Comparative lophotrochozoan neurogenesis and larval neuroanatomy: recent advances from previously neglected taxa.
Recently, a number of neurodevelopmental studies of hitherto neglected taxa have become available, contributing to questions relating to the evolution of the nervous system of Lophotrochozoa (Spiralia + Lophophorata). As an example, neurogenesis of echiurans showed that these worm-shaped spiralians, which as adults do not exhibit any signs of segmentation, do show such traits during ontogeny, e.g. by segmentally arranged perikarya and commissures. Similarly, sipunculan worms, which have a single ventral nerve cord in the adult stage, develop this nerve cord by gradual fusion of a paired larval nerve during metamorphosis, and show transitional stages of segmentation. These findings indicate that echiurans, annelids and sipunculans stem from a segmented ancestor. By contrast, no traces of body segmentation are present during neurogenesis of basal molluscs. However, a tetraneurous condition (i.e. one pair of ventral and one pair of lateral nerve cords), as is typical for Mollusca, and a serotonergic larval apical organ that matches the complexity of polyplacophoran apical organs, were found in larval entoprocts, thus strongly supporting a mollusc-entoproct clade. Within the Lophophorata (Ectoprocta + Phoronida + Brachiopoda), data on nervous system development for any of the 3 lophophorate phyla are as of yet too scarce for profound phylogenetic inferences. Taking into account the most recent advances in molecular phylogenetics and developmental neurobiology, a scenario emerges that proposes a clade comprising Sipuncula + Annelida (including Echiura) on the one hand and a monophyletic assemblage of Entoprocta + Mollusca on the other.
Wanninger
A
A
Department of Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark. awanninger@bio.ku.dk
D000818:Animals; D000845:Annelida / Q000033:anatomy & histology / Q000254:growth & development; D005075:Biological Evolution; D002013:Bryozoa / Q000033:anatomy & histology / Q000254:growth & development; D007448:Invertebrates / Q000033:anatomy & histology / Q000145:classification / Q000254:growth & development*; D007814:Larva / Q000033:anatomy & histology / Q000254:growth & development; D008974:Mollusca / Q000033:anatomy & histology / Q000254:growth & development; D009348:Nematoda / Q000033:anatomy & histology / Q000254:growth & development; D009420:Nervous System / Q000033:anatomy & histology / Q000254:growth & development; D010802:Phylogeny; D010986:Platyhelminths / Q000033:anatomy & histology / Q000254:growth & development; D013045:Species Specificity