Which came first? A pivotal position in the plant tree of life

Amborella trichopoda

The question of which extant angiosperm (flowering plant) lineage “came first” (i.e., is basal in the flowering plant tree of life) has long puzzled biologists. This question is fascinating and important in its own right, but the answer also has potentially profound ramifications including plant gene and genome evolution (which, for example, has implications for crop improvement). Such information is also important for understanding habit and habitat evolution and for the inference of ancestral character states in the angiosperms (e.g., the ancestral flower as well as the ancestral angiosperm genome). Although great 20^th century plant taxonomists such as Arthur Cronquist, Armen Takhtajan, and Robert Thorne generally agreed that taxa from the subclass Magnoliidae comprised the “basal” angiosperm lineage, there was no way to “prove”, one way or another, which extant angiosperm lineage came first until the advent of molecular systematics towards the end of the 20^th century.

With the aid of modern molecular phylogenetic techniques it is now known that the major groups they recognized, such as Magnoliidae sensu Cronquist and Takhtajan, are typically polyphyletic. Most research now indicates instead that Amborellaceae, Nymphaeales (water lilies), and Austrobaileyales are the earliest branching extant angiosperm lineages. However, the relative branching order of these three lineages, particularly in regards to Amborella trichopoda (the sole species within Amborellaceae) and Nymphaeales, was, until recently, somewhat contentious.

While most molecular analyses during the past 20 years have recovered Amborella as the earliest-diverging angiosperm lineage, some studies have suggested a clade comprising Amborella + Nymphaeales, or even Nymphaeales alone, as the root of all angiosperms. Recently, at the University of Florida, Soltis lab postdoc Bryan Drew and colleagues (including AVATOL team member Stephen Smith at the University of Michigan) endeavored to definitively answer the longstanding question of which angiosperm came first—that is, what living angiosperm is sister to all other living angiosperms in the angiosperm tree of life. Using a plastid data set consisting of 236 taxa, 78 genes, and ~58,000 nucleotides, Drew et al. performed a myriad of analyses with the express purpose of discerning the first-diverging angiosperm lineage; this study by Drew et al. was just accepted by Systematic Biology and will be viewable online in the coming months. Their results: Virtually every analysis conducted found Amborella as the earliest-diverging living angiosperm lineage with high internal support, and every plastid analysis performed using their original datasets recovered a topology in which Amborella alone is sister to all other living angiosperms.

These findings lend strong affirmation to the Amborella sister hypothesis, and should help guide future research regarding angiosperm character (including genomic features) and habitat evolution. Although the “first” angiosperms are long extinct, a better understanding of Amborella will aid in our understanding of angiosperm evolution as a whole. This was the impetus behind the Amborella Genome Project. As a result of this ongoing project, the Amborella nuclear genome has recently been fully sequenced (www.amborella.org; Amborella Genome Project, Science, in press), and this major achievement should lead to unprecedented insights within flowering plants.

 

Doug Soltis is a distinguished professor at the University of Florida.

Bryan Drew is a post-doctoral researcher in the Soltis lab at the University of Florida.