Simple Summary
During asexual reproduction by paratomic fission, the new head and tail ends of the two zooids develop prior to the physical separation of the fusioning individual. This process is accompanied by remodeling of the original ventral nerve cord, development of the new brain, ventral ganglia, peripheral nerves, and sensory organs, including the eyes. Results of our study suggest that the involvement of Pax6 in development and patterning of the nervous system is evolutionary conserved, despite the variability and evolution of the developmental trajectories.
Abstract
Pax6 plays a highly conserved role in the formation of the eye, development, and patterning of the nervous system across bilaterians. Nevertheless, there are no studies focusing on the role of Pax6 during asexual reproduction, a developmental trajectory that is widespread among metazoans. The present study represents the first investigation of Pax6 gene expression during agametic propagation in annelids. We identified in the asexually reproducing annelid Nais communis four homologs of Pax6 and examined their developmental patterns by in situ hybridization. To establish a morphological basis for the expression patterns, we used immunohistochemistry and confocal laser scanning microscopy to describe the nervous system architecture of the growing adults and remodeling of the original ventral nerve cord, development of the new brain, ventral ganglia, peripheral nerves, and sensory organs in asexually reproducing worms. Our results support the hypothesis of an evolutionarily conserved function of Pax6 genes in the development of the eye and other sensory organs, as well as the central nervous system, among bilaterians, regardless of developmental trajectory. On the other hand, identified Pax6 homologs show differential expression within the developing new head and tail ends. Differences in spatiotemporal expression patterns may be evidence of functional diversification of duplicated homologs.