Abstract
Two hypotheses for the early radiation of the metazoans are vividly discussed in recent phylogenomic studies, the ‘Porifera-first’ hypothesis, which places the poriferans as the sister group of all other metazoans, and the ‘Ctenophora-first’ hypothesis, which places the ctenophores as the sister group to all other metazoans. It has been suggested that an analysis of morphological characters (including specific molecules) could throw additional light on the controversy, and this is the aim of this paper. Both hypotheses imply independent evolution of nervous systems in Planulozoa and Ctenophora. The Porifera-first hypothesis implies no homoplasies or losses of major characters. The Ctenophora-first hypothesis shows no important synapomorphies of Porifera, Planulozoa and Placozoa. It implies either independent evolution, in Planulozoa and Ctenophora, of a new digestive system with a gut with extracellular digestion, which enables feeding on larger organisms, or the subsequent loss of this new gut in the Poriferans (and the re-evolution of the collar complex). The major losses implied in the Ctenophora-first theory show absolutely no adaptational advantages. Thus, morphology gives very strong support for the Porifera-first hypothesis.
Highlights
Until recently, the poriferans have almost unanimously been regarded as the sister group of all other animals
Some recent phylogenomic studies conclude that the position of deep-diverging lineages, such as the ctenophores, may exceed the limit of resolution afforded by the traditional phylogenomic analyses [4,10,11]
Ctenophora adults sessile with aquiferous system with choanocyte chambers, larvae lecithotrophic with effective-stroke cilia apical organ, nervous system, neurotransmitters A, myoepithelial muscles flattened body, creeping on endoderm, fibre cells perlecan, nidogen, HIF respiratory pathway aboral organ, nervous system, neurotransmitters B, gap junctions, mesoderm with myocytes, ctenes gastraea advanced choanoblastaea ectoderm–endoderm, extracellular digestion, loss of collar complex, eumetazoan genes basement membrane, collagen IV, oogamy, diploidy, metazoan genes ancestral choanoblastaea choanoflagellates multicellular, epithelia unicellular, isogamy, haploidy, collar complex, intracellular digestion in the lower epithelium secrete digestive enzymes into the sealed space between the epithelium and the substratum, and digested material becomes absorbed [26]
Summary
The poriferans have almost unanimously been regarded as the sister group of all other animals. The position of the poriferans as the sister group of the eumetazoans became challenged when large phylogenomic studies including two ctenophore genomes showed the Ctenophora as the sister group to all the other metazoans [2,3]. Several later phylogenomic studies have found support for this ‘Ctenophora-first’ theory [4,5]. Some recent phylogenomic studies conclude that the position of deep-diverging lineages, such as the ctenophores, may exceed the limit of resolution afforded by the traditional phylogenomic analyses [4,10,11]. The review is almost exclusively based on papers which contain information about the Choanoflagellata and the four major metazoan groups, Porifera, Cnidaria (as a representative of the Planulozoa, i.e. Cnidaria + Bilateria), Placozoa and Ctenophora
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