Molecular cloning and characterization of homologs of achaete-scute and hairy-enhancer of split in the olfactory organ of the spiny lobster Panulirus argus

Insights into the genetic regulatory network underlying neurogenesis in the parthenogenetic marbled crayfish Procambarus virginalis

Nervous system development has been intensely studied in insects (especially Drosophila melanogaster), providing detailed insights into the genetic regulatory network governing the formation and maintenance of the neural stem cells (neuroblasts) and the differentiation of their progeny. Despite notable advances over the last two decades, neurogenesis in other arthropod groups remains by comparison less well understood, hampering finer resolution of evolutionary cell type transformations and changes in the genetic regulatory network in some branches of the arthropod tree of life. Although the neurogenic cellular machinery in malacostracan crustaceans is well described morphologically, its genetic molecular characterization is pending. To address this, we established an in situ hybridization protocol for the crayfish Procambarus virginalis and studied embryonic expression patterns of a suite of key genes, encompassing three SoxB group transcription factors, two achaete-scute homologs, a Snail family member, the differentiation determinants Prospero and Brain tumor, and the neuron marker Elav. We document cell type expression patterns with notable similarities to insects and branchiopod crustaceans, lending further support to the homology of hexapod-crustacean neuroblasts and their cell lineages. Remarkably, in the crayfish head region, cell emigration from the neuroectoderm coupled with gene expression data points to a neuroblast-independent initial phase of brain neurogenesis. Further, SoxB group expression patterns suggest an involvement of Dichaete in segmentation, in concordance with insects. Our target gene set is a promising starting point for further embryonic studies, as well as for the molecular genetic characterization of subregions and cell types in the neurogenic systems in the adult crayfish brain.

Distribution and function of splash, an achaete-scute homolog in the adult olfactory organ of the Caribbean spiny lobster Panulirus argus

achaete-scute complex (ASC) genes, which encode basic helix-loop-helix transcription factors, regulate embryonic and adult neurogenesis in many animals. In adult arthropods, including crustaceans, ASC homologs have been identified but rarely functionally characterized. We took advantage of the recently identified crustacean homolog, splash (spiny lobster achaete scute homolog), in the olfactory organ of the Caribbean spiny lobster Panulirus argus to examine its role in adult neurogenesis. We tested the hypothesis that splash is associated with but not restricted to sensory neuron formation in the olfactory organ, the antennular lateral flagellum (LF), of adult spiny lobsters. We demonstrated splash labeling in epithelial cells across LF developmental zones (i.e., proliferation and mature zones), in auxiliary cells surrounding dendrites of olfactory receptor neurons (ORNs), and in immature and mature ORNs, but not in granulocytes or chromatophores. Since ORN proliferation varies with molt stage, we examined splash expression across molt stages and found that molt stage affected splash expression in the ORN mature zone but not in the proliferation zone. In vivo incorporation of bromodeoxyuridine (BrdU) showed no correlation in the cellular pattern of splash expression and BrdU labeling. The intensity of splash labeling was dramatically enhanced in the proliferation zones following LF damage, suggesting enhanced splash expression during repair and/or regeneration. We conclude that splash is not closely associated with the formation of sensory neurons under normal physiological conditions, and we propose that splash is involved in repair and regeneration. We also propose that splash has additional roles other than neurogenesis in adult crustaceans.

Cytoarchitecture and ultrastructure of neural stem cell niches and neurogenic complexes maintaining adult neurogenesis in the olfactory midbrain of spiny lobsters, Panulirus argus

New interneurons are continuously generated in small proliferation zones within neuronal somata clusters in the olfactory deutocerebrum of adult decapod crustaceans. Each proliferation zone is connected to a clump of cells containing one neural stem cell (i.e., adult neuroblast), thus forming a "neurogenic complex." Here we provide a detailed analysis of the cytoarchitecture of neurogenic complexes in adult spiny lobsters, Panulirus argus, based on transmission electron microscopy and labeling with cell-type-selective markers. The clump of cells is composed of unique bipolar clump-forming cells that collectively completely envelop the adult neuroblast and are themselves ensheathed by a layer of processes of multipolar cell body glia. An arteriole is attached to the clump of cells, but dye perfusion experiments show that hemolymph has no access to the interior of the clump of cells. Thus, the clump of cells fulfills morphological criteria of a protective stem cell niche, with clump-forming cells constituting the adult neuroblast's microenvironment together with the cell body glia processes separating it from other tissue components. Bromodeoxyuridine pulse-chase experiments with short survival times suggest that adult neuroblasts are not quiescent but rather cycle actively during daytime. We propose a cell lineage model in which an asymmetrically dividing adult neuroblast repopulates the pool of neuronal progenitor cells in the associated proliferation zone. In conclusion, as in mammalian brains, adult neurogenesis in crustacean brains is fueled by neural stem cells that are maintained by stem cell niches that preserve elements of the embryonic microenvironment and contain glial and vascular elements.