Systematics Section / ASPT
Rothwell, Gar W. , Nixon, Kevin .
What do we really know about the overall pattern of plant phylogeny?
Recently there has been a dramatic shift from morphological characters to gene sequence data for resolving deep internal nodes of the embryophyte tree. The popular expectation has been that this change will increase the resolution of tree topology in a manner that parallels improvements achieved at more terminal nodes. However, a review of recent studies reveals that this expectation is not being met. Up to the present, analyses of gene sequences have produced a large number of trees with strongly supported internal nodes, but hypotheses generated from different data sets are often wildly discordant or have high levels of internal conflict. A conservative consensus of recent results resolves only five well supported deep internal nodes for the embryophyte tree of living species. These are an Embryophyte Node (where embryophytes form a polytomy with two or more clades of charophyceans), a Polysporangiophyte Node (where polysporangiophytes form a polytomy with the three bryophytic clades), a Lycophyte Node (where the lycophyte/zosterophyll clade diverges from the stem of the tree), a Euphyllophyte Node (sensu Kenrik and Crane, where five clades of free-sporing plants form a polytomy with seed plants) and a Spermatophyte Node (where four to six clades of seed plants diverge from one another). Analyses that include extinct species confirm that at least four of the five embryophyte nodes are robust. They also reveal that support values from any single data set, particularly when different data sets produce different results, cannot be realistically used to select among competing topologies. The use of gene sequence data, morphological characters of living and fossil species, and other data (e.g., rare genetic markers, developmental pathways, and mechanisms of developmental regulation) as reciprocal hypothesis tests, and the simultaneous analyses of all available data sets encompassing large samples of diversity need to be undertaken.
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1 - Ohio University, Department of Environmental & Plant Biology, Porter Hall, Richland Avenue, Athens, Ohio, 45701-2979, USA
2 - Cornell University, LH Bailey Hortorium, 462 Mann Library, Ithaca, New York, 14853-4301, USA
Presentation Type: Oral Paper
Date: Tuesday, August 16th, 2005
Time: 3:00 PM