The chloroplast genome of Nymphaea alba: whole-genome analyses and the problem of identifying the most basal angiosperm

Angiosperms (flowering plants) dominate contemporary terrestrial flora with roughly 250,000 species, but their origin and early evolution are still poorly understood. In recent years, molecular evidence has accumulated suggesting a dicotyledonous origin of monocots. Phylogenetic reconstructions have suggested that several dicotyledonous groups that include taxa such as Amborella, Austrobaileya, and Nymphaea branch off as the most basal among angiosperms. This has led to the concept of monocots, "eudicots," "basal dicots," and "ANITA" groupings. Here, we present the sequence and phylogenetic analyses of the chloroplast DNA of Nymphaea alba. Phylogenetic analyses of our 14-species data set, consisting of 29,991 aligned nucleotide positions per chloroplast genome, revealed consistent support for Nymphaea being a divergent member of a monophyletic dicot assemblage. Three distinct angiosperm lineages were supported in the majority of our phylogenetic analyses-eudicots, Magnoliopsida, and monocots. However, the monocot lineage leading to the grasses was the deepest branching. Although analyses of only one individual gene alignment (out of 61) is consistent with some recently proposed hypotheses for the paraphyly of dicots, we also report observations that nine genes do not support paraphyly of dicots. Instead, they support the basal monocot-dicot split. Consistent with this finding, we also report observations suggesting that the monocot lineage leading to the grasses has the strongest phylogenetic affinity to gymnosperms. Our findings have general implications for studies of substitution model specification and analyses of concatenated genome data.