Conserved sequence patterns in the non-coding ribosomal ITS-1 of distantly related snail taxa

The ensuing contribution gives an insight into the evolution of non-coding ribosomal DNA of two distantly related groups of snails. The non-coding sequences of the ribosomal DNA comprise the First Internal Transcribed Spacer (ITS-1), the Second Internal Transcribed Spacer (ITS-2), and the NonTranscribed Spacer (NTS). These spacers are known to evolve faster than the coding 18S, 5.8S and 28S ribosomal DNA1,2,3. The fast evolution of the spacers can be used to investigate the genetic relationships of closely related genera, species and other lower taxonomic units with a divergence time of less than 50 million years3,4,5. The focus of our analysis is on the evolution of the ITS-1 of land snails (Stylommatophora) and of freshwater snails (Basommatophora). These two snail groups have their origin in the Mesozoic. Consequently, we expected their ITS-1 sequences to show strong differences. However, the ITS-1 of stylommatophoran and basommatophoran gastropods is consecutively divided into conserved and variable regions. The detection of conserved ITS-1 regions among distantly related animals gives further insight into the functional significance of non-coding ribosomal RNA. In the analysis, all published ITS-1 sequences of stylommatophoran and basommatophoran gastropods are taken into consideration. These include the sequences of two species of Albinaria, two species of Isabellaria (Stylommatophora; Clausiliidae)6, three species of Bulinus (Basommatophora; Planorbidae)7, and four species of Stagnicola (Basommatophora; Lymnaeidae)8. In addition, we sequenced the ITS-1 of the following five species (protocols are available on request): three species of Cochlicopa (Stylommatophora; Cochlicopidae), Azeca goodalli (Stylommatophora; Cochlicopidae), and two specimens of Arianta arbustorum (Stylommatophora; Helicidae) (accession numbers: Y16755–Y16765; AF124052–AF124053). The sequences were aligned with the program CLUSTAL W, version 1.5, in order to find homologous regions in all taxa see 9. The sequence alignment clearly shows that conserved sequence regions and variable regions occur inthe ITS-1 of stylommatophoran and basommatophoran gastropods. Only a few alignment positions had to be corrected manually. Six conserved blocks, I to VI, ranging in size from 15 to 28 nucleotides were found. Both the relative position (Figure 1) and the respective sequences are highly similar among the taxa (over 80 percent sequence similarity in pairwise RESEARCH NOTES