A PRELIMINARY STUDY ON PHYLOGENY OF NINE SPECIES OF PHILOMETRIDS IN CHINA

Because of difficulties in finding males of philometrids, they are usually recognized by females only. The character of the female is lacking,so many arguments arise on the present classification system of this nematode. To reveal the phylogenetic relationships of the philometrids collected in China,the intemal transcribed spacer region rDNA(ITS rDNA)of 8 species and the partial small subunit rDNA(18S rDNA)of 9 species were sequenced. Due to great divergence in the sequences, ITS rDNA was not considered suited for the phylogentic studies in philometrids. Regarding 18S rDNA,an alignment of 943 nucleotide sites was obtained ,of which, variable sites were 140 including 69 parsimony informative sites. Sequence divergence among the philometrids analyzed ranged from 0 between close species to 11.99% between distant species. In contrast with ITS rDNA, 18S rDNA was the more reliable marker for analyzing the phylogenetic position of the philometrids, thereby a phylogenetic tree based on the sequences of 18S rDNA was constructed. It was found that the tree strongly supported the philometrids in China to be a monophyly. The three species, Philometroides fulvidraconi, P. ganzhounensis and Philometra clavaeceps were very close in relationship,and might be appeared lately in history. The structure of the Cuticular bosses of philometrids was suggested to be lost easily in rapid evolution of the nematodes, thus it should not to be regarded as one of the most important distinct characteristics of the genus Philometrodes from other genera. According to the present data and the previous opinions of other researchers, it was suggested that the genus Philometroides should be divided into different genera.