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The development of Neochildia fusca supports the position of the acoels as basal bilaterians
Boyer, B.C.; Henry, J.Q.; Martindale, M.Q. (2001). The development of Neochildia fusca supports the position of the acoels as basal bilaterians. Belg. J. Zool. 131(Suppl.): 59-60
In: Belgian Journal of Zoology. Koninklijke Belgische Vereniging voor Dierkunde = Société royale zoologique de Belgique: Gent. ISSN 0777-6276, more
Peer reviewed article  

Also published as
  • Boyer, B.C.; Henry, J.Q.; Martindale, M.Q. (2001). The development of Neochildia fusca supports the position of the acoels as basal bilaterians, in: Saló, E. et al. (Ed.) Proceedings of the 9th International Symposium on the Biology of the Turbellaria, Barcelona, Spain, June 2000 [CD-ROM]. Belgian Journal of Zoology, 131(Suppl. 1): pp. 59-60, more

Available in  Authors 
Document type: Conference paper

Keyword
    Marine

Authors  Top 
  • Boyer, B.C.
  • Henry, J.Q.
  • Martindale, M.Q.

Abstract
    The phylogenetic position of the acoel flatworms has long been controversial. They have traditionally been placed within the Platyhelminthes, either at the base of the metazoan tree or in the Lophotrochozoa. However, recent evidence suggests that they belong in a separate phylum of basal, direct-developing triploblastic metazoans (Science 283: 1919- 1923). Acoel embryos exhibit a unique form of development that previously has been related to that found in polyclad turbellarians and coelomate spiralians, which display typical quartet spiral cleavage. Because developmental characteristics can provide evidence of relationships among metazoan groups, we used modern lineage tracers to generate the cell lineage of the acoel Neochildia fusca. Cleavage occurs in a duet pattern in which the second cleavage plane is leiotrop- ically oblique relative to the animal vegetal axis. At the four-ce1l stage, the plane off first cleavage corresponds to the plane of bilateral symmetry, and subsequent cleavages are symmetrical across the sagittal plane. The first three micromere duets generate only ectodermal derivatives ; there is no ectomesoderm. Both third duet macromeres produce the endomesoderm, including the complex musculature, as well as the peripheral and central parenchymas. The cleavage pattern, cell lineage, and mesodermal origins of N. fusca share little similarity with those of other metazoans, including the quartetcleaving Platyhelminthes such as the polyclads. If acoels flatworms belong to the lophotrochozoan clade, their development appears to represent a degenerate condition related to the abandonment of larval development. Alternatively, however, we suggest that the acoel developmental program may be related to that of ancestral bilaterians, which were represented by small direct-developing, acoelomate animals exhibiting a form of bilateral (or biradial) cleavage, with mesodermal tissues arising solely from endodermal lineages.

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