. "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
the U.S. and in Japan on I. purpurea, which is a member of the same subgenus as I. nil. Both species appear to be quite closely related at the DNA sequence level. 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that the two species diverged roughly three million years ago based on synonymous divergence at DFR and CHS genes (unpublished data). These two closely related species provide a useful comparative framework for the analysis of genetic change over a relatively short period of evolutionary time.
A 3.9-kb Ac/Ds-like element (Tip100) has been identified in the intron of CHS-D near the 5′ junction in flaked (af) mutants of I. purpurea (Habu et al., 1998). The af flower is white with pigmented sectors on the corolla. Another CHS-D mutant phenotype (a12), which has white flowers and no pigmented sectors, has been shown to carry two copies of Tip100 in the intron in the opposite orientation (Habu et al., 1998). In another stable white phenotype (a), a rearrangement of DNA sequence between exon I and an adjacent Tip100 element has occurred (unpublished data). In yet another mutant (a*) of I. purpurea, which has a white corolla with very few pigmented sectors, an additional copy of Tip100 was found in the 5′ flanking region of CHS-D (unpublished data). The discovery that disruption of the CHS-D gene results in an unpigmented phenotype confirms that CHS-D is the only CHS gene family member that is responsible for pigment production in the floral limb (Durbin et al., 2000; Johzuka-Hisatomi et al., 1999). Another Ac/Ds-like element, Tip201, has also been discovered in I. purpurea (Morita et al., 1999) (S. Iida, personal communication).‡ Tip201 is found inserted in exon III of F3′H, resulting in a pink phenotype rather than the wild-type blue color corresponding to the P/p locus (Morita et al., 1999) (S. Iida, personal communication).‡ A point mutation in F3′H in I. nil also results in a red phenotype instead of blue (S. Iida, personal communication). Interestingly, this is the only phenotypic change involving flower color that has been characterized at the molecular level in either I. nil or I. purpurea that is not attributable to the presence of a transposable element.
An En/Spm-like mobile element termed Tpn1 was characterized by Inagaki et al. (1994) in the I. nil genome. Tpn1 is a 6.4-kb non-autonomous element inserted within the second intron of DFR-B in a mutant termed flecked (a-3flecked). This phenotype has predominantly white flowers with colored sectors. An unusual feature of Tpn1 is that it contains a segment of DNA consisting of four exons that encode part of an HMG-box (High Mobility Group DNA-binding proteins) (Takahashi et al., 1999). This finding is consistent with the observation that transposable elements can cause
Morita, Y., Hoshino, A., Tanaka, Y. Kusumi, T., Saito, N. & Iida, S. (1999) Plant Cell Physiol. 40, Suppl., 124 (abstr.).