Illuminating the Base of the Annelid Tree Using Transcriptomics

Annelida is one of three animal groups possessing segmentation and is central in considerations about the evolution of different character traits. It has even been proposed that the bilaterian ancestor resembled an annelid. However, a robust phylogeny of Annelida, especially with respect to the basal relationships, has been lacking. Our study based on transcriptomic data comprising 68,750–170,497 amino acid sites from 305 to 622 proteins resolves annelid relationships, including Chaetopteridae, Amphinomidae, Sipuncula, Oweniidae, and Magelonidae in the basal part of the tree. Myzostomida, which have been indicated to belong to the basal radiation as well, are now found deeply nested within Annelida as sister group to Errantia in most analyses. On the basis of our reconstruction of a robust annelid phylogeny, we show that the basal branching taxa include a huge variety of life styles such as tube dwelling and deposit feeding, endobenthic and burrowing, tubicolous and filter feeding, and errant and carnivorous forms. Ancestral character state reconstruction suggests that the ancestral annelid possessed a pair of either sensory or grooved palps, bicellular eyes, biramous parapodia bearing simple chaeta, and lacked nuchal organs. Because the oldest fossil of Annelida is reported for Sipuncula (520 Ma), we infer that the early diversification of annelids took place at least in the Lower Cambrian.