Search for the missing unicellular relatives of animals

ongoing

How animals emerged from their unicellular ancestors remains a major evolutionary question. Work done on diverse unicellular relatives of animals demonstrated that the unicellular ancestor of animals had a larger repertoire of genes associated with multicellularity than previously thought. These include “animal-specific” genes such as protein tyrosine kinases, integrins and Brachyury. This suggests a latent genetic potential in place at the origin of animals and hints at a much more gradual transition in their emergence. However, a comparison of extant early-branching animals and their unicellular relatives still reveals an abrupt difference between protists and the body plans of extant animals. This gap could be due to intermediate lineages going extinct, or that descendants of key lineages have not been found yet. Recent DNA environmental surveys suggest the latter, revealing several novel kingdom-level lineages that branch close to animals and remain unknown. We will not make progress in understanding the origin of animals until we have explored the real diversity of animals’ closest relatives and isolated the major lineages that remain uncharacterized. Indeed, some of the answers to animal origins that we can not currently address with our taxon sampling are likely hiding in hindsight in those unsampled lineages. Notably, a targeted survey of animal relatives has not been done. I propose to do this by exploiting recent developments in long-read metabarcoding. We will screen different environments and isolate the novel lineages using fixation-free labeling methods. We will culture them and get their genomes. We will provide a complete picture of the diversity among animal relatives, which will also be relevant to ecologists. Notably, the novel lineages will allow us to address fundamental questions about the origin of animals that cannot be answered with the current taxon sampling, including the origin of embryogenesis and spatial cell differentiation.