Chromosome number evolution in Hymenophyllum (Hymenophyllaceae), with special reference to the subgenus Hymenophyllum

With about 100 species distributed worldwide, Hymenophyllum subg. Hymenophyllum is the largest subgenus of filmy ferns. It also displays morphological disparity and extreme chromosome numbers variation, with n = 11, 12, 13, 14, 18, 21, 22, 26, 28, and 34. We use DNA sequences from the genes rbcL, part of accD, rps4, and the intergenic spacers rbcL-accD, rps4-trnS, and trnG-trnR to infer relationships within Hymenophyllum, with a focus on this subgenus. In the subgenus Hymenophyllum, two main clades are retrieved together with several minor clades whose placement within the subgenus remains ambiguous. We then investigate the evolution of chromosome numbers in the genus and the subgenus, using a maximum likelihood approach taking into account phylogenetic uncertainty. We provide evidence that Hymenophyllum experienced descending aneuploidy during its evolutionary history, especially within the subgenus Hymenophyllum. Reduction in chromosome numbers is particularly extreme in one clade of the subgenus, with the lowest number reported for homosporous ferns. In addition, this group of species displays a high instability in its chromosome number. Both the reduction and the instability in chromosome number may coincide with the distribution of these species in either temperate areas or at high elevations.