This survey emphasizes the possibilities of isozymes as marker genes in solving a variety of problems important to population geneticists, evolutionists, systematists, and plant breeders.

Isozymes in plant biology

During the past decade, molecular techniques have provided a wealth of data that have facilitated the resolution of several controversial questions in polyploid evolution. Herein we have focused on several of these issues: (1) the frequency of recurrent formation of polyploid species; (2) the genetic consequences of multiple polyploidizations within a species; (3) the prevalence and genetic attributes of autopolyploids; and (4) the genetic changes that occur in polyploid genomes following their formation. Molecular data provide a more dynamic picture of polyploid evolution than has been traditionally espoused. Numerous studies have demonstrated multiple origins of both allopolyploids and autopolyploids. In several polyploid species studied in detail, multiple origins were found to be frequent on a local geographic scale, as well as during a short span of time. Molecular data strongly suggest that recurrent formation of polyploid species is the rule, rather than the exception. In addition, molecul...

Molecular data and the dynamic nature of polyploidy

This chapter discusses the use of enzyme electrophoresis for addressing questions in plant systematics. The present treatment updates previous reviews (Gottlieb, 1977a, 1981a, 1982, 1983, 1984a, 1986; Crawford, 1983, 1985; Giannasi and Crawford, 1986) and also discusses: (1) the factors involved in the initial applications of enzyme electrophoresis to plant systematics; (2) the kinds of studies employing electrophoresis to be emphasized in the future; and (3) the future of enzyme electrophoresis in plant systematics in light of the increasing emphasis on comparative studies of DNA.

Enzyme Electrophoresis and Plant Systematics

Electrophoretic variation of isoenzyme characters of eleven enzymes (ADH, A AD, MDH, SKD, AAT, SOD, PRX, PGD, PGI, LAP, EST) has been studied with the use of PAGE in 21 annual brome species: six of the section Genea, fourteen of the section Bromus and one of the section Boissiera of the genus Bromus. Most species could be distinguished from each other by diagnostic isoenzyme phenotypes. Only two pairs of morphologically similar species, B. diandrus B. rigidus of sect. Genea and B. japonicus B. squarrosus of sect. Bromus, revealed identical zymograms. The other two morphologically related species pair of the type section, B. hordeaceus B. interruptus, and B. arvensis B. intermedius, showed partially overlapping isozyme phenotypes. All diploid species lacked heterozygous allozyme phenotypes, indicating prevalent autogamy and self-fertilization. The two Genea diploids, B. sterilis and B. tectorum, proved clearly differentiated by alternate allozymes of seven isozymes. Populations of B. tectorum and B. sterilis from different sites of their Eurasian ranges had low genetic diversity but showed differentiation into eleven (B. tectorum) and six (B. sterilis) multilocus allozyme lineages. Geo­ graphically marginal European populations had only some of the multilocus allozyme lineages distributed in the Near East region, indicating weedy coloni­ zation of the two species by only a few successful genotypes. Phylogenetic relationships and genetic differentiation among eleven diploid brome species were evaluated by cladistic and phenetic analysis of allozyme diversity. The placement of B. pumilio in its own section Boissiera was supported by its basal position in a separate clade. Morphologically uniform B. intermedius was the most polymorphic species, revealing six isoenzyme lineages. The diploids of the section Genea were distinguished in a separate cluster on both cladistic and phenetic allozyme trees. All polyploids studied revealed different fixed heterozygosities of several heterozymes with codominant homoeozymes, suggesting their alloploid nature. Morphologically similar polyploids B. madritensis and B. rubens differed in seven heterozymes out of fourteen studied, supporting their specific recogni­ tion. One genome of the polyploid species of the section Genea corresponded closely to that of B. sterilis. Tetraploids B. secalinus and B. commutatus had very similar enzyme phenotypes, indicating their strong genetic affinity, but still differed in PRX-F homoeozymes of one composite genome. Diploid B. alopecuros appears to be the donor of one genome in the tetraploid B. lanceolatus. Bromus pseudosecalinus could not be one of the diploid pro­ genitors for B. secalinus. 6-PGD gene is duplicated and diverged in several diploids of the section Bromus, but species of the section Genea had no 6-PGD gene duplication.

Isoenzyme diversity and phylogenetic affinities among the Eurasian annual Bromus (Bromus L., Poaceae)

Cytology, genetics, and evolution in the genus Dactylis.

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.

Roselyne Lumaret

Papers

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