EVOLUTION OF METAMERISM IN ARTHROPODA - DEVELOPMENTAL AND MORPHOLOGICAL PERSPECTIVES

Processes governing metamerization and the subsequent differentiation of metameres are well known in Drosophila. It is little known, however, to what extent the metameric patterns in Drosophila can be extended to other arthropods and related metameric animals, how the known diversity of metameric patterns and processes should be evolutionarily interpreted, and what the relationship is between their morphological and developmental features. We review the aspects of Drosophila development that involve compartmentalization, parasegmentation, (meta)segmentation, patterns of muscle development, clonal composition of metameric domains, and the correspondence between clonally and/or genetically defined boundaries and adult structures. These regularities are compared with what little is known of these phenomena in other insects, crustaceans, millipedes, centipedes, onychophorans, and polychaete and clitellate annelids. Both parasegmental and metasegmental metamerism are probably characteristic of all the arthropods and annelids. Developmentally, the annelid segments (as well as segments of the hypothetical soft-bodied prearthropod ancestor) cannot be identified with parasegments (sensu Minelli and Bortoletto). The alleged primary segments (sensu Snodgrass) do not correspond to any identified developmental body metameres in arthropods, moreover, they are not recapitulated during ontogeny, and ''primary segmentation'' of longitudinal muscles (myosegmentation) seems to be evolutionarily as ''primary'' as parasegmentation and metasegmentation, while developmentally the latest of them. The anteriormost areas of the definite segments (metasegments) do not show any traces of being secondarily incorporated in these metasegments, as required by the established hypothesis of concurrent phylogenetic and ontogenetic switch from primary to secondary body segmentation.