Matthew C. Gibson

Throughout eukaryotes, the synaptonemal complex (SC) is a supramolecular structure essential for meiotic chromosome dynamics and sexual reproduction. While metazoan SC proteins display substantial sequence divergence, the lack of functional analyses beyond bilaterian models has obscured a mechanistic understanding of SC evolution. Here, we report that the sea anemone Nematostella vectensis exhibits synapsis of homologous chromosomes in a classic zipper-like manner and expresses at least eight orthologs of 10 known vertebrate SC proteins. Unexpectedly, mutagenesis of the core SC components sycp1 , syce2 , and sycp3 resulted in divergent and sexually dimorphic phenotypes, indicating the functional diversification of these conserved factors. Combined, these findings challenge the assumption that the conservation of protein sequence or ultrastructure implies mechanistic homology.

Coral reefs are constructed by colonial cnidarians whose survival depends on the coordinated growth and physiological integration of thousands of interconnected polyps. While coral skeletons have been extensively studied, the internal three-dimensional organization of coral tissues remains poorly resolved, limiting our understanding of how reef-building corals function as integrated modular organisms. In this study, we established a contrast-enhanced X-ray tomography (XRT) workflow for decalcified coral tissues, enabling detailed visualization and quantitative comparison of internal polyp architecture across four reef-building species with distinct colony forms: Acropora cervicornis, Acropora millepora, Montipora capitata, and Pocillopora damicornis . Importantly, this methodology resolved previously inaccessible patterns of tissue organization and structural connectivity among neighboring polyps. The two Acropora species shared a conserved axial – radial organization but differed in mesenterial morphology, whereas M. capitata exhibited complex, entangled mesenterial networks that connected both neighboring and distant polyps. In contrast, P. damicornis displayed superficial connectivity restricted to the coenosarc. Together, these results suggest that internal tissue architecture is an evolutionarily flexible trait, shaped by ecological and developmental pressures rather than strictly by shared ancestry. Our XRT workflow thus provides a new comparative framework for understanding how corals function as integrated living colonies.

Cnidae are explosive harpoon-like organelles localized within stinging cells, or cnidocytes, of the phylum Cnidaria (jellyfish, hydroids, sea anemones, and corals). These unique Golgi-derived structures define the phylum and are prominent examples of evolutionary novelty at the subcellular level. While recent studies have focused on the developmental specification of cnidocytes, less is known about the genetic basis for structural variation and differential toxin expression between distinct cnidae subtypes. Here we determine the transcriptional profile of developing nematocytes and spirocytes, the two major cnidae subtypes in the sea anemone Nematostella vectensis . We first interrogate the gene expression patterns and cell type-specificity of three known and three uncharacterized minicollagen genes, which encode a family of cnidae-specific structural proteins. We report that four minicollagen genes are broadly expressed in ectodermal cnidocytes while two others are restricted to tentacular cnidocytes. Leveraging whole adult scRNA-seq analysis as well as nematocyte- and spirocyte-specific transgenic reporter lines, we then identify the tentacle-restricted cnidocytes as spirocytes distinguished by expression of the minicollagen NvNcol5 . To deepen our understanding of the gene expression differences between nematocytes and spirocytes, we next developed a customized RNA-FACS-seq pipeline. This approach identified a suite of differentially expressed genes, illuminating spatial and temporal gene expression dynamics in both cell types. Altogether, our experiments provide fundamental and novel insights into the specialization of cnidarian stinging cells while establishing a rich set of molecular resources for further investigation.

The emergence of centralized nervous systems reflects a major inflection point in evolution, enabling animals to integrate diverse inputs and coordinate complex behaviors. Neural centralization is typically associated with Bilateria, whereas their sister group, Cnidaria (jellyfish, anemones, and corals), has long been thought to rely on diffuse nerve nets mediating simple reflexes. This view, reinforced by limited anatomical and molecular data, has left unresolved whether cnidarians can form localized centers for neural processing, a question sharpened by the growing recognition of their diverse behavioral repertoires. Here we show that the sea anemone Nematostella vectensis possesses an oral nerve ring composed of ganglion-like condensations, a hallmark of centralized organization. These neurons are enriched for excitatory, inhibitory, and modulatory receptors but lack sensory or ciliary markers, yielding a molecular profile most consistent with bilaterian interneurons. Genetic disruption of a conserved inhibitory receptor subunit predominantly expressed in the oral nerve ring delayed the initiation of swallowing in a novel feeding paradigm, demonstrating a potential role in behavioral regulation. Together, these findings provide converging anatomical, molecular, and functional evidence that cnidarians can assemble localized integrative centers, suggesting that key elements of neural centralization predated the cnidarian–bilaterian split.

