Conserved meiotic mechanisms in the cnidarian Clytia hemisphaerica revealed by Spo11 knockout

From conservation to adaptation: understanding the synaptonemal complex's evolutionary dynamics

The synaptonemal complex (SC) is structurally conserved across eukaryotes and is essential for a proper progression of meiosis. Despite this conservation, SC protein sequences diverge drastically. In this review, we explore findings on SC protein evolution, highlighting key differences and commonalities among lineages like the Caenorhabditis and the Drosophila genera. We further explore known cases where the SC and its proteins adopt novel functional roles and discuss why knowledge of these cases could be important for the study of canonical SC biology. The existing studies demonstrate that work on the evolutionary biology of SC proteins and functional studies in more diverse meiotic research organisms should play a major role in aiding our understanding of SC structure and functions.

ARPP19 phosphorylation site evolution and the switch in cAMP control of oocyte maturation in vertebrates

cAMP-PKA signaling initiates the crucial process of oocyte meiotic maturation in many animals, but inhibits it in vertebrates. To address this 'cAMP paradox', we exchanged the key PKA substrate ARPP19 between representative species, the vertebrate Xenopus and the cnidarian Clytia, comparing its phosphorylation and function. We found that, as in Xenopus, Clytia maturing oocytes undergo ARPP19 phosphorylation on a highly conserved Gwl site, which inhibits PP2A and promotes M-phase entry. In contrast, despite a PKA phosphorylation signature motif recognizable across most animals, Clytia ARPP19 was only poorly phosphorylated by PKA in vitro and in vivo. Furthermore, unlike Xenopus ARPP19, exogenous Clytia ARPP19 did not delay Xenopus oocyte maturation. We conclude that, in Clytia, ARPP19 does not intervene in oocyte maturation initiation because of both poor recognition by PKA and the absence of effectors that mediate vertebrate oocyte prophase arrest. We propose that ancestral ARPP19 phosphorylated by Gwl has retained a key role in M-phase across eukaryotes and has acquired new functions during animal evolution mediated by enhanced PKA phosphorylation, allowing co-option into oocyte maturation regulation in the vertebrate lineage.

VitelloTag: a tool for high-throughput cargo delivery into oocytes

Delivering molecular tools into oocytes is essential for developmental and reproductive biology. Microinjection, the conventional method, is equipment intensive, often technically challenging and has a low yield, and is impractical in species with delicate oocytes or restricted spawning seasons. To overcome these limitations, we developed VitelloTag, a cost-effective, high-throughput system using vitellogenin-derived fusion proteins to enable efficient cargo delivery via receptor-mediated endocytosis. We demonstrate its utility by delivering Cas9/sgRNA complexes in two distantly related species for gene knockout.

Transcriptomic analysis of meiotic genes during the mitosis-to-meiosis transition in Drosophila females

Germline cells produce gametes, which are specialized cells essential for sexual reproduction. Germline cells first amplify through several rounds of mitosis before switching to the meiotic program, which requires specific sets of proteins for DNA recombination, chromosome pairing, and segregation. Surprisingly, we previously found that some proteins of the synaptonemal complex, a prophase I meiotic structure, are already expressed and required in the mitotic region of Drosophila females. Here, to assess if additional meiotic genes were expressed earlier than expected, we isolated mitotic and meiotic cell populations to compare their RNA content. Our transcriptomic analysis reveals that all known meiosis I genes are already expressed in the mitotic region; however, only some of them are translated. As a case study, we focused on mei-W68, the Drosophila homolog of Spo11, to assess its expression at both the mRNA and protein levels and used different mutant alleles to assay for a premeiotic function. We could not detect any functional role for Mei-W68 during homologous chromosome pairing in dividing germ cells. Our study paves the way for further functional analysis of meiotic genes expressed in the mitotic region.

The genome of the colonial hydroid Hydractinia reveals that their stem cells use a toolkit of evolutionarily shared genes with all animals

Hydractinia is a colonial marine hydroid that shows remarkable biological properties, including the capacity to regenerate its entire body throughout its lifetime, a process made possible by its adult migratory stem cells, known as i-cells. Here, we provide an in-depth characterization of the genomic structure and gene content of two Hydractinia species, Hydractinia symbiolongicarpus and Hydractinia echinata, placing them in a comparative evolutionary framework with other cnidarian genomes. We also generated and annotated a single-cell transcriptomic atlas for adult male H. symbiolongicarpus and identified cell-type markers for all major cell types, including key i-cell markers. Orthology analyses based on the markers revealed that Hydractinia's i-cells are highly enriched in genes that are widely shared amongst animals, a striking finding given that Hydractinia has a higher proportion of phylum-specific genes than any of the other 41 animals in our orthology analysis. These results indicate that Hydractinia's stem cells and early progenitor cells may use a toolkit shared with all animals, making it a promising model organism for future exploration of stem cell biology and regenerative medicine. The genomic and transcriptomic resources for Hydractinia presented here will enable further studies of their regenerative capacity, colonial morphology, and ability to distinguish self from nonself.

