Plasticity and genetic basis of cichlid gill arch anatomy reveal novel roles for Hedgehog signaling

Gill arch and raker morphology of common carp (Cyprinus carpio, Linnaeus, 1758) sampled in aquaculture system

The study aimed to investigate the morphologic aspects of common carp's gill arch and gill rakers (Cyprinus carpio, Linnaeus, 1758), an omnivore and highest-produced aquaculture species. The study used 10 common carp (395.35 ± 45.06 g) grown entirely under aquaculture conditions. The fish tissues were fixed with Glutaraldehyde (2.5%) for scanning electron microscopy and with formalin (10%) for stereomicroscopic examination. In the SEM examination, two types of taste papillae (Type II and Type III) were observed in the pharyngeal mucosa. Microridge-like structures in the epithelial layer were found to have two forms. The study findings indicate a significant decrease in gill arch lengths from cranial to caudal and a significant increase in rakers per unit area, as determined through digital calliper measurements and stereomicroscopic examinations (p < 0.05). However, there was no significant difference in measurements of gill arches and raker numbers between the bilateral symmetry of the gill arches (p > 0.05). In conclusion, it was observed that the epithelial structure on the common carp gill arch contained two types of microridge-like structures: the gill arch length decreased from cranial to caudal, and the rake density on these arches increased caudally.

The transcriptional state and chromatin landscape of cichlid jaw shape variation across species and environments

Adaptive phenotypes are shaped by a combination of genetic and environmental forces, but how they interact remains poorly understood. Here, we utilize the cichlid oral jaw apparatus to better understand these gene-by-environment effects. First, we employed RNA-seq in bony and ligamentous tissues important for jaw opening to identify differentially expressed genes between species and across foraging environments. We used two Lake Malawi species adapted to different foraging habitats along the pelagic-benthic ecomorphological axis. Our foraging treatments were designed to force animals to employ either suction or biting/scraping, which broadly mimic pelagic or benthic modes of feeding. We found a large number of differentially expressed genes between species, and while we identified relatively few differences between environments, species differences were far more pronounced when they were challenged with a pelagic versus benthic foraging mode. Expression data carried the signature of genetic assimilation, and implicated cell cycle regulation in shaping the jaw across species and environments. Next, we repeated the foraging experiment and performed ATAC-seq procedures on nuclei harvested from the same tissues. Cross-referencing results from both analyses revealed subsets of genes that were both differentially expressed and differentially accessible. This reduced dataset implicated notable candidate genes including the Hedgehog effector, KIAA0586 and the ETS transcription factor, etv4, which connects environmental stress and craniofacial morphogenesis. Taken together, these data provide novel insights into the epigenetic, genetic and cellular bases of species- and environment-specific bone shapes.

Arsenic (III) induces oxidative stress and inflammation in the gills of common carp, which is ameliorated by zinc (II)

Arsenic (As) is widely present in the environment in form of arsenite (As^III) and arsenate (As^V). Oxidative stress and inflammation are believed to be the dominant mechanisms of As^III toxicity in vivo and in vitro. The aim of this study was to investigate whether zinc (Zn²⁺) alleviates exogenous gill toxicity in carp induced by As^III and to gain insight into the underlying mechanisms. Exposure of carp to 2.83 mg As₂O₃/L for 30 days reduced superoxide dismutase activity by 4.0%, catalase by 41.0% and glutathione by 19.8%, while the concentration of malondialdehyde was increased by 16.4% compared to the control group, indicating oxidative stress. After the exposure of carp to As^III the expression of inflammatory markers, such as interleukin-6, interleukin-8, tumor necrosis factor α and inducible nitric oxide synthase in gill tissue were significantly increased. In addition, the phosphorylation of nuclear factor kappa-B (NF-κB) was increased by 225%. 1 mg ZnCl₂/L can relieve the toxicity of As^III based on histopathology, antioxidase activity, qRT-PCR and western results. Zn²⁺ attenuated As^III-induced gill toxicity that suppressed intracellular oxidative stress and NF-κB pathway by an upregulation of metallothionein. Therefore, the toxic effect of As^III on the gill cells of carp was reduced. This study provides a theoretical basis for exploring the alleviation of the toxic effects of metalloids on organisms by heavy metals and the biological assessment of the effects.

Ciliary Rootlet Coiled-Coil 2 (crocc2) Is Associated with Evolutionary Divergence and Plasticity of Cichlid Jaw Shape

Cichlid fishes exhibit rapid, extensive, and replicative adaptive radiation in feeding morphology. Plasticity of the cichlid jaw has also been well documented, and this combination of iterative evolution and developmental plasticity has led to the proposition that the cichlid feeding apparatus represents a morphological "flexible stem." Under this scenario, the fixation of environmentally sensitive genetic variation drives evolutionary divergence along a phenotypic axis established by the initial plastic response. Thus, if plasticity is predictable then so too should be the evolutionary response. We set out to explore these ideas at the molecular level by identifying genes that underlie both the evolution and plasticity of the cichlid jaw. As a first step, we fine-mapped an environment-specific quantitative trait loci for lower jaw shape in cichlids, and identified a nonsynonymous mutation in the ciliary rootlet coiled-coil 2 (crocc2), which encodes a major structural component of the primary cilium. Given that primary cilia play key roles in skeletal mechanosensing, we reasoned that this gene may confer its effects by regulating the sensitivity of bone to respond to mechanical input. Using both cichlids and zebrafish, we confirmed this prediction through a series of experiments targeting multiple levels of biological organization. Taken together, our results implicate crocc2 as a novel mediator of bone formation, plasticity, and evolution.