How to make a protostome

Abstract

Abstract. The origin and radiation of the major metazoan groups can be elucidated by phylogenomic studies, but morphological evolution must be inferred from embryology and morphology of living organisms. According to the trochaea theory, protostomes are derived from a holoplanktonic gastraea with a circumblastoporal ring of downstream-collecting compound cilia (archaeotroch) and a nervous system comprising an apical ganglion and a circumblastoporal nerve ring. The pelago-benthic life cycle evolved through the addition of a benthic adult stage, with lateral blastopore closure creating a tubeshaped gut. The archaeotroch became differentiated as prototroch, metatroch and telotroch in the (trochophora) larva, but was lost in the adult. The apical ganglion was lost in the adult, as in all neuralians. Paired cerebral ganglia developed from the first micromere quartet. The circumblastoporal nerve became differentiated into a pair of ventral nerve cords with loops around mouth (the anterior part of the blastopore) and anus. Almost all new information about morphology and embryology fits the trochaea theory. The predicted presence of a perioral loop of the blastoporal nerve ring has now been demonstrated in two annelids. Alternative 'intercalation theories' propose that planktotrophic larvae evolved many times from direct-developing ancestors, but this finds no support from considerations of adaptation.

Larval nervous systems: true larval and precocious adult

“…The eyes found in many annelid and mollusk larvae develop from cells of the cerebral ganglia (Conklin, 1897;Ackermann et al, 2005). (Nielsen, 2012)], and the lower illustration shows the CNS of a late lecithotrophic larva of Capitella as demonstrated through a cell-lineage study (see Meyer et al, 2010). The shape of the circumblastoporal nerve ring in Capitella is exactly as predicted, except that the anal loop is missing; it may have been overlooked, and it is present in many other annelids.…”

“…Both development and morphology of the adult central nervous system is quite well known in many spiralians (reviews in Nielsen, 2004;Nielsen, 2005), and its evolution has been explained by the trochaea theory (Nielsen, 2012). Typically, it consists of a pair of cerebral ganglia and a blastoporal nerve cord, which differentiates into the paired or fused ventral nerve cord with a perioral and a perianal loop (Fig.…”

“…The following discussions are based on the framework of the trochaea theory (Nielsen, 2012), which appears to be the only theory that explains both the origin of the ciliary bands of the trochophora larva and the morphology of the protostomian CNS in a continuous series of adaptational modifications of existing structures.…”

“…New nervous centers developed in the benthic adults and gradually became established in the larval stages, through the evolutionary process called adultation (Jägersten, 1972), so that two types of nervous centers can be recognized in the larvae, viz. the exclusively larval apical organ and the larval-adult nervous system.The following discussions are based on the framework of the trochaea theory (Nielsen, 2012), which appears to be the only theory that explains both the origin of the ciliary bands of the trochophora larva and the morphology of the protostomian CNS in a continuous series of adaptational modifications of existing structures. …”

Genetic Mechanisms Regulating the Spatiotemporal Modulation of Proliferation Rate and Mode in Neural Progenitors and Daughter Cells during Embryonic CNS Development

“…Irrespective of the driving mechanisms of anterior expansion, many of which have been resolved in this thesis, the evolutionarily-conserved features of anterior CNS development and expansion are even more intriguing in light of the theories suggesting that the two parts of the CNS; brain and the nerve cord, might have existed separately in the common ancestor of bilateria and annelids as ANS (apical nervous system) and BNS (basal nervous system) and have merged together throughout evolution to form the modern CNS [159,161,164,165].…”