Multicellularity arose from unicellular ancestors multiple times independently in the history of life. During the evolution of multicellular lineages, multicellular and unicellular life-cycles have diverged significantly from each other: Some differences are lineage specific, for example, in contrast with plant and fungal cells, animal cells are motile, and animal development involves massive cellular rearrangement. While other differences between multicellular and unicellular organisms are shared across the different multicellular lineages, and form core features of multicellularity.

This research proposal deals with a core multicellular feature: directional development. Multicellular life-cycles invariably involve developmental stages, where starting from a single celled zygote, cells divide and differentiate into distinct cell-types and give rise to a multicelled adult body. Multicellular development is directional: cell-types that arise early in development (such as embryonic cells) are not seen again in later stages (such as the adult). There are many molecular mechanisms which maintain developmental directionality, for example epigenetic mechanisms, and mechanisms based on cellular signaling. The directionality of multicellular development is in sharp contrast with unicellular cell-type switching programs which are cyclic and contain bidirectional differentiations: This contrast leads us to ask why directional development repeatedly arose in multicellular lineages that evolved independently from unicellular ancestors?

Our understanding of directionality of development comes piecemeal from experimental observations, and we lack a measure of the extent to which multicellular development is directional. We also do not understand how and why multicellular development has evolved to be directional. The main objectives of this research project are: (a) using the recent explosion in single cell RNA sequencing data to empirically quantify the extent to which multicellular developmental programs are directional across different multicellular lineages, and (b) theoretical modeling of developmental evolution in order to understand evolutionary pressures that drove multicellular development to become directional. Overall, this project aims to deliver a new perspective for developmental evolution, and forges a connection between research on cell fate restriction during development with questions around the origins and evolution of multicellularity.