Insights into arthropod head evolution. Two heads in one: The end of the "endless dispute"?

Molecular evolutionary genetics provides a powerful approach to the evoluti on of developmental pathways and to the origin and modification of morpholo gical structures. This approach led to a growing interest in understanding the evolution of the arthropod body plan. Comparative anatomy and evolution ary studies based on morphological or embryological head segments have fail ed to determine the precise organization of cephalic structures. In recent years multiple evidence showed that developmental genes involved in head pa tterning are conserved throughout the Bilateria. Studies carried out in Dro sophila, in numerous hexapods and in crustaceans demonstrate that two major developmental pathways are involved in the formation and differentiation o f the arthropod head: the 'head gap genes system' (belonging to otd/Otx, em s/Emx and btd families) in the rostral part of the embryo (procephalon and mandibular segment) and the 'Hox genes' in the posterior part of the head ( maxillae and labium) as well as in the trunkal segments. Together with othe r morphological and developmental evidence this suggests that the arthropod head consists of two major domains, the 'primitive' boundary subdividing t he mandibulate cephalon into an 'anterior head domain' (pregnathal region a nd mandibular segment) and a 'posterior head domain' comprising trunkal-lik e segments. The pivotal issue concerning the head metamery or segmentation is whether these developmental modes of head formation are mutually related or completely independent. Another interesting dilemma is whether they rep resent convergence of pattern-forming mechanisms or are derived from a comm on ancestor. The relationships between the two major mandibulate phyla, Hex apoda and Crustacea, are discussed on the basis of a comparative study of t he head patterning process.