Phylogenetics of Cystopteridaceae: Reticulation and Divergence in a Cosmopolitan Fern Family

Rothfels, Carl

The fern family Cystopteridaceae has been a thorn in the side of fern phylogeneticists, on many levels. Until this thesis, its basic existence (as a deeply isolated clade) and composition were unrecognized, hypotheses as to the relationships of its constituents within the broader fern tree-of-life were wildly inconsistent, the relationships of its genera to each other were contested, the species limits within those genera weakly understood, and the relationships among those species unknown. This thesis first establishes the broad evolutionary context for the family, which is that it is the first-diverging branch in Eupolypods II (it is sister to the rest of the eupolypod II clade). Eupolypods II is a large clade, containing nearly a third of extant fern species; this is a position pivotal to a full understanding of fern evolution. The evolution of the Eupolypods II is marked by an “ancient, rapid radiation” at the base of the clade, which helps to explain the difficulty that this broad group has historically posed to evolutionary biologists. Molecular data from five plastid loci show that Eupolypods II consists of 10 deeply divergent lineages, each worthy of recognition at the rank of family: Cystopteridaceae, Rhachidosoraceae, Diplaziopsidaceae, Hemidictyaceae, Aspleniaceae, Thelypteridaceae, Woodsiaceae, Onocleaceae, Blechnaceae, and Athyriaceae. The ancestors of Cystopteridaceae diverged from those of the rest of the clade approximately 100 million years ago, and the family is now composed of five extant genera: Acystopteris, Cystoathyrium (the only genus for which we lack molecular data—it may be extinct), Cystopteris, Gymnocarpium, and ×Cystocarpium. Within the family, the relationships of Cystoathyrium are unknown. Acystopteris is sister to Cystopteris, and those two genera, together, are sister to Gymnocarpium. Gymnocarpium is the maternal parent of ×Cystocarpium, so that genus falls within Gymnocarpium in phylogenetic trees based on maternally transmitted loci (i.e., plastid or mitochondrial loci). Plastid data resolve a basal trichotomy in Gymnocarpium, among the G. disjunctum clade, the G. robertianum clade, and core Gymnocarpium. The earliest diverging branch of core Gymnocarpium is the morphologically anomalous G. oyamense, followed by a split that separates G. appalachianum and G. jessoense parvulum (on one side) from G. remotepinnatum and G. jessoense jessoense, on the other. In Acystopteris, the first division surprisingly separates A. taiwaniana (which is frequently treated as a variety of A. japonica) from A. japonica + A. tenuisecta (which are morphologically very distinct from each other). The evolution of Cystopteris is, as expected, more complex. The first lineage to diverge from the rest of the genus is the one that gave rise to C. montana. The next division, however, is unclear; molecular data infer a trichotomy among the sudetica clade (containing C. sudetica, C. moupinensis, and C. pellucida), the bulbifera clade (containing C. bulbifera and its related allopolyploids C. tennesseensis and C. utahensis), and the C. fragilis complex. Within the C. fragilis complex relationships (and species limits) get particularly messy. The diploid species of eastern North America—C. protrusa—is sister to the rest of the complex, but after that point the major named species (including C. fragilis and C. tenuis) cease to be monophyletic, being found on both sides of a major split, alongside such taxa as the Australian/New Zealand C. tasmanica, the Hawaiian C. douglasii, and the Mexican C. membranifolia and C. millefolia. In the context of the deep divergence of Gymnocarpium from Cystopteris, and the complicated species-level patterns of relationship within each genus, it is particularly surprising that molecular data confirm that ×Cystocarpium is a hybrid between Gymnocarpium dryopteris and a European... This thesis ends with a tentative synopsis of the Cystopteridaceae (Appendix E)....

Phylogenetics of Cystopteridaceae: Reticulation and Divergence in a Cosmopolitan Fern Family

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