Zoology (Book Chapters)

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Evolutionary origin of vertebrate cranial placodes
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 6 addresses the question of how cranial placodes evolved as novel structures in vertebrates by redeploying pre-existing and sometimes evolutionarily ancient cell types. The chapter integrates insights from comparative studies with our knowledge on vertebrate placode development. After briefly summarizing the evolutionary history of sensory and neurosecretory cells, the chapter discusses how these cell types may have become concentrated in non-neural ectoderm adjacent to the neural plate and how they may have become segregated into an anterior and posterior proto-placodal territory in the last common tunicate-vertebrate ancestor. It is then discussed how an increase in progenitor expansion may have converted these proto-placodal territories into proper placodes, which give rise to larger and complex sense organs, in ancestral vertebrates. It is argued that rewiring of the gene regulatory network upstream and downstream of the transcriptional regulators Six1/2, Six4/5 and Eya probably played a central role for the evolution of placodes. A detailed scenario of placode evolution is then presented, which summarizes the proposed regulatory changes and links them to functional changes of life style during chordate and vertebrate evolution. The chapter ends with general conclusions on the evolution of novelties as illustrated by placodes and placodal cell types.
Evolution of neurosecretory cell types
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 5 investigates the evolutionary history of the neurosecretory cells derived from cranial placodes in vertebrates. These comprise the hormone-producing cells of the adenohypophysis and the olfactory neuropeptidergic (GnRH) neurons. The chapter argues that specialized neurosecretory cell types evolved many times independently. Apart from an anteromedial population of neurosecretory cells (specified by Otp and other transcription factors), there is little evidence for neurosecretory cell types with a deep evolutionary history. The placodal neurosecretory cells probably originated as novel cell types only in vertebrates or the last common tunicate-vertebrate ancestor. Data from amphioxus suggest that in ancestral chordates neurosecretory cells were concentrated in rostral endomesodermal pouches (forming Hatschek’s pit in amphioxus). These probably secreted protein hormones allowing the regulation of metabolic and reproductive processes in response to environmental cues. A new regulatory environment was then established in the non-neural ectoderm anterior to the neural plate in the lineage leading to tunicates and vertebrates by the recruitment of transcription factors from the endoderm (FoxA, GATA2/3) and anterior neuroectoderm (FoxG, SP6-9, DMRT4/5). This facilitated the recruitment of neurosecretory cells (specified by Pitx, Lhx3/4, Islet, POU1f1, Six1/2, Eya) from the rostral endomesodermal pouches to this domain and the origin of a new type of neurosecretory (GnRH) neuron.
Evolution of photosensory cell types
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 4 reviews the evolution of photoreceptors in animals. It is shown that two lineages of photoreceptors, known as rhabdomeric and ciliary photoreceptors, respectively, are present throughout bilaterians. Ciliary photoreceptors use ciliary opsins and their specification depends on a core regulatory network (CoRN) involving transient Pax6, Rx and Otx transcription factors. Rhabdomeric photoreceptors instead use rhabdomeric opsins and depend on a CoRN involving persistent Pax6, Atonal, POU4 expression. Whereas shared dependence on Pax6 and similarities in phototransduction mechanisms suggests that ciliary and rhabdomeric photoreceptor originated by duplication and divergence of an ancestral photoreceptor, shared dependence on Atonal, POU4 and other transcription factors instead suggests a common origin of rhabdomeric photoreceptors and mechanoreceptors. Evidence from cnidarians indicates that both “ciliary” and rhabdomeric phototransduction mechanisms are present, but that there are no distinct “ciliary” and rhabdomeric photoreceptor cell types. Moreover, Atonal, POU4 and PaxB are expressed in mechano- as well as photosensory cells in cnidarians and many sensory cells appear to be multimodal. This suggests that rhabdomeric and ciliary photoreceptors and mechanoreceptors originated as distinct cell types from a common sensorineural precursor cell type only in the stem lineage of bilaterians.
