A Phylogenomic Solution to the Origin of Insects by Resolving Crustacean-Hexapod Relationships.

Insects, the most diverse group of organisms, are nested within crustaceans, arguably the most abundant group of marine animals. However, to date, no consensus has been reached as to which crustacean taxon is the closest relative of hexapods. A majority of studies have proposed that Branchiopoda (e.g., fairy shrimps) is the sister group of Hexapoda [1-7]. However, these investigations largely excluded two equally important taxa, Remipedia and Cephalocarida. Other studies suggested Remipedia [8-11] or Remipedia + Cephalocarida [12, 13] as potential sister groups of hexapods, but they either did not include Cephalocarida or used only Sanger sequence data and morphology [9, 12]. Here we present the first phylogenomic study specifically addressing the origins of hexapods, including transcriptomes for two species each of Cephalocarida and Remipedia. Phylogenetic analyses of selected matrices, ranging from 81 to 1,675 orthogroups and up to 510,982 amino acid positions, clearly reject a sister-group relationship between Hexapoda and Branchiopoda [1-7]. Nonetheless, support for a hexapod sister-group relationship to Remipedia or to Cephalocarida-Remipedia was highly dependent on the employed analytical methodology. Further analyses assessing the effects of gene evolutionary rate and targeted taxon exclusion support Remipedia as the sole sister taxon of Hexapoda and suggest that the prior grouping of Remipedia + Cephalocarida is an artifact, possibly due to long branch attraction and compositional heterogeneity. We further conclude that terrestrialization of Hexapoda probably occurred in the late Cambrian to early Ordovician, an estimate that is independent of their proposed sister group [4, 8, 12, 14].