DERIVATION OF THE SECONDARY STRUCTURE OF THE ITS-1 TRANSCRIPT IN VOLVOCALES AND ITS TAXONOMIC CORRELATIONS

Knowledge of secondary structure, formed by the gene spacer regions of the primary transcript of nuclear rDNA cistrons, is lacking for most phyla of eukaryotes. We have sequenced the first internal transcribed spacer region (ITS-1) of multiple representatives of the Volvocales, a nd from comparisons of these, derived a secondary structure common to the entire group. The secondary structure model is supported by numero us compensating base pair changes located within the paired regions of the stem-loops. Within the morphological species, such as those of As trephomene and Gonium, the three basal nucleotide pairs of helices are highly conserved in primary sequence, and the single stranded region rich in CCAA is identical in sequence, even when isolates come from al l continents of the earth. In other Volvocacean species known to inclu de many pairs of mating types, this same level of conservation is foun d to correlate with the mating subgroups of the species. Thus a compar able degree of sequence similarity appears to characterize all isolate s of a ''biological'' species; this is valid for taxonomic species onl y where the biological and taxonomic species levels coincide. In addit ion, the ITS-1 contains information useful for population analyses, an d spacer secondary structure may have additional phylogenetic utility at the level of class or subclass when that information becomes availa ble for other protistan groups.