Secondary structure of some elements of 18S rRNA suggests that strongylid and a part of rhabditid nematodes are monophyletic

Analysis of the secondary structure of 18S rRNA molecules in nematodes revealed some new traits in the secondary structure peculiar to their hairpin 17, Some of them are characteristic of all the nematodes, whereas others are characteristic exclusively of the order Rhabditida. The loss of a nucleotide pair in the highly conservative region of hairpin 17 distinguishes 18S rRNA of tire Strongylida and some species of the Rhabditida from other nematodes and, moreover, from all other organisms. Hence, it is possible to regard the Strongylida and a part of the Rhabditida including Caenorhabditis elegans as a nem monophyletic taxon. (C) 1998 Federation of European Biochemical Societies.