Evolution of mechano- and chemosensory cell types
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 3 opens a series of three chapters, which compare the sensory and neurosecretory cell types of the vertebrate head with similar cell types in other animals to get insights into their evolutionary origins. Chapter 3 reviews the evolution of mechano- and chemosensory cells throughout the animal kingdom. Cells mediating these two sensory modalities are considered together because substantial evidence suggests that chemosensory cells have evolved repeatedly from mechanosensory cells. Chapter 3 and the following two chapters use comparisons between vertebrates and their sister group, the tunicates, to infer, which cell types most likely were present in their last common ancestor. Similar comparisons are then made with increasingly distantly related groups (amphioxus, hemichordates/echinoderms, protostomes, cnidarians, sponges) to elucidate cell types present in ancestors of more and more inclusive groups (chordates, deuterostomes, bilaterians, eumetazoans, metazoans). This survey suggests that mechanosensory cell type identity was specified by a core regulatory network (CoRN) of Atonal or Ascl, Pax2/5/8, POU4, Islet, Gfi, Prox and BarH already in the last common ancestor of bilaterians. A CoRN including some of these or related transcription factors (e.g POU4, Islet and PaxB-like) may already have been required to establish the identity of possibly multimodal sensory cell types in the eumetazoan ancestor.
Teaching old cells new tricks
(CRC Press, 2021-06-18) Schlosser, Gerhard
This second introductory chapter clarifies conceptual issues relating to homology and evolutionary innovation and illustrates them with examples from sensory evolution. The first part of the chapter introduces the general concepts of character identity, homology and evolutionary innovation, while the second part applies these general concepts to the evolution of cell types. It is argued that characters can only be tracked through evolving lineages (allowing to identify homologs), if characters are conceptualized as independently evolving units. This in turn requires that characters are genetically individuated, meaning that they have at least a partially different genetic basis from other characters. Cell types are then introduced as a particular kind of independently evolving unit (or character). The identity of a cell type is typically determined by a core regulatory network (CoRN) of cross-regulating genes (often encoding transcription factors). Cell types in different species remain the same (homologous) as long as they retain the same CoRN, even though downstream target genes may change. Novel cell types originate, when a new CoRN is established either by duplication and divergence of a preexisting CoRN or by recombination and redeployment of genes previously used for other functions.
The evolutionary origin of vertebrates
(CRC Press, 2021-06-18) Schlosser, Gerhard
The first introductory chapter of the second volume summarizes our current understanding of vertebrate evolution. After providing a short primer on the principles of phylogenetic systematics, the chapter provides an overview of the phylogenetic relationships of vertebrates with other chordates and deuterostomes and discusses how deuterostomes are phylogenetically related to other metazoans. Recent evidence is discussed suggesting that within the chordates tunicates are most closely related to vertebrates, with amphioxus being more distantly related. The chordates together with their sister group, the ambulacrarians (hemichordates and echinoderms), comprise the deuterostomes. The chapter then introduces vertebrates and their fossil relatives and presents a brief survey of body plans and embryonic development in other deuterostome groups (tunicates, amphioxus, hemichordates and echinoderms). The final section of the chapter provides a brief history of ideas on the evolution of vertebrates and discusses different scenarios on vertebrate origins from Haeckel until today.
Differentiation of cell types from non-neurogenic placodes
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 8 reviews how cell types derived from the non-neurogenic placodea, i.e. the lens and adenohypophyseal placodes, differentiate. The lens placode gives rise to lens fiber cells, which are rendered transparent by the accumulation of crystallin proteins. They are specified by a core regulatory network (CoRN) of Pax6, FoxE3, SoxB1 together with proteins of the large Maf family. Because some large Mafs also are essential for the specification of retinal photoreceptors and crystallins are also expressed in photoreceptors (where they have a neuroprotective effect), it is proposed that lens fiber cells may have evolved from ciliary photoreceptors. The adenohypophyseal placode generates the neurosecretory cells of the anterior pituitary. These produce hormones belonging to three different families (peptide hormones, dimeric glycoprotein hormones and four-helix cytokine-like proteins). While Pitx1/2 and Lhx3/4 are important members of the CoRN regulating the specification of all types of neurosecretory cells in the anterior pituitary, additional transcription factors are required in defining the different subtypes of neurosecretory cells. Comparative studies suggest that three subtypes of neurosecretory cells were probably present at the base of vertebrates, each dedicated to the production of one hormone class. Subsequently, subtypes diversified further into the six neurosecretory cell types found in gnathostomes.
