The Molecular Mechanism of Body Axis Induction in Lampreys May Differ from That in Amphibians

Lamprey homologues of the classic embryonic inducer Noggin are similar in expression pattern and functional properties to Noggin homologues of jawed vertebrates. All noggin genes of vertebrates apparently originated from a single ancestral gene as a result of genome duplications. nogginA, nogginB and nogginC of lampreys, like noggin1 and noggin2 of gnathostomes, demonstrate the ability to induce complete secondary axes with forebrain and eye structures when overexpressed in Xenopus laevis embryos. According to current views, this finding indicates the ability of lamprey Noggin proteins to suppress the activity of the BMP, Nodal/Activin and Wnt/beta-catenin signaling pathways, as shown for Noggin proteins of gnathostomes. In this work, by analogy with experiments in Xenopus embryos, we attempted to induce secondary axes in the European river lamprey Lampetra fluviatilis by injecting noggin mRNAs into lamprey eggs in vivo. Surprisingly, unlike what occurs in amphibians, secondary axis induction in the lampreys either by noggin mRNAs or by chordin and cerberus mRNAs, the inductive properties of which have been described, was not observed. Only wnt8a mRNA demonstrated the ability to induce secondary axes in the lampreys. Such results may indicate that the mechanism of axial specification in lampreys, which represent jawless vertebrates, may differ in detail from that in the jawed clade.

Evolutionary conservation of maternal RNA localization in fishes and amphibians revealed by TOMO-Seq

Asymmetrical localization of biomolecules inside the egg, results in uneven cell division and establishment of many biological processes, cell types and the body plan. However, our knowledge about evolutionary conservation of localized transcripts is still limited to a few models. Our goal was to compare localization profiles along the animal-vegetal axis of mature eggs from four vertebrate models, two amphibians (Xenopus laevis, Ambystoma mexicanum) and two fishes (Acipenser ruthenus, Danio rerio) using the spatial expression method called TOMO-Seq. We revealed that RNAs of many known important transcripts such as germ layer determinants, germ plasm factors and members of key signalling pathways, are localized in completely different profiles among the models. It was also observed that there was a poor correlation between the vegetally localized transcripts but a relatively good correlation between the animally localized transcripts. These findings indicate that the regulation of embryonic development within the animal kingdom is highly diverse and cannot be deduced based on a single model.

Natural History of Model Organisms: The big potential of the small frog Eleutherodactylus coqui

The Puerto Rican coquí frog Eleutherodactylus coqui is both a cultural icon and a species with an unusual natural history that has attracted attention from researchers in a number of different fields within biology. Unlike most frogs, the coquí frog skips the tadpole stage, which makes it of interest to developmental biologists. The frog is best known in Puerto Rico for its notoriously loud mating call, which has allowed researchers to study aspects of social behavior such as vocal communication and courtship, while the ability of coquí to colonize new habitats has been used to explore the biology of invasive species. This article reviews existing studies on the natural history of E. coqui and discusses opportunities for future research.

Phylogeny and evolutionary history of the amniote egg

We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades of extant vertebrates. We discuss 40 characters that are relevant for an analysis of the evolutionary history of the vertebrate egg. Special attention is given to the morphology of the cellular yolk sac, the eggshell, and extraembryonic membranes. Many features that are typically assigned to amniotes, such as a large yolk sac, delayed egg deposition, and terrestrial reproduction have evolved independently and convergently in numerous clades of vertebrates. We use phylogenetic character mapping and ancestral character state reconstruction as tools to recognize sequence, order, and patterns of morphological evolution and deduce a hypothesis of the evolutionary history of the amniote egg. Besides amnion and chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced in most birds), internal fertilization, and delayed deposition of eggs that contain an embryo in the primitive streak or early somite stage. Except for the amnion, chorioallantois, and amniote type of eggshell, these features evolved convergently in almost all major clades of aquatic vertebrates possibly in response to selective factors such as egg predation, hostile environmental conditions for egg development, or to adjust hatching of young to favorable season. A functionally important feature of the amnion membrane is its myogenic contractility that moves the (early) embryo and prevents adhering of the growing embryo to extraembryonic materials. This function of the amnion membrane and the liquid-filled amnion cavity may have evolved under the requirements of delayed deposition of eggs that contain developing embryos. The chorioallantois is a temporary embryonic exchange organ that supports embryonic development. A possible evolutionary scenario is that the amniote egg presents an exaptation that paved the evolutionary pathway for reproduction on land. As shown by numerous examples from anamniotes, reproduction on land has occurred multiple times among vertebrates-the amniote egg presenting one "solution" that enabled the conquest of land for reproduction.

