Origin and Evolution of Metazoan Cell Types

Contents

Series Preface .........................................................................................................vii

Preface........................................................................................................................ix

Acknowledgements...................................................................................................xii

Editor Biographies....................................................................................................xv

List of Contributors.................................................................................................xvii

Chapter 1 What Is a Cell Type?.............................................................................1

Chapter 2 The Protistan Origins of Animal Cell Differentiation........................ 13

Chapter 3 Convergent Evolution of Animal-Like Organelles across the

Tree of Eukaryotes..............................................................................27

Chapter 4 Evolution of the Animal Germline: Insights from Animal

Lineages with Remarkable Regenerating Capabilities........................ 47

Chapter 5 Origin and Evolution of Epithelial Cell Types.................................... 75

Chapter 6 Evolution of the Sensory/Neural Cell Types..................................... 101

Chapter 7 Cell Types, Morphology, and Evolution of Animal Excretory

Organs............................................................................................... 129

Index....................................................................................................................... 165

Biography

Andreas Hejnol is Professor and research group leader of “Comparative

Developmental Biology” at the Department of Biological Sciences (BIO) in Bergen,

Norway. After earning his Ph.D. in Comparative Zoology from the Free University

Berlin, Germany in 2002, he worked as a postdoctoral fellow in the laboratory of

Ralf Schnabel in Braunschweig and at the Kewalo Marine Laboratory in the lab

of Mark Q. Martindale in Hawaii. He led a research group at the Sars Centre from

2009-2019. His research aims to understand the evolutionary origin and diversification

of animal body plans, cell types, and organ systems. He is an ERC Consolidator

Grant holder and received for his achievements in Evolutionary Developmental

Biology and Comparative Zoology the prestigious Alexander O. Kovalevsky Medal

from the St. Petersburg Society for Naturalists in 2018.

Sally P. Leys is Professor in the Department of Biological Sciences at the University

of Alberta, in Edmonton, Canada. She earned her Ph.D. from the University of

Victoria under George Mackie in 1996, for which she received the Canadian Society

of Zoologists Cameron Award 1997. She held a Commander C Bellairs Postdoctoral

Fellowship from McGill University for postdoctoral research in Barbados (1997)

and then won an NSERC PDF which she took to the University Aix Marseille,

France (1998) and later to the University of Queensland, Australia (1998-2000). She

won an NSERC Women’s University Research Award in 2000 and was Assistant

Professor (Limited Term) at the University of Victoria, British Columbia. In 2002,

she was awarded a Canada Research Chair Tier II at the University of Alberta in

“Evolutionary and Developmental Biology.” Her research interests broadly concern

understanding the origin of multicellularity in metazoans and more specifically the

cellular and molecular basis of coordination in non-bilaterian animals, sponges,

ctenophores, placozoans, and cnidarians.

Like many seemingly straightforward questions, What is a cell type? can be remarkably difficult to answer. Since the 19th century cell types have been defined by morphology and function (when possible). More recently transcriptomes have taken their place with single-cell RNA sequencing (scRNA-seq) used to pro- duce atlases of cell types and subtypes from sponges to humans. Neither morphology nor transcriptomes are foolproof and, worryingly, their results are often incongruent. But the rapid spread of scRNA-seq and related techniques has reinvigorated studies of the evolution of animal cell types and questions about whether seemingly similar cell types across phyla are homologous. Indeed, in the first chapter in this collec- tion Minelli critically examines recent views on the nature of cell types, concluding that no universally satisfactory definition is possible. Just as several gener- ations of graduate students suffered through seminars on the definition of a species, I imagine future gener- ations suffering through seminars on the definition of a cell type.

This is one of the latest volumes in the Evolution- ary Cell Biology Series expertly guided by Brian K. Hall and Sally A. Moody. This series captures the re- cent and most welcome extension of evolutionary developmental biology beyond highly conserved regulatory genes into evolutionary dynamics at the cellular level. Although cell types feature in other volumes in the series, here they are the focus, begin- ning with the origins of animal cell types within the complex life cycles of the unicellular holozoan clades that are the cousins of Metazoa, as expertly detailed by Najle and Ruiz-Trillo. Perhaps the most novel chap- ter in this collection explores the convergent evolu- tion of eye-, statocyst- and nematocyst-like organelles across eukaryotes. The remaining four chapters ad- dress germline, epithelial, sensory/neural, and excre- tory cell types. Collectively these chapters summarize the emerging understanding of these cells, and pro- vide a number of important insights. For example, Minelli deftly discusses limitations to homology and Renard et al. illustrate how epithelial cell types depend upon molecular tools that arose before animals. The nature and limitations of cell type homology re- surface in later chapters. For example, in their chapter on sensory and neural cell types, Leys et al. emphasize the importance of cellular context in constructing cell types, effectively arguing for an ensemble definition. Similarly, the excellent chapter by Andrikou et al. on excretory cells closes with an examination of the ho- mology issue and its implications for characterizing cell types.

Although issues such as the role of gene regulatory networks are missing from these chapters and the coverage could have been broader, collectively the papers in this volume provide an excellent overview of current understanding. This field is advancing so rapidly through new experimental and analytical ap- proaches, broader phylogenetic coverage, and con- ceptual refinement that a new edition may soon be necessary.

Douglas H. Erwin, Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC