V. V. Aleshin2, N. S. Vladychenskaya1, O. S. Kedrova1, I. A. Milyutina1, and N. B. Petrov1
1 Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow, 119899 Russia
2 Russian Institute of Livestock Physiology, Biochemistry, and Nutrition,
Russian Academy of Agricultural Sciences, Borovsk, 249010 Russia
Received August 10, 1998
Abstract—Alternative states of the predicted structure of hairpin 17 in the small-subunit ribosomal RNA genes
are described. Each of these structures is characteristic of one or several phylons of animals. For example, one
of the alternatives could be found in the rRNA of most Bilateria, but not in protists, fungi, plants, or diploblastic
animals. A presumable secondary structure of the 18S rRNA hairpin 17 of a lower multicellular animal, Rho-
palura ophiocomae (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 punctuated equilibrium is discussed.
Key words: 18S rRNA, hairpin 17, Rhopalura ophiocomae (Mesozoa: Orthonectida), evolution, molecular phy-
logeny, punctuated equilibrium
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