Phylogeny of six naviculoid diatoms based on 18S rDNA sequences

18S rDNA sequences of six Naviculaceae species [Amphora montana, Gomphonema parvulum, Eolimna minima (syn. Navicula minima), Eolimna subminuscula (syn , Navicula subminuscula), Navicula veneta and Phaeodactylum tricornutum] we re determined in order to assess the monophyly of this important group of d iatoms, to date not included in 18S rDNA databases, and also that of the re cently described genus Eolimna. Phylogenetic trees were constructed using o ther known diatom 18S rDNA sequences, and best tree topologies obtained wer e tested against alternative trees for their reliability, The analyses do n ot reject the monophyly of Naviculaceae and strongly support the separation of the genus Eolimna from Navicula sensu late. The two species of Eolimna, however, do not appear to be each other's closest relatives among the spec ies investigated: rather, E. subminuscula shows affinities to A, montana, a nd E, minima to P. tricornutum. A. montana, a species which it has been pro posed should be transferred into a separate taxon from the other five speci es, was found to have grouped well within them in all analyses.