Variation and Evolution in Plants and Microorganisms Toward a New Synthesis 50 Years After Stebbins (2000) / Chapter Skim
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12 Flower Color Variation: A Model for the Experimental Study of Evolution
12 Flower Color Variation: A Model for the Experimental Study of Evolution
Pages 211-234
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From page 211... ...
CLEGG AND MARY L DURBIN We review the study of flower color polymorphisms in the morning glory as a model for the analysis of adaptation.
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The study of adaptation is also among the most difficult and challenging areas of experimental research because a complete causal analysis of adaptation involves a translation between different levels of biological organization, the ecological, the phenotypic, and the molecular level (Clegg, 2000~. In this article we review more than 20 years of research on flower color polymorphisms in the common morning glory [Ipomoea purpurea (L.)
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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)
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Flavonoid Biosynthetic Pathway To put the phenotypic variation into a biochemical context, it is useful to sketch the main outlines of the flavonoid biosynthetic pathway (Fig.
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Enzymes involved in anthocyanin biosynthesis and side branches leading to related flavonoid pathways are shown. PAL, phenylalanine ammonialyase; C4H, cinnamate 4-hydroxylase; 4C1, coumarate: CoA ligase; CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 30-hydroxylase; F3'H, dihydroflavonol 3' hydroxylase; F3'5'H, dihydroflavonol 3'5' hydroxylase; DFR, dihydroflavonol reductase; ANS, anthocyanidin synthase; UF3GT, UDP-glucose flavonol 3-0-glucosyl transferase; RT, rhamnosyl transferase.
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MOLECULAR CHARACTERIZATION OF THE GENES OF FLAVONOID BIOSYNTHESIS IN IPOMOEA The first committed step in the flavonoid biosynthetic pathway is encoded by the enzyme chalcone synthase (CHS) , which catalyzes the formation of naringenin chalcone from three molecules of malonyl-CoA and one molecule of p-coumaroyl-CoA (Kreuzaler and Hahlbrock, 1975~.
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are expressed, but each displays differential regulatory and developmental control (Durbin et al., 2000; Tohzuka-Hisatomi et al., 1999~. CHS-D is the most abundantly expressed transcript and is now known to be the one CHS gene solely responsible for anthocyanin production in the floral limb (Durbin et al., 2000; Habu et al., 1998~.
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Some of these mobile element insertions cause phenotypic changes, including those responsible for several flower color variants (Table 3~. Much of this work has concentrated on the lapanese morning glory (I.
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Molecular clock calculations indicate that the two species diverged roughly three million years ago based on synonymous divergence at DFR and CHS genes (unpublished data)
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(1994) establish that the observed sectoring phenotype was caused by the movement of this new transposable element, but the finding also established that, of the three DFR genes characterized, only one gene family member (DFR-B)
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The vast majority of the phenotypic variation in Ipomoea characterized to date at the molecular level appears to be caused by the insertion or deletion of transposable elements (Table 3~. It is apparent that a wide variety of mobile elements exist in the Ipomoea genome, and these are evidently quite active based on the relatively modest period of evolutionary time that separates I
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A spatial autocorrelation measures the correlation in state of a system at two points that are separated by x distance units. For example, in the common morning glory case, the state may be the flower color determined by the P/p locus x distance units apart.
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For discrete characters, the convention is to transform the autocorrelation into a standard normal deviate with mean 0 and variance 1, and this transformed function is graphed as the correlogram. Let us again take the concrete example of the common morning glory P/p locus.
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SELECTION ON FLOWER COLOR PHENOTYPES Because flowering plants often depend on insect pollinators for reproductive success, a natural question to investigate is whether pollinators discriminate among the various flower color phenotypes in morning glory populations. A number of experiments conducted in different years and by different investigators in both natural and experimental populations agree in revealing a bias by bumblebee pollinators against visiting white flowers when white is less than 25% of the population (Brown and Clegg, 1984; Epperson and Clegg, 1987a; Rausher et al., 1993~.
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In summary, there is clear and convincing evidence that white phenotypes suffer some disadvantage in natural populations based on spatial autocorrelation analyses, and there is clear and convincing evidence that white genes have a transmission advantage when white maternal plants are infrequent in populations. The transmission advantage is associated with pollinator preferences and a consequent increased rate of self-fertilization among white maternal parents, but this advantage is one-sided and should diminish to zero as the frequency of white maternal types approaches 50%.
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As noted in this article, most of the genes of flavonoid biosynthesis occur in multiple copies, and sorting through this redundancy to find the particular genes responsible for individual flower color phenotypes appeared daunting at first sight. The actual findings are encouraging, in that particular gene copies of CHS and DFR are shown to be causally responsible for particular flower color phenotypes.
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nil are the result of transposon insertions. This strongly implies that transposable elements are the major cause of mutations that yield an obvious phenotype in these plant species.
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populations represent a crude series of experiments that trace to introductions within the last several hundred to one thousand years, and the ability to place temporal limits on the history of these populations is a strength of the morning glory program. Common patterns among populations are indicative of common initial conditions or common selective forces.
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(1999) The role of inbreeding depression in maintaining the mixed mating system of the common morning glory, Ipomoea purpurea.
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(1997) Nucleotide polymorphism in the chalcone synthase-A locus and evolution of the chalcone synthase multigene family of common morning glory Ipomoea purpurea.
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(1997) A floral color polymorphism in the common morning glory (Ipomoea purpurea)
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(1999) Capture of a genomic HMG domain sequence by the En/Spm-related transposable element Tpnl in the Japanese morning glory.
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