DNA rearrangements in the vicinity of ANS are attributed to their presence (Hisatomi et al., 1997b).
A sine-like element (SineIp) has also been identified in the 5′ flanking region of CHS-D (unpublished data) in some lines of I. purpurea. This element is 236 bp and contains the Pol III promoters at the 5′ end of the element. It has 15 bp direct terminal repeats. No phenotypic changes have yet been associated with this element.
Another floral mutation in I. nil, not related to the flavonoid pathway, but which is caused by a transposon insertion, is the duplicated mutant. In this mutant phenotype, sexual organs are replaced by perianth organs (petals and sepals) resulting in a double floral whorl. An En/Spm-like insertion, termed Tpn-botan, was found in a C class MADS-box-like gene (Nitasaka, 1997)* that is evidently responsible for this phenotype. A similar phenotype has also been described in I. purpurea (Epperson and Clegg, 1992), although the molecular basis for the mutation in I. purpurea is unknown.
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. nil and I. purpurea. We now turn to studies of the population genetics of some flower color phenotypes in I. Purpurea.
The geographic distribution of genetic diversity in I. purpurea appears paradoxical. Levels of flower color polymorphism are high in the southeastern U.S. whereas Mexican populations are frequently monomorphic for the blue color form (Epperson and Clegg, 1986; Glover et al., 1996). The situation is reversed for biochemical and molecular variation. Mexican populations have levels of isozyme polymorphism that are similar to other annual plants (Glover et al., 1996; Hamrick and Godt, 1989), but U.S. populations are depauperate in isozyme variation. Surveys of ribosomal DNA restriction fragment variation and samples of gene sequence data for the (CHS-A) locus also reveal reduced levels of variation in U.S. populations relative to Mexican populations (Glover et al., 1996; Huttley et al., 1997). We speculate that this pattern is a consequence of the introduction of horticultural forms selected for flower color diversity into the U.S. However, we do not know the source of these introductions.
Nitasaka, E. (1997) Genes Genet. Syst. 72, 421 (abstr.).