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...terial species diverged to generate the Protostomes (evolving into such animals as flatworms, molluscs and arthropods) and the Deuterostomes (from which the chordates and then the vertebrates evolved)=-=[20,21]-=-. Molluscs emerged more than100Mybeforevertebrateswithabonyskeleton (the current known range of herpesviruses in vertebrates). One hypothesis to explain the diversity of viruses within vertebrates and...

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...ly sponges, cnidarians and ctenophores. The sponges are widely accepted as one of the oldest metazoan lineages, with the unicellular choanoflagellates forming a sister group to the metazoa as a whole =-=[1,2,3,4]-=-. Since sponge choanocytes and choanoflagellates show many similarities, the first step of animal evolution has been proposed to have been the acquisition of multicellularity from a choanoflagellate-l...

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...his era; they are the only extant animals that never develop nervous systems at any stage of their life cycles, and they are also widely regarded as the most ancient animal lineages that still exist (=-=Nielsen, 2008-=-; Dellaporta et al., 2006). All other animal lineages that exist today develop nervous systems. We are interested in studying when animals developed nervous systems, and the circumstances under which ...

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...ulation. In the mouse, beginning at embryonic day (E) 6.5 epiblast cellssmigrate through the primitive streak and, under the influence of cell-cell signaling,sbecome specified as mesoderm or endoderm =-=[1]-=-. The endodermal cells intercalate into thesextraembryonic visceral endoderm on the ventral-most surface of the embryo, formingsthe definitive endoderm. By E9.5 coordinated morphogenic movements conve...

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...es share the same collar cell morphology as the feeding cells of sponges, so called choanocytes, indicating that choanocyte-like cells are likely to have been one of the earliest metazoan cell types (=-=Nielsen, 2008-=-; Nichols et al., 2009). Although all choanoflagellates have a single-celled phase in their life history, many species also form colonies composed of multiple cells (Fig. 1D) (Alegado et al., 2012; Da...

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...homsen, 2000). They are characterized by a remarkably uniform cell body morphology. This comprises a spherical to ovoid cell with a single apical flagellum surrounded by a collar of actin-filled microvilli (Fig. 1B,C). The undulation of the apical flagellum creates fluid currents that draw bacteria into the feeding collar for phagocytosis (Pettitt et al., 2002; Roper et al., 2013). Choanoflagellates share the same collar cell morphology as the feeding cells of sponges, so called choanocytes, indicating that choanocyte-like cells are likely to have been one of the earliest metazoan cell types (Nielsen, 2008; Nichols et al., 2009). Although all choanoflagellates have a single-celled phase in their life history, many species also form colonies composed of multiple cells (Fig. 1D) (Alegado et al., 2012; Dayel et al., 2011; Fairclough et al., 2010). The genomes of the choanoflagellate Monosiga brevicollis and Salpingoeca rosetta were recently sequenced (Fairclough et al., 2013; King et al., 2008). Comparisons of the two choanoflagellate genomes with genomes from metazoans demonstrated their relevance to reconstructing the origin of protein families central to metazoan biology. For example, M. brevic...

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...c ancestor, and that their differentiation was established in connection with the evolution of a new body axis, the anterior-posterior axis, forming at an angle to the primary, apicalblastoporal axis =-=[46,52,69]-=-. This is unlikely because most ciliated bands are not derived from the vicinity of the blastopore, which forms at the vegetal pole [70]. Furthermore, the multiple embryonic origins of ciliary bands i...