Genetics and epigenetics in flower pigmentation associated with transposable elements in morning glories

Among the genus Ipomoea, three morning glories, I. nil (the Japanese morning glory), I. purpurea (the common morning glory), and I. tricolor, were domesticated well for floricultural plants, and many spontaneous mutants displaying various flower pigmentation patterns were isolated. Most of these spontaneous mutations were found to be caused by the insertion of DNA transposable elements in the genes for the anthocyanin pigmentation in flowers, and many of them exhibited variegated flowers, such as white flowers with pigmented spots and sectors. Here, we describe the historical background of the mutants displaying variegated flowers and review the genetic and epigenetic regulation in flower pigmentation associated with transposable elements of these morning glories. The flecked, speckled, r-1, and purple mutations in I. nil were caused by insertions of Tpn1 and its relatives in the En/Spm superfamily, Tpn2, Tpn3, and Tpn4, into the genes for anthocyanin coloration in flowers, i.e., DFR-B, CHI, CHS-D, and InNHX1, respectively. Similarly, the flaked and pink mutants of I. purpurea have distantly related elements, Tip100 and Tip201, in the Ac/Ds superfamily inserted into the CHS-D and F3'H genes, respectively. The flower variegation patterns can be determined by the frequency and timing of the excision of these transposons, and their stable insertions produce plain color flowers without generating pigmented spots or sectors; furthermore, both genetic and epigenetic regulation appeared to play important roles in determining the frequency and timing of the excision of the transposons. However, flower variegation is not always associated with the excision of an integrated DNA transposon from one of the genes for anthocyanin pigmentation. The mutant Flying Saucers of I. tricolor displaying variegated flowers was found to have the transposon ItMULE1 inserted into the DFR-B promoter region, but no excision of ItMULE1 from the DFR-B could be detected in the variegated flower lines. The instable pearly-vrg allele in cv. Flying Saucers is likely to be an epiallele because the DNA methylation in the DFR-B promoter appeared to be associated with flower pigmentation.