On the importance of sequence alignment inspections in plastid phylogenomics – an example from revisiting the relationships of the water-lilies

Abstract

The water-lily clade represents the second earliest-diverging branch of angiosperms. Most of its species belong to Nymphaeaceae, of which the “core Nymphaeaceae”—comprising the genera Euryale, Nymphaea and Victoria—is the most diverse clade. Despite previous molecular phylogenetic studies on the core Nymphaeaceae, various aspects of their evolutionary relationships have remained unresolved. The length-variable introns and intergenic spacers are known to contain most of the sequence variability within the water-lily plastomes. Despite the challenges with multiple sequence alignment, any new molecular phylogenetic investigation on the core Nymphaeaceae should focus on these noncoding plastome regions. For example, a new plastid phylogenomic study on the core Nymphaeaceae should generate DNA sequence alignments of all plastid introns and intergenic spacers based on the principle of conserved sequence motifs. In this investigation, we revisit the phylogenetic history of the core Nymphaeaceae by employing such an approach. Specifically, we use a plastid phylogenomic analysis strategy in which all coding and noncoding partitions are separated and then undergo software-driven DNA sequence alignment, followed by a motif-based alignment inspection and adjustment. This approach allows us to increase the reliability of the character base compared to the default practice of aligning complete plastomes through software algorithms alone. Our approach produces significantly different phylogenetic tree reconstructions for several of the plastome regions under study. The results of these reconstructions underscore that Nymphaea is paraphyletic in its current circumscription, that each of the five subgenera of Nymphaea is monophyletic, and that the subgenus Nymphaea is sister to all other subgenera of Nymphaea. Our results also clarify many evolutionary relationships within the Nymphaea subgenera Brachyceras, Hydrocallis and Nymphaea. In closing, we discuss whether the phylogenetic reconstructions obtained through our motif-based alignment adjustments are in line with morphological evidence on water-lily evolution.