Slow evolution of 1506 group I intron in Spirogyra Link 1820 (Zygnematophyceae, Streptophyta), a fast evolving lineage in the Zygnemataceae.

Abstract

Phylogenetic analyses of SSU rDNA sequences of 130 Spirogyra strains have revealed that these strains were subdivided into eight clades. Approximately 60% of the assessed strains (clades A-D) contain a 1506 group I intron, whereas strains without introns form individual clades (E-H). The Spirogyra intron shared the common insertion site of the Zygnematalean intron (position 1506 relative to the Escherichia coli small-subunit rRNA). Phylogenetic analyses of the Spirogyra group I intron showed the monophyletic origin within the Zygnematophyceae. Therefore, we assume the secondary loss of the intron in clades E-H is caused by the high evolutionary rate of Spirogyra and its long evolutionary history. The Spirogyra intron belongs to the IC group I introns and shares many common features with the intron of other Zygnematophyceae (the typical domain structure (P1-P9), the base composition, the highly conserved regions the U preceding the 5' splice site and the G to which it pairs, and the G preceding the 3' splice site are typical for IC group I intron). Spirogyra group I introns exhibit features of early desmids (optional P2 domain) as well as of later diverging desmids (variation from the typical L5b-GAAA tetraloop). The P2 domain shows an additional optional P2 sub-domain in clade B. Surprisingly, the mutation rate of the Spirogyra SSU rRNA exceeds the rate of the intron by far. Evolutionary rates differ significantly within the Spirogyra SSU rRNA accessions, but not within the respective group I intron sequences.