Holomeric vs. meromeric segmentation: a tale of centipedes, leeches, and rhombomeres

Explaining the origin and evolution of segmentation is central to understan ding the body plan of major animal groups such as arthropods, annelids, and vertebrates. One major shortcoming of current views on segmentation is the failure to recognize the existence of two layers of segmentation. I distin guish here holomeric segmentation, involving the whole body axis (or the wh ole axis of an appendage) and producing " true" segments (eosegments); and meromeric segmentation, producing merosegments within one or more eosegment (s). In terms of developmental mechanisms, meromeric segmentation is probab ly the same as compartmentalization. This process follows two rules: (1) me rosegments are formed from a stereotyped pattern of subdivisions, where onl y the merosegments in contact to the anterior or posterior boundary of the eosegment are allowed to divide; (2) contiguous eosegments undergoing merom eric segmentation generate merosegments according to identical lineage patt erns apart from possible lineage truncation in one or a few terminal eosegm ents. The segmentation model proposed in this paper is mainly supported by evidence from comparative morphology, but it is compatible with known cellu lar and developmental mechanisms. The development of vertebrate rhombomeres , the annulation of leeches, the subdivision of the distal part of insect a ntenna into flagellomeres and the segmentation of centipedes are interprete d here in terms of meromeric segmentation. Some of these phenomena, like ce ntipede segmentation, have thus far defied all attempts at an explanation, both in mechanistic (developmental) and phylogenetic terms. The model prese nted in this paper suggests a rich research agenda at all levels, from mole cular and genetic to morphological and phylogenetic.