Molecular Evolution and Systematic Applications of PGI Sequences in Plants

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The plant genus Clarkia, of annuals native to California, has been investigated intensively for its morphology and its chromosomes to understand how plant species originate and how new morphological and cytological patterns evolve. More recently Dr. Les Gottlieb of the University of California at Davis has studied biochemical and genomic aspects of these plants to describe the extent of their overall genetic divergence, to account for the evolution of particular genes, and to learn how to use this molecular evidence to infer phylogenetic relationships among species. Many species of Clarkia have duplicated isozymes of the glycolytic enzyme phosphoglucose isomerase (PGI), which catalyzes an essential reaction in intermediary metabolism. Dr. Gottlieb recently sequenced the nuclear genes encoding these isozymes in species with and without the duplication. Unexpectedly the genes proved to have two different structures, one with a large number of exons separated by introns of varying lengths and the other with no introns at all. The genes without introns are hypothesized to be descended from a "processed" gene that arose when a copy of the primary messenger RNA transcript of a structured gene was inserted into the genome in a position and orientation that permitted it to be expressed. Dr. Gottlieb will continue his analysis of PGI gene structure in order to identify the taxonomic distribution of the structured and nonstructured PGI genes in Clarkia. In turn, this will allow determination of the origin(s) of the duplication when compared with the phylogeny of these species determined from other morphological and molecular evidence. The studies constitute one of the first analyses of molecular evolution in nuclear genes in wild (noncultivated) plants and are directly relevant to the description of the organization and pattern of sequence variation, the source of evolutionary change.