Abstract
Multicellularity has independently evolved numerous times throughout the major lineages of life. Often, multicellularity can enable complex, macroscopic organismal architectures but it is not required for the elaboration of morphology. Several alternative cellular strategies have arisen as solutions permitting exquisite forms. The green algae class Ulvophyceae, for example, contains truly multicellular organisms, as well as macroscopic siphonous cells harboring one or multiple nuclei, and siphonocladous species, which are multinucleate and multicellular. These diverse cellular organizations raise a number of questions about the evolutionary and molecular mechanisms underlying complex organismal morphology in the green plants. Importantly, how does morphological patterning arise in giant coenocytes, and do nuclei, analogous to cells in multicellular organisms, take on distinct somatic and germline identities? Here, we comparatively explore examples of patterning and differentiation in diverse coenocytic and single-cell organisms and discuss possible mechanisms of development and nuclear differentiation in the siphonous algae.
Highlights
Among the green plants (Viridiplantae), green algae (Chlorophyta) show remarkable diversity in morphology and cellular organization
Several lines of evidence suggest that land plant morphology is specified at an organismal level rather than through emergent properties of cells, lessening the contribution of multicellularity to plant architecture compared to animal morphology
Acetabularia acetabulum (Dasycladales), which is a uninucleate single-celled organism throughout most of its lifecycle, and Caulerpa taxifolia (Bryopsidales), a multinucleate siphonous coenocyte, show striking patterns of local transcript accumulation superimposed on morphological structures convergent with the roots, stems, and leaves of land plants (Vogel et al, 2002; Ranjan et al, 2015)
Summary
Among the green plants (Viridiplantae), green algae (Chlorophyta) show remarkable diversity in morphology and cellular organization. Phylogenetic reconstruction suggests that the common ancestor of these species was a unicellular, uninucleate organism (Cocquyt et al, 2010), demonstrating that in the green plants macroscopic growth forms independently arose from diverse cellular strategies. Land plants (Embryophyta) are truly multicellular organisms, they possess siphonous-like properties at the cellular level (Kaplan and Hagemann, 1991; Niklas et al, 2013). In contrast to animal cell division, cytokinesis in land plants involves phragmoplast formation, which leaves cytoplasmic connections between daughter cells. This symplastic connectivity is further maintained via the formation of plasmodesmata. Irradiation studies and clonal analyses have demonstrated that organ initiation and elaboration are largely independent of cell
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