Collaborative Research: Developmental Mechanisms Underlying Diversity in Arthropod Limbs

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Their studies compare both the molecular and cellular development of a multi-branched limb in Triops to that of the unbranched limb in Drosophila. These two limbs differ not only in number of branches but also in their fundamental structure: Drosophila legs are jointed and rod-shaped, Triops legs have an unjointed paddle-shaped limb with eight branches arising around the margin. Thus, their comparison between them should reveal how very different limbs arise. Their results suggest that there are three critical features of the Drosophila model that vary between species. 1) In Drosophila, the intersection of the anteroposterior (A/P) and dorsoventral (D/V) patterning axes position the limb primordia. Their data suggest that variation in D/V positioning in Triops results in the specification of a larger limb primordia, which they hypothesize is the first step in building a multi-branched limb. 2) In Drosophila, the intersection of the secreted signaling factors, wingless (wg), decapentaplegic (dpp), and hedgehog (hh) specify a point of proximal/ distal (P/D) elongation for leg outgrowth. Cell fates in the distal region of the leg are activated and maintained directly by different levels of Wg and Dpp signals; in the proximal region of the leg they are activated by a different gene network, regulated at least in part by the extradenticle (exd) gene. Their data suggest that the multibranched limb of Triops does indeed have a single P/D axis, potentially controlled by homologues of the same signaling genes. However, the patterning of the branches in Triops is not a simple iteration of the same gene network involved in patterning the Drosophila limb. From their earliest development at least four different branch types are identifiable by distinctive gene expression patterns. 3) Establishment of a single point of P/D outgrowth in Drosophila is not directly linked to cell division patterns. However, in Triops, they find a concentration of mitotically active cells in the region of the putative Triops limb organizer. Their model therefore suggests that, unlike Drosophila, the P/D organizer in Triops may control specific patterns of cell division, in a manner similar to vertebrate limb development.