EVOLUTIONARY CHANGE IN NEURAL DEVELOPMENT WITHIN THE ARTHROPODS - AXONOGENESIS IN THE EMBRYOS OF 2 CRUSTACEANS

It has been previously suggested that there is a conservative program for neural development amongst the arthropods, on the basis that a ste reotyped set of cells involved in establishing the axon tracts in the CNS of insect embryos is also present in crayfish embryos. We have exa mined the spatiotemporal pattern of axon growth from a set of early di fferentiating central neurons in the embryo of two crustaceans, the wo odlouse Porcellio scaber and the freshwater crayfish Cherax destructor , and drawn comparisons with insect neurons whose somata lie in corres ponding positions within the CNS. While many of the woodlouse and cray fish neurons show a similar pattern of axon growth to their insect cou nterparts, the axon trajectories taken by others differ from those see n in insects. We conclude that this aspect of early neural development has not been rigidly conserved during the evolution of the crustacean s and insects. However, the extent of similarity between the insects a nd the crustaceans is consistent with the idea that these groups of ar thropods share a common evolutionary 'Bauplan' for the construction of their nervous systems. While the pattern of early axon growth in the woodlouse and crayfish embryos is sufficiently similar that many neuro ns could be confidently recognised as homologues, several differences were noted in both the relative order of axon outgrowth and axon morph ologies of individual neurons.