A recently silenced, duplicate PgiC locus in Clarkia

Previous electrophoretic analysis showed that 17 diploid species of thewildflower Clarkia (Onagraceae) have two cytosolic isozymes ofphosphoglucose isomerase (PGIC; EC 5.3.1.9), whereas 15 other diploidspecies have a single PGIC. Molecular studies revealed that the twoisozymes in the former species are encoded by duplicate genes, PgiC1 andPgiC2, whereas the single isozyme in the latter is always encoded by PgiC1.Phylogenetic analysis of the nucleotide sequences implied that PgiC2 wassilenced four times independently in the genus. Here we describe a psiPgiC2 from C. mildrediae, a species in which only PgiC1 is expressed. Thediscovery of the psi PgiC2 is significant because it confirms a formalprediction of the phylogenetic analysis. The psi PgiC2 includes 5,039nucleotides corresponding to 18 of the 23 exons of PgiC, as well as theintervening introns and 3' nontranslated region. The absence of an increaseof nucleotide substitutions in its "exons" suggests that the gene wassilenced recently. The present study appears to be the first to establishthat a specific duplicate gene locus regularly expressed in a group ofrelated plant species has been silenced in one of them. The multipleindependent silencings of PgiC2 suggest that it remained functional butinessential in ancestral lineages. We discuss the possibility that PgiC2may have been preserved in these lineages by selection against mutantscausing defective PGIC1- PGIC2 heterodimers.