Phylogenetic evidence for horizontal transmission of group I introns in the nuclear ribosomal DNA of mushroom-forming fungi |
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Authors: | Hibbett DS |
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Institution: | Harvard University Herbaria, Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, USA. dhibbett@oeb.harvard.edu |
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Abstract: | Group I introns were discovered inserted at the same position in the
nuclear small-subunit ribosomal DNA (nuc-ssu-rDNA) in several species of
homobasidiomycetes (mushroom-forming fungi). Based on conserved intron
sequences, a pair of intron-specific primers was designed for PCR
amplification and sequencing of intron-containing rDNA repeats. Using the
intron-specific primers together with flanking rDNA primers, a PCR assay
was conducted to determine presence or absence of introns in 39 species of
homobasidiomycetes. Introns were confined to the genera Panellus,
Clavicorona, and Lentinellus. Phylogenetic analyses of nuc-ssu-rDNA and
mitochondrial ssu-rDNA sequences suggest that Clavicorona and Lentinellus
are closely related, but that Panellus is not closely related to these. The
simplest explanation for the distribution of the introns is that they have
been twice independently gained via horizontal transmission, once on the
lineage leading to Panellus, and once on the lineage leading to Lentinellus
and Clavicorona. BLAST searches using the introns from Panellus and
Lentinellus as query sequences retrieved 16 other similar group I introns
of nuc-ssu-rDNA and nuclear large-subunit rDNA (nuc-lsu-rDNA) from fungal
and green algal hosts. Phylogenetic analyses of intron sequences suggest
that the mushroom introns are monophyletic, and are nested within a clade
that contains four other introns that insert at the same position as the
mushroom introns, two from different groups of fungi and two from green
algae. The distribution of host lineages and insertion sites among the
introns suggests that horizontal and vertical transmission, homing, and
transposition have been factors in intron evolution. As distinctive,
heritable features of nuclear rDNAs in certain lineages, group I introns
have promise as phylogenetic markers. Nevertheless, the possibility of
horizontal transmission and homing also suggest that their use poses
certain pitfalls.
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