Parental origin and genome evolution in the allopolyploid Iris versicolor
Background Aims One of the classic examples of an allopolyploid is Iris versicolor, 'Blue Flag' (2n = 108), first studied by Edgar Anderson and later popularized by George Ledyard Stebbins in cytogenetics and evolutionary textbooks. It is revisited here using modem molecular and cytogenetic tools to investigate its putative allopolyploid origin involving progenitors of I. virginica (2n = 70) and L setosa (2n = 38). Methods Genomic in situ hybridization (GISH), fluorescent in situ hybridization (FISH) and Southern hybridization with 5S and 18-26S ribosomal DNA (rDNA) probes were used to identify the parental origin of chromosomes, and to study the unit structure, relative abundance and chromosomal location of rDNA sequences. Key Results GISH shows that L versicolor has inherited the sum of the chromosome complement from the two progenitor species. In L versicolor all the 18-26S rDNA units and loci are inherited from the progenitor of I. virginica, those loci from the L setosa progenitor are absent. In contrast 5S rDNA loci and units from both progenitors are found, although one of the two 5S loci expected from the L setosa progenitor is absent. Conclusions These data confirm Anderson's hypothesis that L versicolor is an allopolyploid involving progenitors of I. virginica and L setosa. The number of 18-26S rDNA loci in L versicolor is similar to that of progenitor I. virginica, suggestive of a first stage in genome diploidization. The locus loss is targeted at the L setosa-origin subgenome, and this is discussed in relation to other polyploidy systems.