Different sources of data on the evolution of segmentation lead to very different conclusions. Molecular similarities in the developmental pathways generating a segmented body plan tend to suggest a segmented common ancestor for all bilaterally symmetrical animals. Data from paleontology and comparative morphology suggest that this is unlikely. A possible solution to this conundrum is that throughout evolution there was a parallel co‐option of gene regulatory networks that had conserved ancestral roles in determining body axes and in elongating the anterior‐posterior (...) axis. Inherent properties in some of these networks made them easily recruitable for generating repeated patterns and for determining segmental boundaries. Phyla where this process happened are among the most successful in the animal kingdom, as the modular nature of the segmental body organization allowed them to diverge and radiate into a bewildering array of variations on a common theme. (shrink)

One of the most fundamental features of the body plan of arthropods is its segmental design. There is considerable variation in segment number among arthropod groups (about 20‐fold); yet, paradoxically, the vast majority of arthropod species have a fixed number of segments, thus providing no variation in this character for natural selection to act upon. However, the 1000‐species‐strong centipede order Geophilomorpha provides an exception to the general rule of intraspecific invariance in segment number. Members of this group, and especially our (...) favourite animal Strigamia maritima, may thus help us to understand the evolution of segment number in arthropods. Evolution must act by modifying the formation of segments during embryogenesis. So, how this developmental process operates, in a variable‐segment‐number species, is of considerable interest. Strigamia maritima turns out to be a tractable system both at the ecological level of investigating differences in mean segment number between populations and at the molecular level of studying the expression patterns of developmental genes. Here we report the current state of play in our work on this fascinating animal, including our recent finding of a double‐segment periodicity in the expression of two Strigamia segmentation genes, and its possible implications for our understanding of arthropod segmentation mechanisms in general. BioEssays 27:653–660, 2005. © 2005 Wiley Periodicals, Inc. (shrink)