Establishing Bilateral Symmetry in Hydrozoan Planula Larvae, a Review of Siphonophore Early Development

Siphonophores are colonial hydrozoans, characterized by complex colony organization and unparalleled zooid functional specialization. Recent genomic studies have offered an evolutionary perspective on how this morphological complexity arose, but a molecular characterization of symmetry breaking in siphonophore embryonic development is still largely missing. Here, bringing together historical data on early development with new immunohistochemical data, we review the diversity of developmental trajectories that lead to the formation of bilaterally symmetric planula larvae in siphonophores. Embryonic development, up to the planula stage, is remarkably similar across siphonophore phylogeny. Then, with the appearance of the lateral endodermal thickening (= ventral endoderm), larval development diverges between taxa, differing in the location and patterning of the primary buds, chronology of budding, establishment of growth zones, and retention of larval zooids. Our work also uncovers a number of open questions in siphonophore development, including homology of different zooids, mechanisms underlying formation and maintenance of spatially restricted growth zone(s), and molecular factors establishing a secondary dorsal-ventral axis in planulae. By discussing siphonophore development and body axes within the broader cnidarian context, we then set the framework for future work on siphonophores, which is finally achievable with the advent of culturing methods.

Epithelial wound healing in Clytia hemisphaerica provides insights into extracellular ATP signaling mechanisms and P2XR evolution

Epithelial wound healing involves the collective responses of many cells, including those at the wound margin (marginal cells) and those that lack direct contact with the wound (submarginal cells). How these responses are induced and coordinated to produce rapid, efficient wound healing remains poorly understood. Extracellular ATP (eATP) is implicated as a signal in epithelial wound healing in vertebrates. However, the role of eATP in wound healing in vivo and the cellular responses to eATP are unclear. Almost nothing is known about eATP signaling in non-bilaterian metazoans (Cnidaria, Ctenophora, Placozoa, and Porifera). Here, we show that eATP promotes closure of epithelial wounds in vivo in the cnidarian Clytia hemisphaerica (Clytia) indicating that eATP signaling is an evolutionarily ancient strategy in wound healing. Furthermore, eATP increases F-actin accumulation at the edges of submarginal cells. In Clytia, this indicates eATP is involved in coordinating cellular responses during wound healing, acting in part by promoting actin remodeling in cells at a distance from the wound. We also present evidence that eATP activates a cation channel in Clytia epithelial cells. This implies that the eATP signal is transduced through a P2X receptor (P2XR). Phylogenetic analyses identified four Clytia P2XR homologs and revealed two deeply divergent major branches in P2XR evolution, necessitating revision of current models. Interestingly, simple organisms such as cellular slime mold appear exclusively on one branch, bilaterians are found exclusively on the other, and many non-bilaterian metazoans, including Clytia, have P2XR sequences from both branches. Together, these results re-draw the P2XR evolutionary tree, provide new insights into the origin of eATP signaling in wound healing, and demonstrate that the cytoskeleton of submarginal cells is a target of eATP signaling.

The genome of the colonial hydroid Hydractinia reveals their stem cells utilize a toolkit of evolutionarily shared genes with all animals

Hydractinia is a colonial marine hydroid that exhibits remarkable biological properties, including the capacity to regenerate its entire body throughout its lifetime, a process made possible by its adult migratory stem cells, known as i-cells. Here, we provide an in-depth characterization of the genomic structure and gene content of two Hydractinia species, H. symbiolongicarpus and H. echinata, placing them in a comparative evolutionary framework with other cnidarian genomes. We also generated and annotated a single-cell transcriptomic atlas for adult male H. symbiolongicarpus and identified cell type markers for all major cell types, including key i-cell markers. Orthology analyses based on the markers revealed that Hydractinia's i-cells are highly enriched in genes that are widely shared amongst animals, a striking finding given that Hydractinia has a higher proportion of phylum-specific genes than any of the other 41 animals in our orthology analysis. These results indicate that Hydractinia's stem cells and early progenitor cells may use a toolkit shared with all animals, making it a promising model organism for future exploration of stem cell biology and regenerative medicine. The genomic and transcriptomic resources for Hydractinia presented here will enable further studies of their regenerative capacity, colonial morphology, and ability to distinguish self from non-self.

Genetic and Epigenetic Regulation of Drosophila Oocyte Determination

Primary oocyte determination occurs in many organisms within a germ line cyst, a multicellular structure composed of interconnected germ cells. However, the structure of the cyst is itself highly diverse, which raises intriguing questions about the benefits of this stereotypical multicellular environment for female gametogenesis. Drosophila melanogaster is a well-studied model for female gametogenesis, and numerous genes and pathways critical for the determination and differentiation of a viable female gamete have been identified. This review provides an up-to-date overview of Drosophila oocyte determination, with a particular emphasis on the mechanisms that regulate germ line gene expression.