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Abstract Hox transcription factors are conserved regulators of neuronal subtype specification on the anteroposterior axis in animals, with disruption of Hox gene expression leading to homeotic transformations of neuronal identities. We have taken advantage of an unusual mutation in the Caenorhabditis elegans Hox gene lin-39, lin-39(ccc16), which transforms neuronal fates in the C. elegans male ventral nerve cord in a manner that depends on a second Hox gene, mab-5. We have performed a genetic analysis centered around this homeotic allele of lin-39 in conjunction with reporters for neuronal target genes and protein interaction assays to explore how LIN-39 and MAB-5 exert both flexibility and specificity in target regulation. We identify cis-regulatory modules in neuronal reporters that are both region-specific and Hox-responsive. Using these reporters of neuronal subtype, we also find that the lin-39(ccc16) mutation disrupts neuronal fates specifically in the region where lin-39 and mab-5 are coexpressed, and that the protein encoded by lin-39(ccc16) is active only in the absence of mab-5. Moreover, the fates of neurons typical to the region of lin-39-mab-5 coexpression depend on both Hox genes. Our genetic analysis, along with evidence from Bimolecular Fluorescence Complementation protein interaction assays, supports a model in which LIN-39 and MAB-5 act at an array of cis-regulatory modules to cooperatively activate and to individually activate or repress neuronal gene expression, resulting in regionally specific neuronal fates. 

The mechanisms supporting regeneration and successful recovery of function have fascinated scientists and the general public for quite some time, with the earliest description of regeneration occurring in the 8th century BC through the Greek mythological story of Prometheus. While most animals demonstrate the capacity for wound-healing, the ability to initiate a developmental process that leads to a partial or complete replacement of a lost structure varies widely among animal taxa. Variation also occurs within single species based on the nature and location of the wound and the developmental stage or age of the individual. Comparative studies of cellular and molecular changes that occur both during, and following, wound healing may point to conserved genomic pathways among animals of different regenerative capacity. Such insights could revolutionize studies within the field of regenerative medicine. In this review, we focus on several closely related species of Lumbriculus (Clitellata: Lumbriculidae), as we present a case for revisiting the use of an annelid model system for the study of regeneration. We hope that this review will provide a primer to Lumbriculus biology not only for regeneration researchers but also for STEM teachers and their students. 

Examination of algal assemblages from aerial environments around the globe, especially those from pseudoaerial habitats found on moistened rocks underneath waterfalls or around springs and seeps, reveals the presence of unique diatom floras. Yet, diatom assemblages from northern regions like Iceland remain understudied, especially those from the volcanic rock outcrops and boulders that create euaerial habitats where biota receive moisture from the atmosphere or the rock itself. During the summers of 2013 and 2015, we examined the biodiversity of mostly euaerial, but also pseudoaerial, diatom assemblages collected from volcanic rock outcrops or large boulders on the landscape from southwestern Iceland. We used light and scanning electron microscopy to document the biodiversity of common, smaller, new, or interesting specimens, such as Humidophila and Eunotia. We describe one new Humidophila species, H. eldfjallii sp. nov., with triundulate valve margins and include information on another unidentified taxon, Humidophila sp. 1, naviculoid in shape with tapering to rounded ends, continuous striae through the length of the valve, and a circular central area. We formally transfer Diadesmis contenta var. biceps to Humidophila biceps. To correct the nomenclature, we recognized Humidophila parallela at the species level. Relative abundance estimates of diatom populations provided further characterization of the assemblages on these habitats. Humidophila taxa, especially H. gallica dominated the diverse diatom flora. We discuss adaptations for survival with access to mostly atmospheric water. The diatom flora described here adds to the flora for this region, highlights the diversity of diatom assemblages that can inhabit euaerial environments, and provides evidence of adaptive success of diatoms in extreme habitats with limited moisture and nutrients.