Phylogenetic analysis of Asplenium subgenus Ceterach (Pteridophyta: Aspleniaceae) based on plastid and nuclear ribosomal ITS DNA sequences

Phylogenetic relationships among 20 taxa of the fern genus Asplenium subgenus Ceterach (Filicopsida, represented by 73 accessions) were investigated using DNA sequence data from the nuclear ribosomal internal transcribed spacers (ITS nDNA) and plastid trnL-F intergenic spacer. In addition, a single sample per taxon was used in an analysis of the plastid rbcL gene. Chromosome counts were determined for all the samples, and these demonstrated a range from diploid to octoploid. Analyses of the DNA sequence data indicated that Asplenium subgenus Ceterach is polyphyletic, implicating homoplasy in the characters previously used to circumscribe this taxon. Plastid trnL-F and rbcL analyses resulted in identical tree topologies. The trees produced from the separate plastid and nuclear matrices agree in (1) the recognition of identical groups of accessions corresponding to A. dalhousiae, A. ceterach, A. aureum, A. cordatum, A. phillipsianum, and A. haughtonii; (2) the division of A. subg. Ceterach into two subclades, a Eurasian-Macaronesian and a strictly African alliance; (3) the position of A. dalhousiae as a member of the former subclade; (4) the lack of genetic variation in A. cordatum despite its morphological variability; and (5) the clustering of each autopolyploid with their diploid ancestor. However, the plastid and nuclear trees differ in their placement of A. haughtonii and A. dalhousiae, which might be due to different evolutionary histories of nuclear and plastid genomes, and is possibly an indication of ancient hybridization. The analyses confirm the existence of several strictly African taxa. Asplenium phillipsianum and A. cordatum each form species complexes of diploid and autopolyploid taxa, from which a third, morphologically intermediate, allotetraploid species has originated. Asplenium haughtonii is a distinct endemic species from Saint Helena. The maternally inherited plastid sequences support the hypothesis that A. aureum is an ancestor of A. lolegnamense and of A. octoploideum. Because gene conversion did not eliminate divergent ITS alleles in the allopolyploids, their reticulate ancestry could be demonstrated. Biparentally inherited nrITS sequences support the allopolyploid status of A. aureum, A. lolegnamense, and A. punjabense, indicating they share the ancestral A. javorkeanum genome.