title: The Presence of a Functionally Tripartite Through-Gut in Ctenophora Has Implications for Metazoan Character Trait Evolution.
abstract: The current paradigm of gut evolution assumes that non-bilaterian metazoan lineages either lack a gut (Porifera and Placozoa) or have a sac-like gut (Ctenophora and Cnidaria) and that a through-gut originated within Bilateria [1-8]. An important group for understanding early metazoan evolution is Ctenophora (comb jellies), which diverged very early from the animal stem lineage [9-13]. The perception that ctenophores possess a sac-like blind gut with only one major opening remains a commonly held misconception [4, 5, 7, 14, 15]. Despite descriptions of the ctenophore digestive system dating to Agassiz [16] that identify two openings of the digestive system opposite of the mouth-called "excretory pores" by Chun [17], referred to as an "anus" by Main [18], and coined "anal pores" by Hyman [19]-contradictory reports, particularly prominent in recent literature, posit that waste products are primarily expelled via the mouth [4, 5, 7, 14, 19-23]. Here we demonstrate that ctenophores possess a unidirectional, functionally tripartite through-gut and provide an updated interpretation for the evolution of the metazoan through-gut. Our results resolve lingering questions regarding the functional anatomy of the ctenophore gut and long-standing misconceptions about waste removal in ctenophores. Moreover, our results present an intriguing evolutionary quandary that stands in stark contrast to the current paradigm of gut evolution: either (1) the through-gut has its origins very early in the metazoan stem lineage or (2) the ctenophore lineage has converged on an arrangement of organs functionally similar to the bilaterian through-gut.
authors: Department of Biology, University of Miami, Cox Science Center, 1301 Memorial Drive, Miami, FL 33146, USA.
authors: Jason S
authors: JS
authors: Presnell
authors: Department of Biology, University of Washington, 24 Kincaid Hall, Seattle, WA 98195, USA; Benaroya Research Institute at Virginia Mason, 1201 Ninth Avenue, Seattle, WA 98107, USA.
authors: Lauren E
authors: LE
authors: Vandepas
authors: Department of Biology, University of Miami, Cox Science Center, 1301 Memorial Drive, Miami, FL 33146, USA.
authors: Kaitlyn J
authors: KJ
authors: Warren
authors: Department of Biology, University of Washington, 24 Kincaid Hall, Seattle, WA 98195, USA; Friday Harbor Laboratories, University of Washington, Friday Harbor, WA 98250, USA.
authors: Billie J
authors: BJ
authors: Swalla
authors: Department of Biology, University of Washington, 24 Kincaid Hall, Seattle, WA 98195, USA; Benaroya Research Institute at Virginia Mason, 1201 Ninth Avenue, Seattle, WA 98107, USA.
authors: Chris T
authors: CT
authors: Amemiya
authors: Department of Invertebrate Zoology, Smithsonian National Museum of Natural History, MRC-163, PO Box 37012, Washington, DC 20013-7012, USA. Electronic address: webrowne3@gmail.com.
authors: William E
authors: WE
authors: Browne
mesh_terms: D000818:Animals; D005075:Biological Evolution; D037302:Ctenophora / Q000033:anatomy & histology* / Q000502:physiology*; D041981:Gastrointestinal Tract / Q000033:anatomy & histology / Q000502:physiology
keywords: Ctenophora; alimentary canal; anus; ctenophore; evolution; metazoan; non-bilaterian; through-gut