Functional morphology, diversity, and evolution of yolk processing specializations in embryonic reptiles and birds

Evolution of the terrestrial, amniotic egg of vertebrates required new mechanisms by which yolk material could be processed for embryonic use. Recent studies on each of the major extant reptile groups have revealed elaborate morphological specializations for yolk processing, features that differ dramatically from those of birds. In the avian pattern, liquid yolk is housed in a yolk sac whose endodermal lining absorbs and digests yolk material and sends resultant nutrients into the blood circulation. In snakes, lizards, turtles, and crocodilians, as documented herein, the yolk sac becomes invaded by endodermal cells that proliferate and phagocytose yolk material. Blood vessels then invade, and the endodermal cells become arranged around them, forming elongated "spaghetti-like" strands that fill the yolk sac cavity. This pattern provides an effective means by which yolk material is cellularized, digested, and transported by vitelline vessels to the developing embryo. Phylogenetically, the (non-avian) "reptilian" pattern was ancestral for sauropsids and was modified or replaced in ancestors to birds. This review postulates that evolution of the "avian" pattern involved increased reliance on extracellular digestion of yolk, allowing embryonic development to occur more rapidly than in typical reptiles. Comparative studies of yolk processing that draw on morphological, biochemical, molecular approaches are needed to explain how and why the "reptilian" pattern was replaced in birds or their archosaurian ancestors.

Zygotic genome activation in the chicken: a comparative review

Maternal RNAs and proteins in the oocyte contribute to early embryonic development. After fertilization, these maternal factors are cleared and embryonic development is determined by an individual's own RNAs and proteins, in a process called the maternal-to-zygotic transition. Zygotic transcription is initially inactive, but is eventually activated by maternal transcription factors. The timing and molecular mechanisms involved in zygotic genome activation (ZGA) have been well-described in many species. Among birds, a transcriptome-based understanding of ZGA has only been explored in chickens by RNA sequencing of intrauterine embryos. RNA sequencing of chicken intrauterine embryos, including oocytes, zygotes, and Eyal-Giladi and Kochav (EGK) stages I-X has enabled the identification of differentially expressed genes between consecutive stages. These studies have revealed that there are two waves of ZGA: a minor wave at the one-cell stage (shortly after fertilization) and a major wave between EGK.III and EGK.VI (during cellularization). In the chicken, the maternal genome is activated during minor ZGA and the paternal genome is quiescent until major ZGA to avoid transcription from supernumerary sperm nuclei. In this review, we provide a detailed overview of events in intrauterine embryonic development in birds (and particularly in chickens), as well as a transcriptome-based analysis of ZGA.

Simulations of sea urchin early development delineate the role of oriented cell division in the morula-to-blastula transition

The sea urchin morula to blastula transition has long been thought to require oriented cell divisions and blastomere adherence to the enveloping hyaline layer. In a computer simulation model, cell divisions constrained by a surface plane division rule are adequate to effect morphological transition. The hyaline membrane acts as an enhancer but is not essential. The model is consistent with the orientation of micromere divisions and the open blastulae of direct developing species. The surface plane division rule precedes overt epithelization of surface cells and acts to organize the developing epithelium. It is a universal feature of early metazoan development and simulations of non-echinoid cleavage patterns support its role throughout Metazoa. The surface plane division rule requires only local cues and cells need not reference global positional information or embryonic axes.

Embryogenesis of Marsupial Frogs (Hemiphractidae), and the Changes that Accompany Terrestrial Development in Frogs

The developmental adaptations of the marsupial frogs Gastrotheca riobambae and Flectonotus pygmaeus (Hemiphractidae) are described and compared with frogs belonging to seven additional families. Incubation of embryos by the mother in marsupial frogs is associated with changes in the anatomy and physiology of the female, modifications of oogenesis, and extraordinary changes in embryonic development. The comparison of early development reveals that gene expression is highly conserved. However, the timing of gene expression varies between frog species. There are two modes of gastrulation according to the onset of convergent extension. In gastrulation mode 1, convergent extension is an intrinsic mechanism of gastrulation. This gastrulation mode occurs in frogs with aquatic reproduction, such as Xenopus laevis. In gastrulation mode 2, convergent extension occurs after the completion of gastrulation movements. Gastrulation mode 2 occurs in frogs with terrestrial reproduction, such as the marsupial frog, G. riobambae. The two modes of frog gastrulation resemble the two transitions toward meroblastic cleavage of ray-finned fishes (Actinopterygii). The comparison indicates that a major event in the evolution of frog terrestrial development is the separation of convergent extension from gastrulation.

