Molecular punctuated equilibrium in the evolution of Rhopalura ophiocomae (Mesozoa : Orthonectida) 18S rRNA, hairpin 17

Alternative states of the predicted structure of hairpin 17 in the small-su bunit ribosomal RNA genes are described. Each of these structures is charac teristic of one or several phylons of animals. For example, one of the alte rnatives could be found in the rRNA of most Bilateria, but not in protists, fungi, plants, or diploblastic animals. A presumable secondary structure o f the 18S rRNA hairpin 17 of a lower multicellular animal, Rhopalura ophioc omae (Mesozoa: Orthonectida), was constructed. It differs drastically from the one of other lower multicellular animals, i.e., Rhombozoa, Myxozoa, and Acoela. The evolution of rRNA fragments according to the principle of punc tuated equilibrium is discussed.