Phylogenetic relationships of Baikalian species of Prorhynchidae turbellarian worms as inferred by partial 18S rRNA gene sequence comparisons (preliminary report).

The polymerase chain reaction (PCR) and direct sequencing of RNA genes were applied for phylogenetic relationship analysis among Turbellaria (flatworms). Representatives of 10 species of the genus Geocentrophora (Prorhynchidae family) were used to establish homology between nucleotides in the 5'-end portion of the 18S rRNA gene sequences. Sequence data were subdivided into 3 subsets--unpaired and basepaired (according to rRNA secondary structure) and transversion, including only transversion substitutions--that were independently analyzed to construct phylogenetic trees. Distance matrix and maximum parsimony approaches were applied to infer phylogenies. Trees were examined in terms of morphological taxonomy. The DNA sequences support the taxonomy and systematics of Baikalian flatworms. The hypothesis of Geocentrophora origin in Baikal is discussed.