Genetic diversity in the common morning glory

Abstract

Populations of the common morning glory in the southeastern US are characterized by a striking diversity of flower color polymorphisms. This diversity is probably a consequence of horticultural escapes from cultivation in the 18(th) and 19(th) centuries. More than 15 years of research in our laboratory has shown that some color phenotypes are selected by virtue of their differential attraction to insect pollinators. We have studied genetic diversity at isozyme and ribosomal DNA loci and we find reduced diversity in the southeastern US compared to Mexican populations. In an effort to link ecological genetics to molecular evolution, we have cloned and characterized the chalcone synthase (CHS) gene family in morning glory and we have studied the expression of CHS genes in flower development. We have also initiated an investigation of spatial patterns of diversity at CHS genes by sampling and sequencing genes from US and Mexican populations. These investigations reveal (1) that the four CHS genes (CHS A, B, C, and PS) characterized to date evolve rapidly in morning glory and that the gene family in Ipomoea is of relatively recent origin (approximately 21 million years); (2) the duplicate genes in Ipomoea group into two categories (CHS A, C versus CHS B, PS) that may indicate a functional divergence between chalcone synthase and stilbene synthase activities; (3) levels of molecular diversity for CHS A genes sampled from Mexico are much higher than observed in US collections suggesting a major population bottleneck associated with the introduction of morning glory into the southeastern US; and (4) the ratio of amino acid substitution to synonymous substitution between Ipomoea species is remarkably high (about 5.4 synonymous to amino acid substitutions) compared to CHS genes in other plant species. Taken together these data portray a rapidly evolving gene family where functional divergence may arise repeatedly in evolution, despite the central role of chalcone synthase in flavonoid metabolism. Show more