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 diver sity of metameric patterns and processes should be evolutionarily inte rpreted, and what the relationship is between their morphological and developmental features. We review the aspects of Drosophila developmen t that involve compartmentalization, parasegmentation, (meta)segmentat ion, patterns of muscle development, clonal composition of metameric d omains, and the correspondence between clonally and/or genetically def ined boundaries and adult structures. These regularities are compared with what little is known of these phenomena in other insects, crustac eans, millipedes, centipedes, onychophorans, and polychaete and clitel late annelids. Both parasegmental and metasegmental metamerism are pro bably characteristic of all the arthropods and annelids. Developmental ly, 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 Sn odgrass) do not correspond to any identified developmental body metame res in arthropods, moreover, they are not recapitulated during ontogen y, and ''primary segmentation'' of longitudinal muscles (myosegmentati on) seems to be evolutionarily as ''primary'' as parasegmentation and metasegmentation, while developmentally the latest of them. The anteri ormost areas of the definite segments (metasegments) do not show any t races of being secondarily incorporated in these metasegments, as requ ired by the established hypothesis of concurrent phylogenetic and onto genetic switch from primary to secondary body segmentation.