Crustacean Limb Morphogenesis during Normal Development and Regeneration
Crustaceans have been favored in developmental biology for the study of the diversification of body plans and their associated appendages, which exhibit remarkable diversity within and between species. Until recently, because of technical limitations, crustacean studies were restricted in scope to the comparison of appendage morphologies and expression patterns of candidate limb patterning genes already known from classic developmental animal models. To remedy this limitation and explore their full potential, a few select crustacean experimental models have been reinforced with powerful genomic and transcriptomic resources, new methods for forward and reverse genetic investigations, and for live imaging of entire embryos, or cell and tissue-specific markers, with exceptional spatial and temporal resolution. These models include the malacostracan amphipod Parhyale hawaiensis and the branchiopod cladocerans Daphnia magna and Daphnia pulex, which display collectively all the different uniramous, biramous, and phyllopodous crustacean limb types. Within the past couple years, important discoveries have been made on the molecular and cellular basis of embryonic limb development and postembryonic limb regeneration. In Parhyale alone, gain and loss-of-function studies of Hox genes have revealed the combinatorial logic used by these genes for appendage specialization, whereas the reconstruction of single-cell-resolution fate maps of developing and regenerating appendages have identified the lineage restrictions and cellular behaviors driving both morphogenetic processes. Century-old questions regarding the conservation and divergence of appendage patterning mechanisms across arthropods and bilaterians, or how these mechanisms can be used and reused throughout the lifetime of an organism, can now be addressed productively with crustaceans.