The Origin of Animal Multicellularity and Cell Differentiation

Abstract

Over 600 million years ago, animals evolved from a unicellular or colonial organism whose cell(s) captured bacteria with a collar complex, a flagellum surrounded by a microvillar collar. Using principles from evolutionary cell biology, we reason that the transition to multicellularity required modification of pre-existing mechanisms for extracellular matrix synthesis and cytokinesis. We discuss two hypotheses for the origin of animal cell types: division of labor from ancient plurifunctional cells and conversion of temporally alternating phenotypes into spatially juxtaposed cell types. Mechanistic studies in diverse animals and their relatives promise to deepen our understanding of animal origins and cell biology.

Multicellular Features of Phytoplankton

“…This might be a key question in attempts to decipher the onset of phytoplankton aggregates; do individual cells converge or do cells undergo clonal divisions while attached? Both modes of multicellularlike assemblages occur in various microorganisms including choanoflagellates, ciliates, fungi, amoeba, and more (Brunet and King, 2017). Aggregation and clonal division are considered to be ancestral forms of current complex multicellularity, with clonal development being ancestral to all forms (Brunet and King, 2017).…”

“…Interestingly, many genes that are necessary for animal multicellularity are involved in cell-cell adhesion and have evolved prior to animal origins (Brunet and King, 2017). Furthermore, clumping has been used as a trait to experimentally evolve multicellularity in yeast and algae (Ratcliff et al, 2012(Ratcliff et al, , 2013Driscoll and Travisano, 2017).…”

“…Genetic analysis of Volvocine algae revealed the key role of extracellular matrix in the transition to multicellularity (Merchant et al, 2007;Prochnik et al, 2010). It would therefore be fascinating to explore the genetics of phytoplankton exopolymers and modes of attachment and compare them to components with known functions in multicellularity such as cadherins, integrins, and extracellular matrix domains (Brunet and King, 2017). A better characterization of the phytoplankton extracellular polymers is crucial in determining whether these structural components are specific only toward the organisms that produce them, or whether the fibers are generally adhesive thereby promoting multi-species assemblages.…”

“…Here we propose that microalgal populations often display the characteristics of a multicellular-like community rather than a random collection of individuals. Evolution of multicellularity entails a continuum of events starting from single cells that go through aggregation or clonal divisions (Brunet and King, 2017). Phytoplankton may be an intermediate state between single cells and aggregates of physically attached cells that communicate and co-operate; perhaps an evolutionary snapshot toward multicellularity.…”

Novel Diversity of Deeply Branching Holomycota and Unicellular Holozoans Revealed by Metabarcoding in Middle Paraná River, Argentina

“…These unicellular opisthokonts are understudied compared to their multicellular relatives, even though they are essential for better understanding key evolutionary transitions, including those that took place at the origins of both animals and fungi (Ruiz-Trillo et al, 2007;Richter and King, 2013;Brunet and King, 2017;Richards et al, 2017;Sebé-Pedrós et al, 2017).…”