Segment number and segmentation performances in arthropods

In arthropods, developmental mechanisms of segmentation are very diverse and differently associated with growth, both with respect to developmental time and with respect to the spatial localization of segment origination in the embryo or in the larva. Although mechanisms of segment specification have been elucidated in a few model organisms, we know very little about how the number of segments is determined and controlled during ontogeny. In spite of this, morphological data can indicate what arthropod developmental systems must be able to do in order to produce the observed segmental patterns, in other words, their performances in segmentation. Arthropod segmentation performances can be evaluated looking at the results of segmentation processes, i.e. at the observed range of developmental trajectories that in turn produces the variety of arthropod segmental arrangements. Even limiting the inspection to the number of segments produced, considerable differences in segmentation performances across taxa emerge. These involve a) the number of segments per se, b) the level of developmental precision for the number of segments produced, c) the developmental schedule of segmentation, and d) the possible pattern of intra-specific variation. Patterns of intra-specific variation for the number of segments and for the post-embryonic developmental schedule can be related to sexual dimorphism and geographic variation. For some species that exhibit intra-specific variability in segmentation, there is evidence for a relevant hereditary component, but there is no information on the genetic mechanism eventually involved. Developmental performances in segmentation, being related to the mechanisms of segmentation and their genetic control, can suggest possible developmental constraints on the evolution of segmental patterns.