Amborella not a “basal angiosperm”? Not so fast

Abstract

The sequence of the plastid genome of Amborella trichopoda, the putative sister to all other extant angiosperms, was recently reported (Molecular Biology and Evolution 20: 1499-1505). Goremykin et al. used sequence data for 61 plastid genes from Amborella and 12 other embryophytes in phylogenetic analyses and concluded that Amborella is not the sister to the remaining flowering plants; the monocots instead occupy this position. The authors attributed their results, which differ substantially from all recent phylogenetic analyses of angiosperms, to the increased character sampling (30 017 nucleotides in their aligned matrix) in their analysis relative to published studies that included fewer genes but more taxa. We hypothesized that the difference in topology is not due to limited character sampling in previous studies but to limited taxon sampling in the analysis by Goremykin et al. To test this, we conducted a series of phylogenetic analyses using a three-gene, 12 (or more)-taxon data set to evaluate the topological effects of (i) including three vs. 61 genes for (nearly) the same set of taxa, (ii) analyzing different codon positions, (iii) substituting representatives of other basal lineages for Amborella, (iv) replacing the grasses used to represent the monocots with other monocots, selected either for their phylogenetic position or randomly, and (v) adding other basal taxa-Nymphaea, Austrobaileya, magnoliids, and monocots-to the 12-taxon data set. Our results demonstrate that the "monocots basal" topology obtained by Goremykin et al. is not due to increased character sampling of the plastid genome; their topology was obtained using only two plastid genes or two plastid genes and one nuclear gene. This topology was also retained when either Nymphaea or Austrobaileya was substituted for Amborella, demonstrating that any of the three basal lineages will attach to Calycanthus for lack of any other close branch. Furthermore, the "monocots basal" topology is not robust to changes in sampling of monocots. Simply adding Oncidium, for example, places Amborella sister to the other angiosperms. Thus, limited taxon sampling, focusing on organisms with complete genome sequences, can lead to artifactual results.