Expansion and transformation of the minor spliceosomal system in the slime moldPhysarum polycephalum

Spliceosomal introns interrupt nuclear genes and are removed from RNA transcripts (“spliced”) by machinery called spliceosomes. While the vast majority of spliceosomal introns are removed by the so-called major spliceosome, diverse eukaryotes also contain a mysterious second form, the minor spliceosome, and associated introns [1–3]. In all characterized species, minor introns are distinguished by several features, including being rare in the genome (∼0.5% of all introns) [4–6], containing extended evolutionary-conserved splicing sites [4,5,7,8], being generally ancient [9,10] and being inefficiently spliced [11–13]. Here, we report a remarkable exception in the slime mold Physarum polycephalum . The P. polycephalum genome contains > 20,000 minor 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 minor introns. These results reveal an unappreciated dynamism of minor spliceosomal introns and spliceosomal introns in general.