While the vast majority of spliceosomal introns are removed by the so-called U2 (major) spliceosome, diverse eukaryotes also contain a mysterious second form, the U12 (minor) form, and associated introns. In all characterized species, U12-type introns are distinguished by several features, including being rare in the genome, containing extended evolutionary-conserved splicing sites, being generally ancient as judged by conservation between distant species, and being inefficiently spliced. Here,

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... we report a remarkable exception in the slime mold Physarum polycephalum. The P. polycephalum genome contains > 20,000 U12-type introns -- 25 times more than any other species -- with transformed splicing signals that have co-evolved with the spliceosome due to massive gain of efficiently spliced U12-type introns. These results reveal an unappreciated dynamism of minor spliceosomal introns and spliceosomal introns in general.