Throughout eukaryotes, the synaptonemal complex (SC) is a supramolecular structure essential for meiotic chromosome dynamics and sexual reproduction. While metazoan SC proteins display significant sequence divergence, the lack of functional analyses beyond bilaterian models has obscured a mechanistic understanding of SC evolution. Here we report that the sea anemone Nematostella vectensis exhibits synapsis of homologous chromosomes in a classic zipper-like manner and expresses eight orthologs of ten known vertebrate SC proteins. Surprisingly, mutagenesis of the core SC components sycp1, syce2 , and sycp3 resulted in divergent and sexually dimorphic phenotypes, indicating the functional diversification of these conserved factors. Combined, these findings challenge the assumption that the conservation of protein sequence or ultrastructure implies mechanistic homology.

In Drosophila melanogaster females, as in most organisms, the segregation of homologous chromosomes in meiosis depends on the formation of crossovers between them. In most cases, crossovers require the synaptonemal complex (SC), a conserved multi-protein structure that forms between homologous chromosomes in early meiosis. Recent studies leveraging partial-loss-of-function alleles suggest that the SC plays a more direct role in crossover formation. One SC protein that is involved in crossover formation is SYP-4 in nematodes, which we found is a likely ortholog of the D. melanogaster SC protein Corolla. To create a hypomorphic allele of corolla in D. melanogaster, we used CRISPR/Cas9 to replace it with its D. mauritiana ortholog, yielding corolla^mau. Since SC protein sequences are rapidly diverging while maintaining the SC’s structure, we hypothesized that this replacement would enable SC assembly but show defects in crossover formation. Indeed, at 25 °C corolla^mau homozygous females exhibited defects in SC maintenance and crossover formation, resulting in moderate levels of chromosome missegregation. At 18 °C, SC maintenance was rescued, and recombination rates were improved, although they remained significantly lower than observed in wild type. Unexpectedly, in homozygotes we also observed unique polycomplexes composed of the SC proteins Corolla and Corona but lacking the transverse filament protein C(3)G. Overall, we report a novel hypomorphic allele of corolla that suggests Corolla regulates crossover formation. Further, the unique polycomplexes found in mutant flies may provide new insights into SC architecture and protein-protein interactions.

SUMMARY: Animal tissue morphology is determined by the shape, position, and proliferative capacity of individual epithelial cells. Nevertheless, it remains incompletely understood how the dynamic shape transformations implicit in mitotic proliferation influence tissue packing, particularly at the level of basal cell contacts. Here, we use an in silico vertex model to show that epithelial mitotic rounding necessitates a sequence of dynamic basal contact rearrangements, including basal diminution of the mitotic cell volume, transient multicellular rosette assembly, basal reinsertion of daughter cells, and neighbor reorganization. We then leverage a mammalian intestinal organoid model to confirm nearly identical basal cell-contact dynamics as those predicted in silico . Pharmacological inhibition of mitotic progression reveals that two events—basal diminution of the cell body and daughter cell reinsertion—independently drive distinct contact rearrangements. Together, our results uncover a previously underappreciated topological role for basal mitotic cell dynamics in shaping epithelial packing and morphogenesis.

