Genomic and single cell insights into the metamorphosis of the crinoid A. mediterranea

Many marine animals have the striking ability to generate different body plans out of the same genome during their life cycle: the larva and the adult. Planktonic larval stages allow animals to disperse before metamorphosing into usually benthic adult forms. Larval stages are widely distributed across animal phyla (from sponges to molluscs) but one of the most distinctive larvae is that of echinoderms. In this phylum of marine organisms, larval metamorphosis proceeds through a complete reorganisation of the bilateral larval body to give rise to a five-fold symmetrical adult body. This dramatic change makes echinoderms an essential model to study how gene expression can create two different bodies and body axes from the information encoded in the same genome. While larval life can last several weeks in some echinoderms such as sea stars, crinoid larvae only take a few days and will settle easily on glass or plastic substrate making them a great model to study metamorphosis. To help unravel the molecular events involved in this process, we generated a chromosome-scale genome for the crinoid A. mediterranea and applied single-cell transcriptomics to successive stages from larva to juvenile. These datasets allow us to monitor gene expression, changes in cell populations and cell molecular signature through settlement and body plan shift. Moreover, we performed a comparison of cell types of the short-lived larva of crinoid with that of other echinoderms and larvae across metazoan to shed light on the evolutionary origins of larval stages.