Diversification of the fern genus Cryptogramma across time and space

Abstract

I examined diversification, biogeographic history and polyploidy within the parsley ferns (Cryptogramma) across multiple time scales. Cryptogramma is a small circumboreal genus of rock ferns in the large, diverse family Pteridaceae and is most closely related to the Asian genus Coniogramme and the monotypic Central American genus Llavea. I generated a combined six locus plastid sequence alignment (rbcL, rbcL-accD, rbcL-atpB, rps4-trnS, trnG-trnR, and trnPpetG) and a low-copy nuclear marker (gapCp) alignment for 40 accessions. Phylogenetic analysis of these datasets using maximum parsimony, maximum likelihood, and Bayesian inference demonstrate that all three genera are reciprocally monophyletic, with Cryptogramma and Coniogramme most closely related to one another. This analysis also recovered the monotypic Cryptogramma section Homopteris and sect. Cryptogramma as reciprocally monophyletic. Within sect. Cryptogramma, the unambiguously supported phylogeny supported recognizing most described species as reciprocally monophyletic clades that are mostly allopatric and can be delineated by a few morphological characters. The nuclear DNA phylogeny supported the hypothesis that the allotetraploid Cr. sitchensis originated from a hybridization event between the Asian Cr. raddeana and the Beringian Cr. acrostichoides, and the plastid DNA phylogeny revealed that Cr. acrostichoides was the maternal parent. In contrast, the tetraploid Cr. crispa appears to have originated as an autopolyploid from an undiscovered or extinct ancestor. Further phylogenetic investigation of European Cryptogramma species using DNA sequence data from 15 accessions from Europe and southwest Asia revealed that Pleistocene glacial cycles have created genetic partitioning of Cr. crispa into eastern and western clades and have also led to the formation of the Turkish auto-octoploid Cr. bithynica with Cr. crispa as the parental taxon. Divergence time estimates for key nodes were inferred using Bayesian analysis of the plastid data set coupled with secondary time constraints to reveal that crown group Cryptogramma began diversifying in the Oligocene, with most present-day species originating in the Pliocene and Pleistocene. The genus was inferred by likelihood-based ancestral area reconstruction of the chronogram and geographic distribution data to have originated in east Asia, with four colonization events reconstructed by vicariance or dispersal to the New World. My Bayesian Analysis of Macroevolutionary Mixtures (BAMM) showed no significant difference in speciation rates across time or among clades. The morphological stasis of Cryptogramma and its stable speciation rates in response to climate cycles during the Pleistocene suggest it will survive future range shifts caused by anthropogenically induced climate change.