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This document provides a summary of recent research on unicellular holozoans, which are the closest living relatives of animals. The research focuses on two specific organisms, Choanocea flexa and Chromosphaera perkinsii. The first study reveals that sheet colonies of Choanocea flexa can form through both clonal division and aggregation, challenging previous assumptions about multicellularity. The second study shows evidence of directed cell differentiation within the multicellular stages of Chromosphaera perkinsii, similar to animals. These findings suggest that multicellularity and cell differentiation may be more common among unicellular holozoans than previously thought, highlighting the need for further research to understand the similarities and differences to animal development. [Extracted from the article]

Gene expression is tightly controlled during animal development to allow the formation of specialized cell types. Our understanding of how animals evolved this exquisite regulatory control remains elusive, but evidence suggests that changes in chromatin-based mechanisms may have contributed. To investigate this possibility, here we examine chromatin-based gene regulatory features in the closest relatives of animals, choanoflagellates. Using Salpingoeca rosetta as a model system, we examined chromatin accessibility and histone modifications at the genome scale and compared these features to gene expression. We first observed that accessible regions of chromatin are primarily associated with gene promoters and found no evidence of distal gene regulatory elements resembling the enhancers that animals deploy to regulate developmental gene expression. Remarkably, a histone modification deposited by polycomb repressive complex 2, histone H3 lysine 27 trimethylation (H3K27me3), appeared to function similarly in S. rosetta to its role in animals, because this modification decorated genes with cell type-specific expression. Additionally, H3K27me3 marked transposons, retaining what appears to be an ancestral role in regulating these elements. We further uncovered a putative new bivalent chromatin state at cell type-specific genes that consists of H3K27me3 and histone H3 lysine 4 mono-methylation (H3K4me1). Together, our discoveries support the scenario that gene-associated histone modification states that underpin development emerged before the evolution of animal multicellularity.

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., (© 2024 Schnitzler et al.; Published by Cold Spring Harbor Laboratory Press.)

The sea anemone Nematostella vectensis is a genetically tractable cnidarian species that has become a model organism for studying the evolution of developmental processes and genome regulation, resilience to fluctuations in environmental conditions, and the response to pollutants. Gene expression analyses are central to many of these studies, and in situ hybridization has been an important method for obtaining spatial information, in particular during embryonic development. Like other cnidarians, Nematostella embryos are of comparably low morphological complexity, but they possess many cell types that are dispersed throughout the tissue and originate from broad and overlapping areas. These features have made two-color fluorescence in situ hybridization an important method to determine potential co-expression of genes and to generate hypotheses for their functions in cell fate specification. We here share protocols for single and double fluorescence in situ hybridization in Nematostella and for the combination of fluorescence in situ hybridization and immunofluorescence., (© 2024. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

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., Competing Interests: Competing interests: The authors declare no competing financial interests.