involved in the biosynthesis of pigments in the plant species of interest is one of the essential prerequisites for controlled engineering of flower color (Forkmann 1991).
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Materials and Methods |
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The accession OR was found to be true breeding for orange-red corolla and white eye. It was crossed with a white flowered variety, Nirmal. In an earlier study the flower color genotype of Nirmal was found to be rrWW (Kulkarni et al. 1999b). Parent plants were raised in a glasshouse from seeds obtained by artificial self-pollination. Reciprocal crosses were made as described earlier (Kulkarni et al. 2001). The F1 plants (four plants) were selfed and also backcrossed to both parents. Altogether 416, 106, and 85 plants of F2, and two backcrosses, F1 x OR and F1 x Nirmal, respectively, were scored for corolla and eye color, along with 10 plants each of the parental and F1 generations. Chi-square and G tests were used for testing the goodness-of-fit of observed and expected frequencies of different phenotypic classes in the F2 and backcross generations (Sokal and Rohlf 1981).
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionReferences |
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Flowers of F1 plants had pink corolla and red eye similar to the commonly found pink flowered plants in periwinkle, which have pink corolla and red eye. There were no differences in the flower colors of F1 plants of reciprocal crosses.
Plants of the F2 generation could be broadly classified into six flower color categories: pink corolla and red eye, pink corolla and white eye, orange-red corolla and red eye, orange-red corolla and white eye, white corolla and red eye, and white corolla. The pink and orange-red flowered plants, however, showed some differences among themselves in the intensities of their corolla color. The observed frequencies of these six categories of flower colors fit a ratio of 108:36:27:21:9:55 (Table 2).
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The progeny of the backcross, F1 x OR, produced four types of plants: pink corolla and red eye, pink corolla and white eye, orange-red corolla and red eye, and orange-red corolla and white eye. The observed frequencies of plants with these four kinds of flowers fit a ratio of 1:1:1:1 (Table 3). Only two types of plants—pink corolla and red eye, and white corolla—were observed in the ratio of 1:1 in the progeny of the backcross, F1 x Nirmal (Table 3). The model used for developing these ratios in the F2 and backcross generations is discussed below.
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The commonly observed flower colors in periwinkle were earlier explained by a three gene (R, W, and I) model by Milo et al. (1985). According to their model, the R allele produces three anthocyanidins—A1, B1, and C1 (located in the center of the corolla)—and is epistatic to the W and I alleles, which function only in its presence. The W allele produces pigments A2, B2, and C2. The I allele also produces the same pigments as the W allele, but in smaller quantities and mainly in the center of the corolla. The r, w, and i alleles do not produce any pigments. Earlier Simmonds (1960) hypothesized that two complementary genes, A and R, are necessary for the production of colored flowers, and without either of these genes the flowers would be white. The phenotype of the parent OR (orange-red corolla and white eye) and the phenotypes pink corolla and white eye, and orange-red corolla and red eye observed in plants of the F2 and one of the backcross generations (F1 x OR) of the present study did not fit into the models proposed by Milo et al. (1985) and Simmonds (1960).
The phenotypes of plants of the parental, F1, F2, and backcross generations of the present study could be accounted for by incorporating two more genes, proposed symbols E and O, in the models of Milo et al. (1985) and Simmonds (1960). The genotypes of plants observed in the present study were deduced from the following observations: (1) From the flower phenotype of the parent OR (i.e., colored corolla) and earlier genetic studies involving the variety Nirmal (Kulkarni et al. 1999b), it was evident that both parents were homozygous for the A allele. (2) The occurrence of plants with white corolla and red eye, that is, red-eyed flowers (iiwwR-), in the F2 generation suggested that the parent OR was homozygous recessive at the W locus (from earlier studies it is known that the genotype of the variety Nirmal with white flowers is AArrWW). (3) The absence of plants possessing flowers with pale pink centers (I-wwR-) in the F2 generation suggested that both parents had the genotype ii at the I locus. (4) The flower phenotype (pink corolla and red eye) of F1 plants suggested the existence of the R allele in a homozygous condition in the parent OR. According to both Milo et al. (1985) and Simmonds (1960), the R allele produces anthocyanidins, which are located in the eye region. Therefore it may be assumed that the R allele produces pigments in the eye region only in the presence of another gene, E; in the absence of the E gene, flowers would have white eye. With this assumption, the genotypes of parental accessions OR and Nirmal would be AAiieeRRww and AAiiEErrWW, respectively. (5) Another gene (proposed symbol O) responsible for the production of orange-red colored corollas could be assumed to be present in the parent OR. Further, the absence of plants with orange-red corollas in the progeny of the backcross, F1 x Nirmal (Table 3) suggested that the O allele did not express in the presence of the W allele, as all the progeny of this backcross would have at least one W allele. The expected genotypes of plants of the parental, F1, F2, and backcross generations based on the analysis given above are presented in Tables 2 and 3.
As mentioned earlier, the pink and orange-red flowered plants of segregating generations showed minor differences in intensities of their flower color which could not be further classified. Such minor unclassifiable differences in flower color in segregating generations have also been found in other plants, and have been attributed to modifying genes and differences in the genetic background (Bassett et al. 1990; Cornu and Maizonnier 1983; Gottlieb and Ford 1988; Riley 1945).
The phenotypes observed in the present study could also, alternatively, be explained by assuming that the E allele in the homozygous condition inhibits the expression of the R allele in the eye region, leading to the production of flowers with white eye (EER-), and genotypes EeR- and eeR- would produce flowers with red eye (the genotypes of parents OR and Nirmal would then be AAiiEERRwwOO and AAiieerrWWoo, respectively). However, the former explanation appears to be more likely since plants with pink corolla and red eye, as well as plants with white corolla and red eye, are quite common in nature, while those with colored corolla and white eye are rare and may have arisen from a recessive mutation at the E locus. In Pelargonium hortorum, where many varieties have a floret center area different in color from the remainder of the floret, white center has been found to be controlled by a single recessive gene (Nugent and Snyder 1967), as found in the present study. Flower color in many ornamental plants as well as field crops has been found to be governed by the interaction between two or more genes (Ennos and Clegg 1983; Forkmann 1991; Kumar et al. 2000; Negi and Raghava 1990). The results of the present study also suggested the involvement of at least six major interacting and independently inherited genes—A, R, W, I, E, and O—in the determination of flower color in periwinkle.
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Acknowledgments |
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The authors thank Prof. Sushil Kumar, Director, CIMAP, Lucknow, India, for his keen interest and encouragement.
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Footnotes |
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Corresponding Editor: Brandon Gaut
Received January 20, 2001
Accepted November 26, 2001
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References |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionReferences |
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