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Abstract
According to PCR assays and sequencing, we now report the shared presence of two rps3
introns, namely the rps3i74 and the rps3i249, in the mitochondria of all the classes
representing the surviving lineages of gymnosperms, and unveil several lineages experiencing
intron loss. Interestingly, the rps3 intron gains and losses within the four groups
of gymnosperms let us sort out the Pinaceae and the non-Pinaceae into intron (+)-
and intron (-)-lineages, respectively. Worthy of mention is also the finding that
only Gnetum within the Gnetales harbours both the rps3 introns. This intron distribution
pattern is consistent with the hypothesis that the two rps3 introns were likely present
in the common ancestor of the seed plants and, then, independently lost in the non-Pinaceae
during gymnosperm evolution. The derived secondary structural model of the novel group
IIA intron improves our understanding of the significance and origin of the extraordinary
length polymorphisms observed among rps3i249 orthologs. Despite the remarkable structural
plasticity to adopt and reject introns, the rps3 mRNAs undergo accurate processing
by splicing and extensive editing in gymnosperm mitochondria. This study provides
additional insights into the evolutionarily high dynamics of mitochondrial introns
which may come and go in closely related plant species. The turnover of the mitochondrial
rps3 group II introns seen among lineages of seed plants further suggests that these
introns might be an additional signature to discriminate between particularly cryptical
taxonomic groups for which there is a need of a further evaluation of their evolutionary
affiliation.
Copyright 2010 Elsevier Masson SAS. All rights reserved.