Insights into the evolution of epithelium from non-metazoan eukaryotes

The ability to form a single body with distinct organs that perform specialized tasks is one of the defining features of metazoan complex multicellularity. Crucial for this ability is the epithelium, a tissue made of a tightly adhered sheet of cells that is capable of compartmentalizing organs within metazoan bodies. Epithelia are recognizable across metazoans by several major characteristics mediated by conserved epithelial proteins. Cells within epithelial sheets adhere to each other through the interaction of cadherin with beta-catenin, creating a seal of reduced permeability to diffusing molecules. Within the sheet, cells are polarized along the same apical- basal axis. At the basal pole, integrins attach the cell to the basement membrane, which is composed of networks of collagen IV and laminin and serves as a scaffold and signaling hub. At the apical pole, the action of actin and myosin networks allows epithelial cells to undergo apical constriction, a cell-shape change that can cause bending of epithelial sheets. Across metazoans, epithelial sheet bending is crucial for morphogenetic rearrangements, such as gastrulation, during embryonic development.Since epithelia with these characteristics have been identified in all major metazoan phyla from sponges to chordates, it is likely that the last common ancestor of all metazoans already possessed an epithelium. However, key questions remain about the evolution of this crucial tissue type. When did the domains and proteins necessary for the epithelium originate? If they evolved in earlier, single-celled ancestors of metazoans, what might their original functions have been? How did apical constriction and tissue bending evolve? Might there have been precursors for those cell behaviors that predated epithelia and morphogenesis? To answer these questions, one promising avenue of research is to study the biology of metazoans’ extant relatives, which can illuminate events that occurred prior to metazoan origins. The sister taxon of the metazoans is the choanoflagellates, a group of single-celled and colonial microeukaryotes that are found in aquatic environments across the globe. In Chapter 2, we report the discovery of a new choanoflagellate species from Curaçao that forms colonies reminiscent of epithelial sheets: each colony is a cup-shaped monolayer of tightly adhered cells sharing the same apical-basal orientation. Colonies of the new species, which we name Choanoeca flexa, exhibit collective contractility, undergoing reversible inversion from a relaxed, flagella-in state to a contracted, flagella-out state in response to light-to-dark transitions. We show that C. flexa phototransduction is controlled by a rhodopsin photoreceptor with cGMP as a second messenger, similar to the pathway for vision in many metazoans. Furthermore, C. flexa colony inversion occurs through actomyosin-mediated apical constriction, much like the mechanism of epithelial sheet bending in metazoan morphogenesis. Based on observations of apical contractility in C. flexa colonies and in single-celled forms of other choanoflagellate species, we conclude that apical constriction evolved prior to the divergence of choanoflagellates and metazoans, while apical constriction-mediated collective contractility appears to have evolved independently in C. flexa and metazoans. In Chapter 3, we investigate the evolution of collagen, one of the key components of the epithelial basement membrane. Collagens were previously thought to be unique to metazoans until the first choanoflagellate genomes were sequenced and found to encode collagen domains. We conduct the first comprehensive survey of collagens across eukaryotes, and find that collagen domains are not only ubiquitous in choanoflagellates, but in fact are found in almost all major groups of eukaryotes. Furthermore, we find that collagen repeat length and amino acid composition in non-metazoans rivals those of metazoan collagens. Specific types of collagens— including long variants of collagen IV, which scaffolds the epithelial basement membrane— appear to be restricted to metazoans and may therefore explain unique features of metazoan biology.

Advisor: King, Nicole