A new Marine Biological Association study in collaboration with researchers at the University of California, Berkeley has reconstructed detailed 3D images of cells that represent the closest cousins of the animals. This new work peels back the mysteries of how cells differentiated leading to the appearance of animals.

The research, published in PLoS Biology and led by MBA research student Davis Laundon, focused on choanoflagellates, a type of microscopic aquatic organism that is the closest single-celled relative of animals. Choanoflagellates are a free-living form of ‘collar cell’, a type of cell found within many kinds of animals from sponges to starfish, and can exist as single cells and in multicellular form. Studying the structure of the choanoflagellate collar cell can therefore shed light on how animal multicellularity and cell differentiation may have evolved.

This study compared the internal structure of single-celled and multicellular choanoflagellates, as well as the collar cells from a marine sponge, one of the most early-evolving animals. To create these striking images, Davis and colleagues used transmission electron microscopy on ultrathin sections of the cell. The 3D reconstructions were then analysed to reveal their internal structure, and clues to how they may differentiate into other cell types.

Choanoflagellates are aquatic microbes that represent the closest single-celled relatives of the animals. The image shows the internal ‘organelles’ of the cell, and the crown of microvilli (orange) which, along with a whip-like structure known as a flagellum (dark green), are characteristic of collar cells.

A significant finding is that when choanoflagellates divide into ‘colonies’, differentiated cell types are observed. MBA PhD student Davis described the findings “as an important step in our understanding of the biology of the last common ancestor of the animals”.

The first animals originated from the sea, and this work shows how important studies of marine ‘model’ organisms are for advancing our knowledge of fundamental biology.

Choanoflagellates are capable of transitioning into multicellular colonies, making them powerful model organisms to investigate the evolution of animal multicellularity.

Davis Laundon is an EnvEast PhD student in the Cunliffe Group.

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May 6, 2019 By MBA Comms