Abstract
Multicellular eukaryotes are characterized by an expanded extracellular matrix (ECM) with a diversified composition. The ECM is involved in determining tissue texture, screening cells from the outside medium, development, and innate immunity, all of which are essential features in the biology of multicellular eukaryotes. This review addresses the origin and evolution of the ECM, with a focus on multicellular marine algae. We show that in these lineages the expansion of extracellular matrix played a major role in the acquisition of complex multicellularity through its capacity to connect, position, shield, and defend the cells. Multiple innovations were necessary during these evolutionary processes, leading to striking convergences in the structures and functions of the ECMs of algae, animals, and plants.
Highlights
Multicellularity has evolved at least 25 times during the evolutionary history of the eukaryotes [1] but only a handful of these multicellular lineages include what could be considered to be complex multicellular organisms
Potential cell wall (CW)-related proteins of Ulva have been listed in a supplementary table of the sea lettuce genome article [8], but the biosynthesis of extracellular matrix (ECM) components was not discussed by the authors and a thorough analysis remains to be done for green macroalgae
The evidence discussed in the various sections above indicates that—as was the case for animals, land plants, and fungi—the ECMs of macroalgae are important from a structural point of view, providing a matrix that holds cells together within tissues and protecting them from the environment, but they probably play important roles in signaling events related to multicellular development and defense responses
Summary
Multicellularity has evolved at least 25 times during the evolutionary history of the eukaryotes [1] but only a handful of these multicellular lineages include what could be considered to be complex multicellular organisms. We define complex multicellularity as the possession of a macroscopic body plan consisting of multiple cell types that is constructed by developmental programs involving cell division and differentiation [2] Under this definition, animals, land plants, and fungi are considered to exhibit complex multicellularity and the three independently evolved lineages of macroalgae: brown, red and green algae (Figure 1). ECMs are complex supramolecular networks that provide multicellular tissues with both rigidity and flexibility In addition to this structural role, the ECM has important functions in the regulation of development as well as in shielding cells from the outside medium, including protection against abiotic and biotic stresses. There are striking similarities between the functions of the ECMs of algae, animals, plants, and fungi It appears that these are essentially convergent features, which emerged independently in each of the phyla that evolved complex multicellularity
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