King, N.; University of California, Berkeley: Bartholomew Award Lecture: The unicellular ancestry of Metazoa

Over 600 million years ago, the multicellular progenitors of Metazoa evolved from a unicellular protozoan. Both the nature of the last unicellular ancestors of Metazoa and the genetic and developmental events underlying the transition to multicellularity remain obscure. In particular, it is uncertain how and when the molecular machinery for two processes crucial to the integrated multicellularity of Metazoa — cell adhesion and cell signaling — first evolved. The study of choanoflagellates, unicellular and colonial protozoans closely related to Metazoa, has yielded important new insights into the genomic complexity of the metazoan progenitor. In particular, we have found that choanoflagellates express members of a surprising number of cell signaling and adhesion protein families that have not previously been isolated from nonmetazoans. Importantly, the activity of one signaling protein family, the tyrosine kinases, is coupled to changes in nutrient availability and required for robust cell proliferation. Thus, we infer that protein families involved in metazoan cell signaling and adhesion evolved first in protozoa and were later co-opted to function in multicellularity.