BIDIRECTIONAL INTERLOCUS CONCERTED EVOLUTION FOLLOWING ALLOPOLYPLOID SPECIATION IN COTTON (GOSSYPIUM)

Polyploidy is a prominent process in plant evolution; yet few data add ress the question of whether homeologous sequences evolve independentl y subsequent to polyploidization. We report on ribosomal DNA (rDNA) ev olution in five allopolyploid (AD genome) species of cotton (Gossypium ) and species representing their diploid progenitors (A genome, D geno me). Sequence data from the internal transcribed spacer regions (ITS1 and ITS2) and the 5.8S gene indicate that rDNA arrays are homogeneous, or nearly so, in all diploids and allopolyploids examined. Because th ese arrays occur at four chromosomal loci in allopolyploid cotton, two in each subgenome, repeats from different arrays must have become hom ogenized by interlocus concerted evolution. Southern hybridization ana lysis combined with copy-number estimation demonstrate that this proce ss has gone to completion in the diploids and to completion or near-co mpletion in all allopolyploid species and that it most likely involves the entire rDNA repeat. Phylogenetic analysis demonstrates that inter locus concerted evolution has been bidirectional in allopolyploid spec ies-i.e., rDNA from four polyploid lineages has been homogenized to a D genome repeat type, whereas sequences from Gossypium mustelinum have concerted to an A genome repeat type. Although little is known regard ing the functional significance of interlocus concerted evolution of h omeologous sequences, this study demonstrates that the process occurs for tandemly repeated sequences in diploid and polyploid plants. That interlocus concerted evolution can occur bidirectionally subsequent to hybridization and polyploidization has significant implications for p hylogeny reconstruction, especially when based on rDNA sequences.