Differentiation of photoreceptors
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 7 discusses how photoreceptors and other cells found in the retina of the vertebrate eye or in the pineal body differentiate. These cell types do not develop from cranial placodes but from the neural tube. However, part three of the book will reveal that these cell types are evolutionarily closely related to other sensory cell types (e.g. mechanoreceptors) that are placode derived. To prepare for this argument, this chapter reviews vertebrate photoreceptors and related cell types and summarizes how their specification is transcriptionally regulated. This survey shows that these cells fall into two broad classes suggesting an evolutionary origin from two distinct types of photoreceptors in the vertebrate ancestor. The rods and cones of the retina and most other photoreceptors in the brain (e.g of the pineal body) are ciliary photoreceptors, which rely on ciliary opsin for phototransduction and depend on a core regulatory network (CoRN) involving Rx, Otx2 and Otx5/Crx. Bipolar cells may be evolutionarily derived from such ciliary photoreceptors, but have lost photosensitivity. Intrinsically photosensitive retinal ganglion cells (RGCs), on the other hand, are modified rhabdomeric photoreceptors, which rely on rhabdomeric opsin (melanopsin) and depend on a CoRN involving Pax6, Atoh7, POU4f2 and Islet1. Other RGCs, amacrine and horizontal cells may also be evolutionarily derived from rhabdomeric photoreceptors, but have lost photosensitivity.
Differentiation of sensory and neuronal cell types from neurogenic placodes
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 6 discusses, how general mechanisms of sensory/neuronal differentiation are modulated in a placode-specific way to produce the specific sensory and neuronal cell types generated by individual placodes. For each placode, structure and function of its various derivative cell types is first reviewed, followed by an overview of transcription factors regulating specification and differentiation of these cell types. This survey suggests that the specification of general somatosensory neurons (GSNs) developing from the profundal and trigeminal placodes depends on a core-regulatory network (CoRN) of Neurog1/2, POU4f1, Islet1, Tlx1/3, and Pax3. The viscerosensory neurons derived from epibranchial placodes, instead require Phox2b (which represses POU4f1), but share other transcription factors with GSNs (e.g. Neurog1/2, Islet1, Tlx1/3). The CoRNs of the special somatosensory neurons (SSNs) derived from the otic and lateral line placodes (including Neurog1, Islet1 and POU4f1) likewise overlap with those of GSNs, while some members of the CoRNs specifying the hair cells derived from the same placodes (e.g. Atoh1, POU4f3, Gfi1 and Prox1) are shared with other sensory cell types. The CoRNs specifying SSNs and hair cells, therefore, most likely involve additional, regionally confined transcription factors such as Pax2/8. The receptor neurons originating from the olfactory placode, in contrast, rely on a different CoRN (e.g. Ascl1, Lhx2, DMRT4/5).
General mechanisms of sensory and neuronal differentiation
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapters 5-8 review how the differentiation of various placodal cell types is regulated. The first of these, chapter 5, summarizes common mechanisms regulating the formation of neurons and sensory cells in most placodes. The first section discusses how SoxB1 transcription factors cooperate with other proteins (including HES and Id factors) to keep cells in a proliferating progenitor state, while biasing them towards neuronal or sensory fate. The second section then summarizes how bHLH transcription factors of the Achaete-Scute- (Ascl), Atonal- (Atoh), and Neurogenin- (Neurog) subfamilies initiate the neuronal and sensory differentiation program in a subset of cells while repressing differentiation and maintaining progenitor states in adjacent cells (lateral inhibition). Finally, it is briefly sketched, how bHLH transcription factors cooperate with additional, regionally restricted transcription factors including LIM-type (e.g., Islet1), Paired-type (e.g., Phox2a, Phox2b, DRG11), and POU-type (e.g., Brn3a and Brn3c) homeodomain proteins, COE-type bHLH proteins, Runx proteins and Fox proteins (e.g., FoxG1) in the specification of sensory and neuronal cell types specific for individual placodes.
