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Recent work suggested that diploblast embryos (Cnidaria) may have germ layers that are similar to those seen in bilaterians, but how these are specified remained unclear. Here, they use scRNA-seq and functional analyses to identify the signaling involved in germ layer segregation in Cnidaria.

Brachyury is an early mesoderm determinant and neural repressor in vertebrates. Comparative Brachyury target screens between a sea anemone and a sea urchin reveal an ancestral gene regulatory feedback loop involved in axial patterning, with conserved endodermal and neuronal, but not mesodermal, targets.

Different muscle cell types account for specific abilities in animals, yet how their diversification arose remains unclear. Here, the authors show that gene duplications of bHLH transcription factors and effector genes contributed to the diversification of muscle cell types in the sea anemone Nematostella.

This phylogenomic study shows that core muscle proteins were already present in unicellular organisms before the origin of multicellular animals, and supports a convergent evolutionary model for striated muscles in which new proteins are added to ancient contractile apparatus during independent evolution of bilaterians and some non-bilaterians, resulting in very similar ultrastructures.

miRNAs are crucial regulators of normal development in plants and animals, but their origins remain obscure. Exploration of the similarities and differences between different miRNA pathways help to elucidate their origins and role.

The cnidarian endoderm has been considered homologous to the bilaterian endoderm and mesoderm. Here, a gut-like ectoderm is revealed in a sea anemone, corresponding to the bilaterian endoderm, supporting an alternative model of germ layer homology.

The Technau lab provides their protocol for the generation of stably transgenic sea anemones. An expression vector is digested with the meganuclease I-SceI and then microinjected into embryos, where I-SceI mediates stable integration into the genome.

How did organismal complexity evolve at a cellular level, and how does a genome encode it? The answer might lie in differences, not in the number of genes an organism has, but rather in the regulation of gene expression.

Slowly evolving cnidarians are useful models to study genome architecture. This study shows that sea anemones have a high degree of chromosomal macrosynteny, but poor microsynteny conservation. This is correlated with a small genome size and short distances of cis-regulatory elements to genes.

In vertebrate embryos, Wnt/β-catenin signaling induces an organizer area guiding the formation of body axes and inducing extra axes upon transplantation. Here, Kraus et al. show that Wnt ligands also induce an organizer in a sea anemone, indicating that the organizer dates back over 600 million years.

The conservation of ancient metazoan gene order is remarkable, yet the functionality of conserved regions is unclear. Using single-cell expression data for many metazoans, the authors identify conserved genomic regions conferring ancient cell type identity.

The authors show in Nematostella that the more orally expressed β-catenin targets repress the more aborally expressed β-catenin targets, thus patterning the oral-aboral axis. This likely represents the common mechanism of β-catenin-dependent axial patterning shared by Cnidaria and Bilateria.

Whereas vertebrate genomes are highly methylated at CpG positions, invertebrate genomes are typically sparsely methylated. Here, the authors report a highly methylated genome in a marine sponge and show striking similarities with vertebrates.

The Biodiversity Cell Atlas aims to create comprehensive single-cell molecular atlases across the eukaryotic tree of life, which will be phylogenetically informed, rely on high-quality genomes and use shared standards to facilitate comparisons across species.

The genome of the jellyfish Clytia hemisphaerica and the transcriptomes of its planula, polyp and medusa life stages show genetic changes associated with the evolution of complex life cycles.

Hox clusters in the cnidarians Nematostella vectensis and Hydra magnipapillata have genes that correspond to anterior-group Hox genes in Bilateria, but the remainder of the Hox genes shows signs of an independent origin.

The freshwater cnidarian Hydra is a significant model for studies of axial patterning, stem cell biology and regeneration. Its (A+T)-rich genome has now been sequenced. Comparison of this genome with those of other animals provides insights into the evolution of epithelia, contractile tissues, developmentally regulated transcription factors, pluripotency genes and more.