Somatic mutations of Ras , encoding a small GTPase, are detected in a wide range of human cancers. Tumor genome sequencing further reveals a cancer type-dependent mutational spectrum for the Ras gene, suggesting that tissue- and allele-specific effects underlie tumorigenic activity. Although biochemical studies have characterized the GTPase activity of several Ras variants in vitro , precisely how somatic mutations of the endogenous Ras locus differentially affect tissue growth and homeostasis remain elusive. Here we engineered the endogenous Drosophila Ras locus to create a spectrum of inducible oncogenic alleles and then compared their activities in vivo . In the developing wing primordium, somatic clones carrying the oncogenic mutation Ras G12V exhibited a weak activation of downstream MAPK signaling but did not disrupt tissue architecture. However, cell clones carrying the same Ras G12V allele in the adult midgut exhibited a growth advantage and progressively took over the tissue, resulting in intestinal barrier dysfunction. In contrast, cell clones expressing a distinct allele, Ras Q61H , formed aberrant cysts that disrupted epithelial architecture and triggered local cell death. Conversely, when we induced cell clones carrying Ras Q61H in the midgut, hyper-proliferating mutant cells rapidly expanded to occupy the entire tissue. Surprisingly, this population of rapidly expanding mutant cells was eventually eliminated from the midgut, restoring wild-type cells and normal barrier function. Thus, in the midgut, Ras G12V was ultimately more deleterious than Ras Q61H due to the regression of Ras Q61H mutant cells. These results establish a new model for somatic mutagenesis at the Ras locus and illuminate a mechanistic basis for the tissue-specific effects of oncogenic Ras variants. Further, this study provides direct evidence that allele-dependent clonal dynamics may play a critical role in the tissue-selectivity of Ras oncogenic mutations.

Segmentation of the gastric cavity is a synapomorphic trait of cnidarians of the class Anthozoa (corals and sea anemones), with different clades forming distinct numbers of segments. In the starlet sea anemone Nematostella vectensis, for example, eight bilaterally positioned gastric segments are generated by the action of a group of Hox-Gbx genes in the developing larval endo-mesoderm. Still, given the range of segment numbers observed in different anthozoans, it remains unclear whether this Hox-Gbx module is evolutionarily conserved and how it might be deployed to generate different numbers of segments. Here, we systematically interrogate the role of Hox-Gbx genes during development of the rice coral Montipora capitata. We first characterize the temporal sequence of segmentation in M. capitata juveniles and then combine transcriptomic profiling and in situ hybridization to identify three conserved homeobox-containing genes, McAnthox8, McAnthox6a.1 and McGbx, which are collectively expressed in the developing endo-mesoderm prior to and during segment formation. The expression boundaries of these genes prefigure the positions of the first six segment boundaries, similar to their Nematostella homologs. Further, we show that chemical inhibition of BMP activity at the planula stage abolishes the expression of Hox-Gbx genes, leading to the formation of an unsegmented gastric cavity. These findings demonstrate the existence of a functionally conserved Hox-Gbx module in evolutionarily divergent anthozoan species, suggesting that the last common ancestor of all anthozoans likely utilized a similar genetic toolkit to axially pattern the endo-mesoderm into metameric subunits.

Segmentation of the gastric cavity is a synapomorphic trait of cnidarians of the class Anthozoa (corals and sea anemones), with different clades forming distinct numbers of segments. In the starlet sea anemone Nematostella vectensis , for example, eight bilaterally positioned gastric segments are generated by the action of a group of Hox-Gbx genes in the developing larval endo-mesoderm. Still, given the range of segment numbers observed in different anthozoans, it remains unclear whether this Hox-Gbx module is evolutionarily conserved and how it might be deployed to generate different numbers of segments. Here, we systematically interrogate the role of Hox-Gbx genes during development of the rice coral Montipora capitata . We first characterize the temporal sequence of segmentation in M. capitata juveniles and then combine transcriptomic profiling and in situ hybridization to identify three conserved homeobox-containing genes, McAnthox8 , McAnthox6a.1 and McGbx , which are collectively expressed in the developing endo-mesoderm prior to and during segment formation. The expression boundaries of these genes prefigure the positions of the first six segment boundaries, similar to their Nematostella homologs. Further, we show that chemical inhibition of BMP activity at the planula stage abolishes the expression of Hox-Gbx genes, leading to the formation of an unsegmented gastric cavity. These findings demonstrate the existence of a functionally conserved Hox-Gbx module in evolutionarily divergent anthozoan species, suggesting that the last common ancestor of all anthozoans likely utilized a similar genetic toolkit to axially pattern the endo-mesoderm into metameric subunits.