Hemichordate models

Abstract

Hemichordates are marine animals with two different lifestyles. The solitary, free-living enteropneusts or acorn worms resemble polychaetes or earthworms, while the tiny, colonial, sessile pterobranchs are similar to bryozoans and phoronids. Hemichordates, together with echinoderms, comprise the clade Ambulacraria and are a sister group to the Chordata. As adults, they exhibit cardinal chordate characters, such as gill slits. Their embryogenesis and dipleurula-type (tornaria) larvae are very similar to those of echinoderms. Recent advances in comparative genomics and molecular developmental biology of hemichordates, especially the vermiform enteropneusts, have shed light on deuterostome ancestors. This paper briefly reviews the numerous recent studies on the Phylum Hemichordata.

Multiple paedomorphic lineages of soft-substrate burrowing invertebrates: parallels in the origin of Xenocratena and Xenoturbella

“…Recent molecular analyses [107,108] clearly dismiss previous data [109] that the sedentary phylum Pterobranchia is a part of the clade of the free-living Enteropneusta. If Pterobranchia was really modified secondarily from the enteropneust-like groundplan, then the pterobranchs must be nested inside as an internal subclade of the enteropneusts, but pterobranchs instead are confirmed as merely a sister group to Enteropneusta [108]. There is also a large agreement currently that segmentation was not a property of the bilaterian ancestor, but emerged independently in several bilaterian lineages [110][111][112], that undermines a scenario of a free-living segmented last common bilaterian ancestor [113].…”

“…A majority of the confirmed early Cambrian echinoderm representatives are sedentary or semisedentary with Ushaped gut, like modern pterobranchs, but not enteropneusts [104][105][106]. Recent molecular analyses [107,108] clearly dismiss previous data [109] that the sedentary phylum Pterobranchia is a part of the clade of the free-living Enteropneusta. If Pterobranchia was really modified secondarily from the enteropneust-like groundplan, then the pterobranchs must be nested inside as an internal subclade of the enteropneusts, but pterobranchs instead are confirmed as merely a sister group to Enteropneusta [108].…”

The ups and downs of amphioxus biology: a history

“…It has long been thought that the gill slits in hemichordates, amphioxus, tunicates and vertebrates are homologous, and thus that a pharynx perforated by gill slits existed before hemichordates branched from the chordates (Bateson, 1886); reviewed in (Tagawa, 2016). Molecular data have reinforced these homologies (Fig.…”

Regeneration in the enteropneust hemichordate, Ptychodera flava, and its evolutionary implications

“…In addition, a stomochord in the proboscis, a notochord, and a dorsal nerve cord forming a hollow tube in the mesosome, imply a close relationship between hemichordates and chordates. Although structural homologies of these organs are still controversial (Satoh et al., ; Tagawa, ), formation of gill slits and pharyngeal arches in deuterostomes is regulated by an orthologous gene cluster (Gillis, Fritzenwanker, & Lowe, ; Ogasawara, Wada, Peters, & Satoh, ; Simakov et al., ).…”