Development of individual placodes from their common primordium
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 4 discusses how the pre-placodal ectoderm (PPE), once it is established, gives rise to different types of placodes. Whereas adenohypophyseal, olfactory and lens placodes will form from its anterior portion, otic, lateral line and epibranchial placodes will form from the posterior PPE, with profundal and trigeminal placodes sandwiched in between. The subdivision of the PPE into individual placodes is a complex process comprising several phases. The first phase involves the multistep regional specification of different placodes within the PPE due to the region-specific induction of different transcription factors. In a second phase, individual placodes separate from each other by a combination of transcriptional cross-repression, apoptosis, and cell movements. The chapter summarizes the development of placodes during these two phases, placing particular emphasis on the role of transcription factors during the first phase, which is most relevant for our understanding of how different placodal cell types are generated in a region-specific manner.
Origin of cranial placodes from a common primordium
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 3 shows that, in spite of the distinct derivatives and functions of different placodes, all cranial placodes have a common origin in embryonic development. The chapter first highlights some important similarities between the development of different placodes, viz. the generation of neurons or sensory cells by most placodes and the importance of cell shape changes and morphogenetic movements during placode development. It then argues that these shared aspects of placode development are due to the origin of all placodes from a common primordium, the so-called pre-placodal ectoderm (PPE). The PPE is characterized by the expression of Six1/2 and Six4/5 transcription factors and their cofactors of the Eya family. The second part of the chapter reviews how the PPE originates in early vertebrate development in parallel to other ectodermal territories such as neural crest, neural plate and epidermis. This review focusses in particular on the role of transcription factors during this process (including the increasingly dorsally restricted TFAP2a, Msx1, Dlx3/5, Ventx2, an FoxI1-4, and the increasingly ventrally restricted Sox3, Sox11, Zic1/3, and Geminin), since these appear to play central roles for the cell fate decisions that step by step convert pluripotent cells into sensory and neurosecretory cell types.
The cranial placodes of vertebrates – an overview
(CRC Press, 2021-06-18) Schlosser, Gerhard
Chapter 2 opens a series of chapters dedicated to the mechanisms underlying the embryonic development of sensory and endocrine neurosecretory cell types and organs arising from cranial placodes. After a brief introduction to cell types, the second chapter provides an overview of the various cranial placodes and their derivatives in vertebrates. The adenohypophyseal placode develops into the anterior pituitary with six different neurosecretory cell types. The olfactory placode forms the olfactory and vomeronasal epithelia with the olfactory/vomeronasal primary sensory cells and several other cell types (e.g. cells secreting GnRH and other neuropeptides, mucus producing cells of the Bowman glands, sustentacular cells). The lens placode gives rise to the transparent lens fiber cells of the lens. The otic placode invaginates to form the inner ear with its mechanosensory hair cells and the sensory neurons transmitting information from these to the brain. These are involved in the perception of gravity and body position and in hearing. The lateral line placodes produces similar cell types on the surface of the body, which respond to water movements (and in some groups to electric stimuli) in aquatic vertebrates. Finally, the profundal, trigeminal and epibranchial placodes contribute sensory neurons to various cranial ganglia.
The vertebrates’ new head
(CRC Press, 2021-06-18) Schlosser, Gerhard
The first chapter introduces the vertebrate head. The chapter argues that the vertebrate head is an evolutionary novelty that originated only in the vertebrate lineage. This was first proposed by Northcutt and Gans in their “New Head” hypothesis, which suggested that the “New Head” evolved when filter-feeding ancestors adopted a more active and predatory life-style. These authors also highlighted that the novel structures of the vertebrate head develop from only two novel embryonic tissues, the neural crest and the cranial placodes. After sketching this proposal, the chapter gives a brief overview of vertebrate head development (early development, neural crest, cranial placodes, head segmentation). This is followed by a short survey of the sensory (inner ear and lateral line, olfactory organs) and neurosecretory organs (anterior pituitary) of the vertebrate head developing from cranial placodes. The eye is also briefly introduced here, because the lens develops from a placode and the photoreceptors of the retina are evolutionarily related to other, placode-derived sensory cells (as shown later in the book). A final section provides an introduction to cranial nerves, which are also partly placode derived.

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