Some aspects of spiralian development

Abstract

Nielsen, C. 2010. Some aspects of spiralian development. -Acta Zoologica (Stockholm) 91: 20-28 Spiralian development is not only a characteristic early cleavage pattern, with shifting orientations of the cleavage planes, but also highly conserved cell lineages, where the origin of several organs can be traced back to identifiable cells in the lineage. These patterns are well documented in annelids, molluscs, nemertines, and platyhelminths and are considered ancestral of a bilaterian clade including these phyla. Spiral cleavage has not been documented in ecdysozoans, and no trace of the spiral development pattern is seen in phoronids and brachiopods. Origin of the spatial organization in spiralian embryos is puzzling, but much of the information appears to be encoded in the developing oocyte. Fertilization and ''pseudofertilization'' apparently provides the information defining the secondary, anterior-posterior body axis in many species. The central nervous system consists of three components: an apical organ, derived from the apical blastomeres 1a 111 -1d 111 , which degenerates before or at metamorphosis; the cerebral ganglia derived from other blastomeres of the first micromere quartet and retained in the adult as a preoral part of the brain; and the originally circumblastoporal nerve cord, which has become differentiated into a perioral part of the brain, the paired or secondarily fused ventral nerve cords, and a small perianal nerve ring.

Life cycle evolution: was the eumetazoan ancestor a holopelagic, planktotrophic gastraea?

“…Unfortunately, the homologous organ in spiralian larvae is in many species intimately connected with a pair of lateral ganglia to form a compound organ which has also been called an apical organ. To avoid confusion, I have in previously tried to introduce the name “apical ganglion” for the organ [22,126,127], but it is not a real ganglion, and it appears that almost all recent papers use the term apical organ in the strict sense, so I have decided to revert to the old practice, following the recommendations of [128]. But it must be remembered that almost all older papers use the term for the compound organ.…”

Early cleavage in Phoronis muelleri (Phoronida) displays spiral features

“…The complex of spiral cleavage (spiral‐micromere‐quartet‐4d‐cleavage) is generally seen as more than a characteristic early cleavage pattern with oblique spindle orientations. According to this view, spiral cleavage also involves the formation of four (micromere) quartets, a unique fate of various blastomeres—most famously the differentiation of the 4d‐cell as mesoderm precursor, and a certain orientation of the body axes with respect to the arrangement of the blastomeres in the eight‐cell stage (e.g., Dohle ; Ackermann et al ; Nielsen ). In contrast to this, Hejnol () discusses the evolution of spiral cleavage within Spiralia or Lophotrochozoa based on current views of bilaterian phylogeny.…”

Chaetognatha

“…Such a placement was also based on the fact that differentiation of the coelom seemingly resembles enterocoely, but Perez et al ( 2014 ) recently pointed out that other aspects of chaetognath gastrulation, especially the process of mesoderm and coelom formation, seem to be apomorphies of this taxon and thus not suggestive of a close affi nity to any other bilaterian phylum. Gastrulation patterns play essential roles in discussions on bilaterian evolution (e.g., Arendt and Nübler-Jung 1997 ;Nielsen 2001Nielsen , 2005aHejnol and Martindale 2009 ;Martindale and Heijnol 2009 ;Lacalli 2010 ;Nielsen 2010 ;Martín-Duran et al 2012 ). Hejnol and Martindale ( 2009 ) have emphasised that, on closer examination, a considerable variation in gastrulation patterns is evident between bilaterian subtaxa, especially in protostomes, which led these authors to stress the extreme variability in blastopore fates.…”