Trefusiidae are a subtaxon of marine enoplida (Nematoda): Evidence from primary structure of hairpin 35 and 48 loops of SSU rRNA gene

A rare nucleotide substitution was found in the evolutionarily conserved lo op of hairpin 35 of the 18S rRNA gene of marine free-living nematode, Trefu sia zostericola (Nematoda: Enoplida), The same substitution was found in al l the marine Enoplida studied but not in other nematodes. Such a molecular synapomorphy indicates that marine enoplids are more closely related to T z ostericola than to freshwater Triplonchida. Maximum parsimony, neighbor-joi ning, and maximum likelihood analyses of complete nucleotide sequences of t he gene, with the heterogeneity of nucleotide sites in evolution rates take n into account, support this conclusion. Hence, the hypothesis of particula r primitiveness of Trefusiidae among nematodes should be rejected. Phylogen ies based on molecular data support the morphological reduction of metaneme s in Trefusiidae. Alongside with the unique change in hairpin 35 loop among marine Enoplida (including T zostericola), hairpin 48 is also modified by a rare transversion which could be found among Mesorhabditoidea nematodes, in related genera Pelodera, Mesorhabditis, Teratorhabditis, Parasitorhabdit is, Crustorhabditis, and Distolabrellus, and in I I orders of Rhodophyta. R are mutations in hairpins 35 and 48 tend to be fixed correlatively in evolu tion and could be found in all the Acanthocephala species. X-Ray data show that these regions (H31 and H43, in alternative nomenclature) are spatially brought together in native ribosomes. The nature and distribution of molec ular autoapomorphies in phylogenetic trees of high-rank taxa are discussed.