A CHLOROPLAST DNA INVERSION MARKS AN EVOLUTIONARY SPLIT IN THE GENUS OENOTHERA.

Abstract

The small size and conservative evolution of the chloroplast DNA make it an ideal molecule for tracing the evolutionary history of plant species. Most of the over 200 angiosperm chloroplast genomes examined are overwhelmingly similar in size, conformation, repeat structure, and gene content, and have the highly conserved gene order typified by spinach and tobacco. In fact, the chloroplast gene arrangement in spinach and tobacco appears to be the ancestral one among angiosperms (Palmer, 1985). Inversions and other mutations that change the relative order of genes are extremely rare. Where gene order differences are found they can often be accounted for by single large inversional switches. Such rearrangements, therefore, are potentially valuable characters for phylogenetic studies (Jansen and Palmer, 1987). Plants of the family Onagraceae represent a welldefined group of flowering plants that has been studied in biosystematic detail (Raven, 1988). Therefore, they appear an ideal group for the application of modem genetic and biochemical techniques such as the molecular analysis of chloroplast genomes as a way to elucidate their phylogeny. For example, examination of chloroplast DNA variation in the genus Clarkia proved useful in answering phylogenetic questions (Sytsma and Gottlieb, 1986). In the North American subsection Euoenothera of the genus Oenothera section Oenothera (see Stubbe and Raven, 1979) a gene rearrangement as compared to the spinach chloroplast DNA revealed a 45 kb inversion within the large single copy region (Herrmann et al., 1983). Although Epilobium and Fuchsia are closely related to Oenothera, the chloroplast DNA gene arrangement in Epilobium (Schmitz and Kowallik, 1986) and Fuchsia (Palmer, pers. comm.) resembles that in spinach and tobacco, i.e., they clearly lack the large inversion reported for Euoenothera. Therefore, this inversion is considered a derived character marking the subsection Euoenothera. To study further the phylogenetic distribution of the 45 kb Euoenothera inversion we have analyzed the chloroplast genomes of two South American Oenothera species belonging to the subsection Munzia (0. villaricae and 0. picensis). Restriction site maps of the chloroplast DNA of these species have been published previously (vom Stein and Hachtel, 1986). In addition, we evaluated representative species from other subsections of the genus Oenothera.