Lineage analysis as an analytical tool in the insect central nervous system: Bringing order to interneurons

Abstract

The interneuron is a major neuroarchitectural element within the arthropod central nervous system, both in terms of numbers and diverstiy of form. The interneuron can be defined structurally as having its soma and most or all processes within the central nervous system, and functionally, as receiving information from afferents or other interneurons and then distributing this information to other interneurons or motoneurons. The interneuron is the key integrating unit of the nervous system. The fact that in arthropods many interneurons can be identified as individuals, recognizable from preparation to preparation, has allowed great progress to be made in understanding the contribution of interneurons to behavior. Further, structural and functional comparisons of identified neurons can be made both along the segmented central nervous system of an individual and between species. In fact, a number of interneurons have now been found to bear remarkable structural similarities in different arthropods, and even appear to function in similar sensory pathways. But such structural similarities alone cannot provide proof of an evolutionary relationship, that is, of homology between nervous systems. A far stronger argument for homology comes from lineage analysis, which establishes the ontogeny of any interneuron. It is the comparison of lineages that can reveal homologies between interneurons. In this chapter we examine the properties of a number of lineage-related neurons in different insect species. We describe the developmental events that lead to segmental specializations in the structure and function of homologous interneurons and then show how a comparison of lineages in different insect species suggests that some systems of neurons may have even been conserved across large distances of evolutionary time. We believe that the study of lineage-related and homologous interneurons can provide a fresh insight into the phylogenetic relationships among insect nervous systems.