Demonstration of the preoral coelom in the brachiopod Lingula anatina with consideration of its phylogenetic significanceстатья
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Дата последнего поиска статьи во внешних источниках: 20 сентября 2015 г.
Аннотация:The origin of the Bilateria and reconstruction of the last common bilaterian ancestor (LCA) are important problems in fundamental zoology, and a related question is “Was the LCA a coelomic or acoelomic creature?” Insight into the nature of the LCA might be obtained by investigating the coelomic system of poorly studied bilaterians. The Brachiopoda is a relict group of marine invertebrates whose anatomy has been seldom studied with modern methods. For most brachiopods, the coelomic system has been described as bipartite, i.e., as consisting of two parts: the lophophore coelom and trunk coelom. In the present report, a tripartite coelomic system is described for the first time in a brachiopod (in the inarticulate brachiopod Lingula anatina). In addition to a lophophoral and trunk coelom, L. anatina has a preoral coelom. The protocoel is located at the base of the epistome and has its own lining, which consists of non-muscular monociliated epithelial cells connected by desmosomes and tight contacts. An analysis of coelomic system organization in the lophophorates revealed that the trimeric coelom is plesiomorphic for the Brachiozoa. Both phoronids and brachiopods combine trimery and metamery. The metamery is represented by lateral mesenteries in the metacoel. The combination of trimery and metamery can be found in all of the main stems of the Bilateria (the Trochozoa, Ecdysozoa, and Deuterostomia), suggesting that the LCA may have had a tripartite coelom, i.e., a metameric organization of the coelom with specialization of the first two segments. This suggestion is consistent with gene expression, Hox genes are never expressed in the two first segments of the body, whereas Otx genes are expressed in the most anterior segments of some bilaterians.