Internally Symmetrical Stwintrons and Related Canonical Introns in Hypoxylaceae Species

Spliceosomal introns are pervasive in eukaryotes. Intron gains and losses have occurred throughout evolution, but the origin of new is unclear. Stwintrons complex intervening sequences where one sequence elements (5′-donor, lariat branch point element or 3′-acceptor) necessary for excision a U2 intron (external intron) itself interrupted by second (internal) intron. In Hypoxylaceae, family endophytic fungi, we uncovered scores donor-disrupted stwintrons with striking similarity among themselves also canonical introns. Intron–exon structure comparisons suggest that these proliferated within diverging taxa give rise to proliferating some genomes. The (stw)introns integrated seamlessly at novel gene positions. recently appear originate from conserved ancestral stwintron characterised terminal inverted repeats (45–55 nucleotides), highly symmetrical may allow formation double-stranded RNA molecule. No short tandem duplications flank putatively inserted sequences, which excludes DNA transposition-based mechanism proliferation. It tempting this role proliferation (an)other mechanism(s).