Nuclear-encoded rDNA group I introns: Origin and phylogenetic relationships of insertion site lineages in the green algae

Debashish Bhattacharya, Thomas Friedl, Simon Damberger

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132 Scopus citations


Group I introns are widespread in eukaryotic organelles and nuclear- encoded ribosomal DNAs (rDNAs). The green algae are particularly rich in rDNA group I introns. To better understand the origins and phylogenetic relationships of green algal nuclear-encoded small subunit rDNA group I introns, a secondary structure-based alignment was constructed with available intron sequences and 11 new subgroup IC1 and three new subgroup IB3 intron sequences determined from members of the Trebouxiophyceae (common phycobiont components of lichen) and the Ulvophyceae. Phylogenetic analyses using a weighted maximum-parsimony method showed that most group I introns form distinct lineages defined by insertion sites within the SSU rDNA. The comparison of topologies defining the phylogenetic relationships of 12 members of the 1512 group I intron insertion site lineage (position relative to the E. coli SSU rDNA coding region) with that of the host cells (i.e., SSU rDNAs) that contain these introns provided insights into the possible origin, stability, loss, and lateral transfer of IC1 group I introns. The phylogenetic data were consistent with a viral origin of the 1512 group I intron in the green algae. This intron appears to have originated, minimally, within the SSU rDNA of the common ancestor of the trebouxiophytes and has subsequently been vertically inherited within this algal lineage with loss of the intron in some taxa. The phylogenetic analyses also suggested that the 1512 intron was laterally transferred among later-diverging trebouxiophytes; these algal taxa may have coexisted in a developing lichen thallus, thus facilitating cell-to-cell contact and the lateral transfer. Comparison of available group I intron sequences from the nuclear-encoded SSU rDNA of phycobiont and mycobiont components of lichens demonstrated that these sequences have independent origins and are not the result of lateral transfer from one component to the other.

Original languageEnglish (US)
Pages (from-to)978-989
Number of pages12
JournalMolecular biology and evolution
Issue number7
StatePublished - Sep 1996
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Ecology, Evolution, Behavior and Systematics
  • Molecular Biology
  • Genetics


  • green algae
  • group I introns
  • lateral transfer
  • lichens
  • molecular evolution
  • phylogeny
  • secondary structure
  • small subunit ribosomal DNA

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