CRANFILL, RAYMOND. Department of Integrative Biology, University & Jepson Herbaria, University of California, Berkeley, CA 94720. - Has ecological specialization driven a fundamental phylogenetic split in the Polypodiales?
Notwithstanding significant progress in our understanding of the
relationships of basal pteridophytes, the phylogeny of derived
leptosporangiate ferns, the Polypodiales, remains enigmatic. Although
demonstrably monophyletic, the deeper branching patterns within the
group remain uncertain and highly controversial. Accordingly, I
conducted an analysis of 40 genera representing each major lineage of
Polypodiales based on a combination of three molecular data sets
(rbcL, rps4 and nad5). Although results are
preliminary in view of limited taxon sampling, several interesting
observations emerge. The basal position of the dennstaedtioid ferns
is unperturbed. From this group a monphyletic clade of pteridoid
ferns is derived, characterized by the loss of a true indusium and the
migration of sporangia back from the laminar margin along the veins.
Also derived from this group is an as yet unnamed monophyletic clade
of ferns in which the indusium is retained but which is also
characterized by a migration of the sorus from a marginal to medial
position. The indusiate clade is divided into an asplenioid group and
an unnamed sister group that is further divided into two unnamed
groups, each with good phylogenetic support. The first group is
characterized by the presence of mostly linear, indusiate sori; a
fundamentally terrestrial habit; and often strongly dimorphic leaves.
The second group is characterized by having round sori, which in
several subclades become exindusiate; a more or less epilithic to
epiphytic habit; and monomorphic leaves in basal clades which often
become strongly dimorphic in epiphytic lineages. The relationship of
groups within the second clade suggests a directed diversification
related to increasing specialization for the epiphytic habit, while
epiphytism is absent from the first clade entirely
Key words: evolution, nad5, Polypodiales, pteridphyte phylogenetics, rbcL, rps4