Analysis of Nervous System Architecture and the Expression of Arthropod/Vertebrate Appendage-Gene Orthologs in Direct-Developing Juveniles of Neanthes arenaceodentata (Annelida, Nereididae)

• Main Author: Winchell, Christopher Jon
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2010
• Subjects: Evolution and Development; Genetics; Neurobiology; Neurosciences
• ISBN: 1124755004; 9781124755007

In distantly related animals, parallels in the expression of genes that regulate body plan development are most often observed in the nervous system. This suggests that many developmental genes functioned ancestrally in neurogenesis and/or post-mitotic differentiation of the nervous system. Hence, for evo-devo studies emphasizing comparative gene expression, interpretation of the data often benefits from detailed knowledge of developing neuroanatomy. A fundamental evo-devo issue among animals concerns the origins of appendages. Similarity in the genetic regulation of arthropod and vertebrate appendage formation has been interpreted as the product of a plesiomorphic gene network that was primitively involved in bilaterian appendage development and co-opted to build appendages (in modern phyla) that are not historically related as structures. Data from lophotrochozoans are needed to clarify the pervasiveness of plesiomorphic appendage-forming mechanisms. This dissertation describes the neuroanatomy of direct-developing juveniles of the nereidid polychaete Neanthes arenaceodentata and examines their expression of several genes that are known to function in appendage development in arthropods and vertebrates. The spatial pattern of immunolabeled acetylated α-tubulin was analyzed via confocal laser scanning microscopy to reconstruct the developing nervous system. Various strategies of polymerase chain reaction were used to isolate Neanthes orthologs of Distal-less (Dll), dachshund, optomotor blind, apterous, and lim1. The expression of these genes was detected via in situ hybridization using riboprobes, and analyzed via light microscopy of wholemount and histologically sectioned specimens. These investigations reveal among-nereidid variation in major neural features, and show that the above genes are primarily active in the developing nervous system. Contrary to their roles in outgrowing arthropod and vertebrate appendages, these genes lack comparable expression in Neanthes appendages, implying independent evolution of annelid appendage development. It is inferred that parapodia and arthropodia are not structurally or mechanistically homologous (but their primordia might be), that Dll's ancestral bilaterian function was in sensory and central nervous system differentiation, and that locomotory appendages possibly evolved from sensory outgrowths. Lastly, expression of the LIM homeobox genes apterous and lim1, each in a different set of parapodial muscle precursors, suggests a "LIM code" for myoblast diversification.