Limb development in a primitive crustacean, Triops longicaudatus: subdivision of the early limb bud gives rise to multibranched limbs.

Recent advances in developmental genetics of Drosophila have uncovered some of the key molecules involved in the positioning and outgrowth of the leg primordia. Although expression patterns of these molecules have been analyzed in several arthropod species, broad comparisons of mechanisms of limb development among arthropods remain somewhat speculative since no detailed studies of limb development exist for crustaceans, the postulated sister group of insects. As a basis for such comparisons, we analysed limb development in a primitive branchiopod crustacean, Triops longicaudatus. Adults have a series of similar limbs with eight branches or lobes that project from the main shaft. Phalloidin staining of developing limbs buds shows the distal epithelial ridge of the early limb bud exhibits eight folds that extend in a dorsal ventral (D/V) arc across the body. These initial folds subsequently form the eight lobes of the adult limb. This study demonstrates that, in a primitive crustacean, branched limbs do not arise via sequential splitting. Current models of limb development based on Drosophila do not provide a mechanism for establishing eight branches along the D/V axis of a segment. Although the events that position limbs on a body segment appear to be conserved between insects and crustaceans, mechanisms of limb branching may not.