Comparative transcriptomics analysis of the anteroposterior organisation of adult bilaterian guts

A through-gut is one of the major features of bilaterians. Comparative work among bilaterians has identified common molecular mechanisms during early gut patterning, but the primordial gut later often undergoes different degrees of reorganization in each lineage to form a fully differentiated adult gut with specializations along its anteroposterior (AP) axis. Yet, how the conserved embryonic gut AP pattern relates to the adult guts in diverse bilaterians after metamorphosis is still poorly understood. To unravel the molecular subdivisions of adult guts, we investigated the gut through transcriptomic analyses of five phylogenetically informative species, an annelid, a sea urchin, a hemichordate, a cephalochordate, and a vertebrate. Our study inferred conserved and modified adult AP patterning modules along bilaterian guts enabling the reconstruction of ancestral bilaterian features with profound implications for the evolution of the bilaterian body plan.We considered five marine organisms across main branches of the bilaterian evolutionary tree. As a representative of the protostomes, the annelid worm Platynereis dumerilii ("Pd", Dumeril's clam worm; Audouin et al., 1833). Among deuterostomes, within the Ambulacraria, the echinoderm Strongylocentrotus purpuratus ("Sp", purple sea urchin; Stimpson, 1857) and the hemichordate Ptychodera flava ("Pf", acorn worm; Eschscholtz, 1825). For the chordate branch, the cephalochordate Branchiostoma floridae ("Bf", Florida lancelet; Hubbs, 1922), and the vertebrate Amphiprion ocellaris ("Ao", common clownfish; Cuvier & Valenciennes, 1830). We then dissected out the entire adult gastrointestinal tract from each of the five species under consideration (with associated digestive organs if applicable), and further subdivided each gastrointestinal tube into subsegments taken to span the entire length of each tract, from anterior to posterior.