Evolution of Arthropod Axis Elongation

Arthropods are the most diverse animal group in terms of species numbers, ecological variability, and morphological disparity. Like all members of the bilaterally symmetrical animals, arthropods have a distinct anterior-posterior axis, which is defined very early in development. Once defined embryologically, the anterior-posterior axis of most bilaterians undergoes a process of axial elongation. This general bilaterian body organization apparently predates that of segmented arthropods. Consequently, it is believed that axis elongation is the ancestral developmental mode of bilaterally symmetric animals, and lies at the basis of the sequential addition of segments seen in arthropod development. While the formation of body segments is tightly linked to axial growth in all extant arthropods, it is not just simply a growth of tissue; rather, it is a highly complex developmental process involving the interaction of different germ layers coming from different sources. These germ layers need to act together in space and in time, and signal to each other in order to maintain the complex orchestration of cell and tissue movements. This challenge is solved in different arthropod groups via different developmental mechanisms and this raises a number of questions. The most central one is: how are the processes of axis elongation and segment formation linked to each other? More specifically, is the phylogenetically older process of axis elongation independent of or directly interacting with segmentation? Are segments really budded from the posterior growth zone, or is the growth zone just responsible for the formation of cell material for subsequent segmentation process? Is the concept of a posterior growth zone an appropriate description of what actually happens in insects and non-malacostracan crustaceans? What was the relative contribution of tissue proliferation vs. cell movements? Which germ layers were central to the process of tissue reorganization in the growth zone? What molecular pathways were active in coordinating these processes? The diversity of the segmentation process and the limitation of techniques mean that no single model organism is sufficient for understanding the mechanisms of arthropod segmentation and axis elongation in general. Thus, our approach involves looking at axis elongation in several different species from two points of view; one is the morphological view through the cell lineage pattern, and the other is the molecular point of view through analyzing gene expression and function. The ultimate combination of data from different sources and approaches will present a novel synthesis, and will hopefully offer us new insights into this fascinating topic.