Abstract Detail

Horsetails Through Form, Space, and Time

Nixon, Kevin [1].

Is Equisetum really a fern? Evaluation of possible Long Branch Attraction in resolution of the "monilophyte" clade.

The extant genus Equisetum shows little similarity to modern ferns, and morphological and anatomical features strongly suggest that it was descended from a common ancestor with Paleozoic sphenopsids. Recent maximum likelihood (ML) and parsimony analyses of large molecular sequence data sets such as the “full” chloroplast genome place Equisetum within the crown group or on the stem group branch of what is called the “monilophyte” clade, proposed to include Equisetum, Psilotum, Marattiaceae and other modern ferns. However, for the same sequence data, trees placing Equisetum in various positions outside of the fern clade are only minutely less optimal. Likelihood results are similar, but suboptimal trees and alternative branching patterns are more difficult to evaluate because of computational issues associated with ingrained poor search performance in ML programs and limited exploration of tree space due to intentionally impaired branch-swapping algorithms; these issues also hamper “Bayesian” analysis. New simulations using real data sets (including the chloroplast full genome) instead of 4-taxon contrivances show that ML is susceptible to long branch attraction (LBA) at a rate equal to or greater than parsimony, depending on parameters such as overall branch length and nearness of “attracted” branches. Generally, ML and parsimony fail due to LBA in different areas, but both fail equally in situations where long branches are separated by relatively short internodes (i.e., the alternative position of Equisetum vs. the ferns). Resampling methods that reduce LBA coding artifacts show that the weakest areas of the chloroplast analysis in terms of both support and potential LBA occur in three regions: around the algal-land plant transition (i.e., Zygnema/Chara), the position of Equisetum relative to "monilophytes," and branching at the base of the angiosperm clade (e.g., the relative positions of Magnoliales sensu lato, eudicots, monocots and Ceratophyllum). In the case of Equisetum, the branch to the common ancestor with ferns likely spans more than 350 million years, and the analytical consequences of this long branch are exacerbated by limited modern diversity/sampling in bryophytes, Equisetum, Psilotum and some collateral fern groups. The addition of a much richer sample of higher algae, bryophytes, lycopods and ferns will undoubtedly improve statistical support among the early land plant divergences (if LBA artifacts can be eliminated), but differential extinction, limited modern diversity, and extremely long branches will always pose a problem in analyzing these data sets.

1 - Cornell University, L.H. Bailey Hortorium, Section of Plant Biology, Ithaca, NY, 14853, USA

Keywords:
Equisetum
Long Branch Attraction.

Presentation Type: Symposium Presentation
Session: SY03, Horsetails through form, space and time
Location: Oglethrope Auditorium/Savannah International Trade and Convention Center
Date: Monday, August 1st, 2016
Time: 4:30 PM
Number: SY03009
Abstract ID:676
Candidate for Awards:None