Doubled Duplication of the Structural Gene for Cytosolic Phosphoglucose Isomerase in the Dactylis glomerata L. Polyploid Complex

Abstract

Evolution of genes that specify cytosolic phosphoglucose isomerases (PGIs) has been studied in a natural intraspecific polyploid complex (Dactylis glomerata L.). Analyses of genetic control, polymorphism, and gene expression have been carried out in plants collected from numerous populations that belong to 14 diploid subspecies originating in the middle or at the end of the Tertiary period and to three tetraploid subspecies that very likely developed during the last Ice Age. The results show that in each one of the 17 Dactylis subspecies, cytosolic PGIs are specified by two tightly but not completely linked polymorphic loci. Moreover, these show considerable structural similarity since the numerous subunits that each locus specifies associate with those coded by the other locus and most of the dimeric enzymes formed by subunits specified at each locus show exactly the same electrophoretic mobilities. Consequently, one of the two genes is considered to have originated from the other by a process of gene duplication (very likely a tandem one). In this particular case, the duplicated gene has not been distinguished from the ancestral one. The PGI duplication is present in the whole Dactylis monospecific genus but has never been found in the other genera of the same tribe. Since the fixation of a particular gene duplication is considered to be a rare or even unique event, it might have occurred in the primitive forms of diploid Dactylis. Tetraploids show a doubled duplication consequent on the polyploidization process of diploids that already possessed the PGI duplication. The reasons why this duplication has been conserved with the full expression of all the genes throughout evolution of this polyploid complex are examined separately for the two ploidy levels. In diploids, the main reasons could be gene recombination that slows down the rate of gene silencing, unregulated enzyme synthesis, and also differences in enzyme properties. These may confer a selective advantage when distinct alleles can be present, because of the gene duplication, in a single individual. The quite recent origin of the tetraploids and their autopolyploid genetic structure-which does not show f&d heterozygosity and, consequently, possible negative heterodimeric interactions between homeoalleles such as would be the case in allopolyploids-could explain why full expression of the cytosolic PGIs’ doubled duplication is maintained in tetraploid Dactylis.