Institute of Botany
Sect. Plant Ecology
Molecular Evolution of ITS1 rDNA
RNA is embedded in the ribosome complex as structural component, e.g. the 18S rRNA and the 28S rRNA. Spacer RNA, like the ITS1 rRNA, is also transcribed from the rRNA gene cluster but it is „digested“ during maturation of the long rRNA precursor molecule. Thus, the ITS1 is often believed to evolve without functional constraints. The relatively fast sequence evolution of the ITS1 helps in the reconstruction of phylogenetic relationships of next related taxa (e.g., of species or genera). Hundreds of scientific papers are based on ITS1 spacer information in phylogenetic and systematic approaches. We put our focus on the ITS1 evolution of two distantly related groups of gastropods: the Stylommatophora (typical lung-breathing land snails) and the Basommatophora (lung-breathing freshwater snails). Time of divergence of both groups was in the Mesozoic, i.e. the last common ancestor lived about 150 million years ago. Interestingly, we found six highly conserved ITS1 regions in the sequence alignment of stylommatophoran and basommatophoran snails. This is a conclusive hint for structural significance of the ITS1 spacer in the precursor molecule during rRNA processing. For another rRNA spacer (the so-called ITS2), the same phenomenon has been documented in green algae and flowering plants (Mai & Colman 1997; J Mol Evol 44:258-271) and in yeast and vertebrates (Joseph et al. 1999; Nucleic Acids Res 27:4533-4540).
In additional projects, we cloned single ITS1 rDNA molecules from PCR amplicons to check for intra-individual nucleotide variation in the rRNA gene cluster. Some land snail individuals showed high nucleotide polymorphism in the gene cluster, whilst PCR products of individuals of other species do not have sequence variation at all. This leads to a range of questions in the molecular evolution of gastropods’ rRNA genes (Power of concerted evolution and molecular drive of the rRNA gene cluster? Existence of polymorphic pseudogene/paralogous variants in the genome?). Moreover, I suggest that ITS1 rRNA sequences should NOT be analysed with particular software using the energy-optimization rule to find structurally important RNA loops, stemloops or RNA helices in the RNA folding architecture.
Armbruster G (2001) -- Temperature-based variation of rRNA secondary structure models: a case study in the insect Drosophila simulans, the land snail Isabellaria adriani and the crustacean Daphnia pulex -- Canadian Journal of Zoology 79: 334-345
Armbruster G, van Moorsel HM & Gittenberger E (2000) -- Conserved sequence patterns in the non-coding ribosomal ITS-1 of distantly related snail taxa -- Journal of Molluscan Studies 66: 570-573
In additional projects, we cloned single ITS1 rDNA molecules from PCR amplicons to check for intra-individual nucleotide variation in the rRNA gene cluster. Some land snail individuals showed high nucleotide polymorphism in the gene cluster, whilst PCR products of individuals of other species do not have sequence variation at all. This leads to a range of questions in the molecular evolution of gastropods’ rRNA genes (Power of concerted evolution and molecular drive of the rRNA gene cluster? Existence of polymorphic pseudogene/paralogous variants in the genome?). Moreover, I suggest that ITS1 rRNA sequences should NOT be analysed with particular software using the energy-optimization rule to find structurally important RNA loops, stemloops or RNA helices in the RNA folding architecture.
References
Armbruster GFJ & Korte A (2006) -- Genomic nucleotide variation in the ITS1 rDNA spacer of land snails -- Journal of Molluscan Studies 72: 211-213Armbruster G (2001) -- Temperature-based variation of rRNA secondary structure models: a case study in the insect Drosophila simulans, the land snail Isabellaria adriani and the crustacean Daphnia pulex -- Canadian Journal of Zoology 79: 334-345
Armbruster G, van Moorsel HM & Gittenberger E (2000) -- Conserved sequence patterns in the non-coding ribosomal ITS-1 of distantly related snail taxa -- Journal of Molluscan Studies 66: 570-573