Spliceosomal introns are ubiquitous in nuclear transcriptomes but gener- ation of
new U2 introns is a vexing mystery. The availability of complete genomes of more than
a thousand fungi allows to study intron gain. St- wintrons consist of nested U2 introns
excised by consecutive splicing re- actions. In a [D1,2] stwintron, an internal intron
interrupts the 5’-donor of an external intron between the first and second nucleotide.
One can classify [D1,2] stwintrons in two groups. Uniquely occurring stwintrons are
typical- ly integrated at gene positions occupied by an apparently sequence-un- related
[D1,2] stwintron in a broad range of related fungal taxa (genus, family, order). On
the other hand, sequence-similar “sister” stwintrons, cross-identified by blastn,
occur at new intron positions in very narrow taxa, like Hypoxylon sp. CO27-5 and EC38.
Four of the 25 sister stwintrons are unique to either CO27-5 or EC38, strongly suggesting
recent prolifer- ation events.When such blastn screens were performed in genomes of
more remotely related Xylariales taxa, series of new sister stwintrons were identified.
Those of Hypoxylon pulicicidum all localize in genes the orthologs of which do not
harbor stwintrons in CO27-5/EC38, suggest- ing they arose after divergence of the
taxa. We have identified some 230 sister stwintrons in published genomes of Hypoxylaceae
and Xylariace- ae species. Some species contain more than 50 sister stwintrons, others
harbor less than ten. Some genomes investigated also specify a group of related, smaller
canonical introns that derive from the fusion of the oppo- site terminal quarters
of a sister stwintron, and propagate as canonical introns. We analyzed all genuine
sister stwintrons, their internal structure, their respective integration sites and
the bounding exonic sequences. In addition, the interrelations were estimated for
the ensemble of collected [D1,2] sister stwintrons which may allow to reflect on the
deeper origin(s) of the Xylariales sister stwintron(s).
