Ferns share mitochondrial rps3 introns with seed plants

Plant mitochondrial (mt) genome evolution has seen a frequent coming and going of group II introns before the establishment of the angiosperm lineages. The evolutionary history of introns and the relative contribution of intron loss and acquisition in shaping land plant mt genes and genomes remains on the whole poorly understood. The mt gene for the ribosomal protein S3 (rps3) has caught our attention because of the multiple changes in intron content that its locus has experienced during plant evolution. The rps3, generally, harbors one group II intron, namely rps3i74, at the same insertion site from algae up to the angiosperms analyzed so far, except in Beta and Marchantia. Surprisingly, in the mitochondria of some gymnosperm groups the rps3 reading frame harbors a downstream second group II intron, the rps3i249. Here, we report the remarkable genomic organization and expression of the rps3 encoded by the mt genome of the previously undocumented fern Adiantum capillus veneris L. Furthermore, a broader survey of additional ferns was carried out to identify the point of acquisition of the rps3i249 and the evolutionary depth of the rps3 intron flow in neglected land plants.