Fine structure of the ventral nerve centre and interspecific identification of individual neurons in the enigmatic Chaetognatha

Abstract

The enigmatic arrow worms (Chaetognatha) are marine carnivores and among the most abundant planktonic organisms. Their phylogenetic position has been heavily debated for a long time. Most recent molecular studies still provide a diverging picture and suggest arrow worms to be some kind of basal protostomes. In an eVort to understand the organization of the nervous system in this clade for a broad comparison with other Metazoa we analysed the ultrastructure of the ventral nerve centre in Spadella cephaloptera by transmission electron microscopy. We were able to identify six diVerent types of neurons in the bilateral somata clusters by means of the cytoplasmic composition (regarding the structure of the neurite and soma including the shape and eu-/heterochromatin ratio within the nucleus) as well as the size and position of these neurons. Furthermore, our study provides new insights into the neuropil composition of the ventral nerve centre and several other Wne structural features. Our second goal was to examine if individually identiWable neurons are present in the ventral nerve centres of four chaetognath species, Sagitta setosa, Sagitta enXata, Pterosagitta draco, and Spadella cephaloptera. For that purpose, we processed whole mount specimens of these species for immunolocalization of RFamiderelated neuropeptides and analysed them with confocal laser-scanning microscopy. Our experiments provide evidence for the interspeciWc homology of individual neurons in the ventral nerve centres of these four chaetognath species suggesting that the potential to generate serially arranged neurons with individual identities is part of their ground pattern.

Evolution, divergence and loss of the Nodal signalling pathway: new data and a synthesis across the Bilateria

“…Chaetognaths have retained ancestral genomic features, such as the conservation of certain genes like that encoding for the Guanidinoacetate N-methyltransferase (GAMT) enzyme, and at the same time, have acquired some derived features, such as a trans-splicing mechanism of mRNA processing (shared with many other organisms, including several spiralians). This makes them a key group for understanding early evolutionary events in the Bilateria (Marletaz et al, 2008;Harzsch and Müller 2007;Harzsch et al, 2009;Harzsch and Wanninger 2010;Rieger et al, 2010). Further studies on the expression patterns and function of Nodal in this group will be particularly useful to understand the early role of this pathway in protostomes.…”

Evolution of invertebrate nervous systems: the Chaetognatha as a case study

“…1D). Several recent studies have used immunohistochemical techniques to examine the distribution of serotonin and RFamide-like immunoreactive neurons (Bone et al 1987;Goto et al 1992;Harzsch and Müller 2007;Harzsch et al 2009), as well as aspartate immunoreactivity (Duvert et al 1997) in the central and peripheral parts of the nervous systems (and see Figs 1 and 2).…”

The central and peripheral nervous system of Cephalodiscus gracilis (Pterobranchia, Deuterostomia)

“…Additionally, nervous systems are interesting because they are constitutive elements of all eumetazoan species and intrinsically correlated with all animal behaviors (PXüger and Stevenson 2005). Technical advances have increased the scientiWc interest in the evolution of animal nervous systems, especially with the widespread use of confocal laser scanning microscopy (Harzsch 2007;Harzsch et al 2009;Loesel and Heuer 2010;Wanninger et al 2007) and molecular studies of developmental processes (Arendt and Nübler-Jung 1999;Holland 2009;Holland et al 2008;Lowe et al 2003Lowe et al , 2006.…”