Landscape of histone modifications in a sponge reveals the origin of animal cis-regulatory complexity

Animals come in many shapes and sizes, and vary in how they move, grow and reproduce. The long-held thought that animal complexity is related to the number of genes that are in the animal’s DNA has now been largely dismissed; simple animals like sponges and cnidarians (for example, jellyfish, anemones and corals) have comparable gene numbers to vertebrates, insects, mollusks and other complicated bilaterians (animals that feature a plane of symmetry, meaning that they have a top, a bottom, a front and a back). This observation led to the idea that gene regulation (how and when genes are turned off and on) is responsible for the evolution of animal diversity. Genomic DNA packs into cells by winding around proteins called histones. Histones themselves can bear certain chemical marks, which in turn determine if the genes contained in the DNA associated with the histones are going to be turned on or off. In bilaterians and cnidarians these marks substantially contribute to gene regulation. Some of these marks predate the evolution of multicellular animals from single-celled organisms. However, the origin of the marks that associate with the gene regulatory elements that are essential for animals to be multicellular remained unknown. In other words, does the evolution of histone marks underpin animal complexity? Gaiti et al. turned to the marine sponge Amphimedon queenslandica to address this question. Sponges are one of the morphologically simplest animals, lacking a gut, nerves and muscles. By analyzing histone marks in this sponge, Gaiti et al. found they were remarkably similar to the networks of histone marks seen in more complex animals. This is consistent with this form of gene regulation being present at the dawn of the animal kingdom. Indeed, this mode of gene regulation may have been necessary for multicellular animals to first evolve. It now appears that most of the genes and regulatory mechanisms underlying the formation of complex animals, like ourselves, had an unexpected early origin – probably as early as the first steps in the evolution of multicellular animals from single-celled organisms. Further studies of animals that are close relatives of sponges, such as comb jellies, and their single-celled cousins, may further improve our understanding of how these simple single-celled organisms became multicellular animals.