RIBOSOMAL DNA EVOLUTION AT THE POPULATION-LEVEL - NUCLEOTIDE VARIATION IN INTERGENIC SPACER ARRAYS OF DAPHNIA-PULEX

Nucleotide variation was surveyed in 21 subrepeat arrays from the ribo somal DNA intergenic spacer of three Daphnia pulex populations. Eighte en of these arrays contained four subrepeats. Contrary to expectations , each of the four positions within the array had a different consensu s sequence. However, gene conversion, involving sequences less than th e length of a subrepeat, had occurred between subrepeats in different positions. Three arrays had more than four subrepeats and were undoubt edly generated by unequal crossing over between standard-length arrays . The data strongly suggested that most unequal exchanges between arra ys are intrachromosomal and that they occur much less frequently than unequal exchanges at the level of the entire rDNA repeat. Strong assoc iations among variants at different positions allowed the recognition of five groups of arrays, two of which were found in more than one pop ulation. Five of the seven individuals surveyed had arrays from more t han one group. Analysis of the distribution of nucleotide variation su ggested that the populations were quite divergent, a result that is co ncordant with previous surveys of allozyme and mitochondrial DNA varia tion. It was suggested that some of the subrepeat array types are quit e old, at least predating the recolonization of pond habitats in the m idwestern United States after the last glaciation.