Flagellar apparatus structure of choanocyte in Sycon sp. and its significance for phylogeny of Porifera
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- Pozdnyakov, I.R. & Karpov, S.A. Zoomorphology (2013) 132: 351. doi:10.1007/s00435-013-0193-4
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The problem of the origin of Metazoa has been one of the most discussed for nearly the last one and a half centuries. Some 20 years ago, morphological approaches were replaced with molecules, but the problem then became more complex. At the same time, morphological data were incomplete, and therefore, this approach can still help in comparison with choanoflagellates and sponges—two sister groups having uniflagellated cells with a collar. The structure of the flagellar apparatus has phylogenetic significance, but sponge choanocytes are poorly studied in this respect, and we still do not know what the ancestral kinetid of Porifera looks like. The kinetid structure of choanocytes in Sycon sp. is investigated here for the first time, and a 3D reconstruction of the kinetid provided. It is composed of a flagellar kinetosome with a nuclear fibrillar root and a basal foot with a few microtubules; the accessory centriole lies orthogonal to and just below the foot of the kinetosome, and a dictyosome is near the centriole. This kinetid is similar to that of the choanocyte of Corticium candelabrum (Homoscleromorpha) and is considered to be the ancestral type for the whole branch Calcarea + Homoscleromorpha.