Meeting Abstract
Recently placed within a clade containing segmented annelids, sipunculans provide a unique opportunity to study the evolutionary loss or gain of an important feature of animal body plans: segmentation. Neural segmentation is evident in ventral nerve cord (VNC) formation in many annelids, including Capitella teleta. On the other hand, neural development in sipunculans has not been well-studied, and reports range from no evidence of segmentation to vestigial segmentation based on a few pairs of serially-repeated neuronal cell bodies in the VNC. We performed an in-depth comparative analysis of neural development in the indirect-developing sipunculan Themiste lageniformis and the segmented annelid C. teleta using a combination of in situ hybridization (ISH) and immunohistochemistry to examine pan-neuronal, neuronal sub-type and axonal markers. We hypothesize that sipunculans do not show signs of segmentation during development and that serially-repeated neuronal subtypes or neurites are not in phase with each other or other tissues (i.e. are irregularly distributed from anterior to posterior). Homologs of C. teleta and T. lageniformis synaptotagmin1 were cloned and used for ISH. Cte-syt1 is expressed throughout the CNS (brain and serially-reiterated VNC ganglia) and in peripheral neurons. Tla-syt1 is also expressed throughout the CNS, but expression in the VNC does not appear segmented. Two paired clusters of serotonergic neurons in the VNC are not spatially restricted. Our initial results are congruent with our hypothesis, but data from ongoing experiments will further elucidate the possible loss of segmentation in sipunculans.
