Walking on insect paths? Early ommatidial development in the compound eye of the ancestral crustacean, Triops cancriformis

Abstract

 Ommatidia (the compound eye's functional units) in insects are formed by the recruitment of undifferentiated cells under the control of signalling factors. During this process, a sequence of "preclusters" composed of specifically arranged precursor cells is followed. In the growth zone of the eye of Triops, an ancestral crustacean, we observed a patterning process that corresponds well with that of insects. In both taxa, clusters with arc-like, five-cell and eight-cell patterns are found, and the sequence in which the photoreceptor or R-cells of each ommatidium become identifiable is basically the same. The first to appear are R8-like and R2/5-like cells, second are R3/4-like, and third are R1/6- and R7-like cells (if the fly's cell-numbering system is used). Thus, the morphogenetic steps during which the cell identities and the cellular architecture of the ommatidia develop appear to be conserved between these arthropod groups. Furthermore, the individual cells and cell pairs which build an insect ommatidium seem to have their homologues in crustaceans. In the evolution of developmental processes, intercellular recruitment seems to be a mechanism operating on the level of single cells even in distantly related species.