Primary and secondary structure of the 18S ribosomal RNA of the bird spider Eurypelma californica and evolutionary relationships among eukaryotic phyla.

The primary structure of the 18S rRNA of the bird spider Eurypelma californica has been determined in the framework of a study of metazoan phylogeny on the basis of ribosomal RNA structure. A secondary-structure model was derived by comparison of the sequence with that of 43 other eukaryotic small-ribosomal-subunit RNA sequences presently available. This comparison allows a rather detailed secondary-structure pattern to be postulated for a eukaryote-specific area of highly variable sequence and length for which no consensus model has hitherto been attained. A dendrogram, reflecting evolutionary relationships among the 40 eukaryotic species of known 18S rRNA structure, was constructed by a matrix method selecting the best-fitting tree on the basis of a least-squares criterion. The tree shows an early divergence of a microsporidium, an euglenoid, kinetoplastids and a slime mold. Among the remaining species, two main clusters are distinguishable, one comprising the Ciliata, the other comprising Metazoa, green plants, fungi and several protists. Among the Metazoa, the three phyla presently investigated, viz. Chordata, Arthropoda and Nemathelminthes, are distinguishable as three separate lines of descent.