Molecular Biology and Evolution, Vol 13, 978-989, Copyright © 1996 by Society for Molecular Biology and Evolution
D Bhattacharya, T Friedl and S Damberger
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 ICI 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 ICI 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 ARTICLE
Nuclear-encoded rDNA group I introns: origin and phylogenetic relationships of insertion site lineages in the green algae
Department of Biochemistry, Max Planck Institute for Biophysical Chemistry, Gottingen, Germany. dbhatta@gwdgvl.gwdg.de
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