A possible billion-year-old holozoan with differentiated multicellularity

• Published in: Current biology Vol. 31; no. 12; pp. 2658 - 2665.e2
• Main Authors: Strother, Paul K., Brasier, Martin D., Wacey, David, Timpe, Leslie, Saunders, Martin, Wellman, Charles H.
• Format: Journal Article
• Language: English
• Published: England Elsevier Inc 21-06-2021
• Subjects: Cell Differentiation; DAH; Diabaig Formation; Fossils; Holozoa; Ichthyosporea; lake deposits; multicellularity; OWMs; Phylogeny; Proterozoic; Holozoa; multicellularity; Diabaig Formation; lake deposits; Proterozoic; DAH; OWMs; Ichthyosporea
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2021.03.051

Sediments of the Torridonian sequence of the Northwest Scottish Highlands contain a wide array of microfossils, documenting life in a non-marine setting a billion years ago (1 Ga).1–4 Phosphate nodules from the Diabaig Formation at Loch Torridon preserve microorganisms with cellular-level fidelity,5,6 allowing for partial reconstruction of the developmental stages of a new organism, Bicellum brasieri gen. et sp. nov. The mature form of Bicellum consists of a solid, spherical ball of tightly packed cells (a stereoblast) of isodiametric cells enclosed in a monolayer of elongated, sausage-shaped cells. However, two populations of naked stereoblasts show mixed cell shapes, which we infer to indicate incipient development of elongated cells that were migrating to the periphery of the cell mass. These simple morphogenetic movements could be explained by differential cell-cell adhesion.7,8 In fact, the basic morphology of Bicellum is topologically similar to that of experimentally produced cell masses that were shown to spontaneously segregate into two distinct domains based on differential cadherin-based cell adhesion.9 The lack of rigid cell walls in the stereoblast renders an algal affinity for Bicellum unlikely: its overall morphology is more consistent with a holozoan origin. Unicellular holozoans are known today to form multicellular stages within complex life cycles,10–13 so the occurrence of such simple levels of transient multicellularity seen here is consistent with a holozoan affinity. Regardless of precise phylogenetic placement, these fossils demonstrate simple cell differentiation and morphogenic processes that are similar to those seen in some metazoans today.
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•The multicellular microfossil Bicellum brasieri possesses two distinct cell types•3D preservation in phosphate preserved different life cycle stages•Differential adhesion may have contributed to cell segregation during morphogenesis•This billion-year-old freshwater protist shows evidence of holozoan affinity
Strother et al. describe life cycle morphogenesis in a new billion-year-old microfossil, which may provide clues to the evolutionary roots of embryonic development in animals.