Insights into fern evolution from mapping chloroplast genomes

Within the leptosporangiate ferns, chloroplast DNA (cpDNA) structural mutations have been identified by physical mapping. Earlier work revealed that members of four disparate ''higher'' fern families all possess cpDNAs that share the same set of complex rearrangements in the inverted repeat. Osmunda cpDNA lacks these rearrangements. We have examined the chloroplast genomes of additional ferns and determined that Polypodium and the schizaeaceous ferns share the rearrangements previously found in Cyathea, Pteridium, Adiantum, and Polystichum. Two additional cpDNA structural types also have been identified. A partial duplication and relocation of the gene chit usually occurs with the other gene rearrangements found in ''higher'' ferns but may be lacking in the heterosporous water fern, Marsilea. Also, a structural type lacking any of these complex rearrangements, but still different from Osmunda, occurs in Gleichenia cpDNA. The phylogenetic implications of these findings conflict, at least in part, with previously proposed evolutionary hypotheses.