Molecular Studies of PgiC Evolution in Allotetraploid Species of Clarkia (Onagraceae)

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9615037 Gottlieb The genus Clarkia of the evening-primrose family (Onagraceae) is a group of 42 species, all but one native to western North America, that has been examined for more than 50 years as a model for understanding speciation and genetic divergence and to assess the value of different sources of evidence for inferring phylogeny. The genealogical relationships among the species have been resolved by analyses of many classes of characters, including nucleotide sequences of the gene PgiC which encodes the cytosolic isozyme of phosphoglucose isomerase, an essential enzyme of sugar metabolism in plants. PgiC was duplicated in the basal stock that gave rise to Clarkia, and about half the extant diploid species express both duplicated genes (have two PgiC isozymes). In the other diploid species, one PgiC gene has been "silenced" (there is only one isozyme present for enzyme function, but there are two detectable DNA loci). Clarkia also includes a number of allotetraploid species, that combine through hybridization the genomes (chromosome sets) of two diploid species and are thus expected to possess four PgiC genes. However, in all tetraploid Clarkias tested only two PgiC genes have been found. The major goal of Prof. Gottlieb's research is to determine through DNA nucleotide sequencing which PgiC genes are expressed and whether the genes that are not expressed have been mutated or lost. In this study, the naturally occurring tetraploid species will be compared to experimentally synthesized hybrids between their diploid progenitors. In addition, hybrids between the diploids and tetraploids will be studied. The synthesized hybrids are expected to express all the PgiC genes of their parents. The extent and rate of nucleotide divergence between the duplicate genes will be compared between the diploid and tetraploid genomes. The proposed studies are an attempt to describe at the molecular level the fate of particular genes encoding enzymes of known function during the formation and establishm ent of allotetraploid plant species. Such basic information should also help guide future analysis of the molecular causes of gene silencing and loss. The study will also provide information about the use of nuclear genes from polyploids in analyzing phylogeny, a highly relevant issue in plant systematics because more than half of the flowering plants are polyploid.