Comparison of oocyte mRNA localization patterns in sterlet Acipenser ruthenus and African clawed frog Xenopus laevis

In oocytes, RNA localization has critical implications, as assembly of proteins in particular subcellular domains is crucial to embryo development. The distribution of mRNA molecules can identify and characterize localized transcripts. The goal of this study was to clarify the origin of primordial germ cells in the oocyte body plan and to reveal the generation of cell lineages by localized RNAs. The distribution of 12 selected mRNAs in sterlet Acipenser ruthenus oocytes was investigated by qPCR tomography and compared with known patterns of mRNA localization in Xenopus laevis. We investigated the distribution of two gene clusters in the ooplasm along the animal-vegetal axis of the sturgeon oocyte, both of which showed clearly defined intracellular gradient pattern remarkably similar to their distribution in the frog oocyte. We elucidated the localization of sturgeon egg germplasm markers belonging to the vegetal group of mRNAs. The mRNAs coding otx1, wnt11, and veg1 found to be localized in the sturgeon animal hemisphere are, in contrast, distributed in the vegetal hemisphere in amphibian. Actinopterygii and Sarcopterygii, two major lineages of osteichthyan vertebrates, split about 476 Ma (Blair & Hedges, ), albeit basal lineages share conserved biological features. Acipenseriformes is one the most basal living lineages of Actinopterygii, having evolved about 200 Ma (Bemis, Birstein, & Waldman, ), contemporaneous with modern amphibians (Roelants et al., ).

Vertebrate Embryonic Cleavage Pattern Determination

The pattern of the earliest cell divisions in a vertebrate embryo lays the groundwork for later developmental events such as gastrulation, organogenesis, and overall body plan establishment. Understanding these early cleavage patterns and the mechanisms that create them is thus crucial for the study of vertebrate development. This chapter describes the early cleavage stages for species representing ray-finned fish, amphibians, birds, reptiles, mammals, and proto-vertebrate ascidians and summarizes current understanding of the mechanisms that govern these patterns. The nearly universal influence of cell shape on orientation and positioning of spindles and cleavage furrows and the mechanisms that mediate this influence are discussed. We discuss in particular models of aster and spindle centering and orientation in large embryonic blastomeres that rely on asymmetric internal pulling forces generated by the cleavage furrow for the previous cell cycle. Also explored are mechanisms that integrate cell division given the limited supply of cellular building blocks in the egg and several-fold changes of cell size during early development, as well as cytoskeletal specializations specific to early blastomeres including processes leading to blastomere cohesion. Finally, we discuss evolutionary conclusions beginning to emerge from the contemporary analysis of the phylogenetic distributions of cleavage patterns. In sum, this chapter seeks to summarize our current understanding of vertebrate early embryonic cleavage patterns and their control and evolution.

Novel technique for visualizing primordial germ cells in sturgeons (Acipenser ruthenus, A. gueldenstaedtii, A. baerii, and Huso huso)

Primordial germ cells (PGCs) are the origin of all germ cells in developing embryos. In the sturgeon embryo, PGCs develop from the vegetal hemisphere, which mainly acts as an extraembryonic source of nutrition. Current methods for studying sturgeon PGCs require either killing the fish or using costly and time-consuming histological procedures. Here, we demonstrate that visualization of sterlet (Acipenser ruthenus>) PGCs in vivo is feasible by simply labeling the vegetal hemisphere with fluorescein isothiocyanate (FITC)-dextran. We injected FITC-dextrans, with molecular weights varying between 10 000 and 2 000 000, into the vegetal pole of 1- to 4-cell stage embryos. At the neurula to tail-bud developmental stages, FITC-positive PGC-like cells appeared ventrally around the developing tail bud in the experimental group that received a high-molecular-weight FITC-dextran. The highest average number of FITC-positive PGC-like cells was observed in embryos injected with FITC-dextran having a molecular weight of 500 000 (FD-500). The pattern of migration of the labeled cells was identical to that of PGCs, clearly indicating that the FITC-positive PGC-like cells were PGCs. Labeled vegetal cells, except for the PGCs, were digested and excreted before the embryos starting feeding. FITC-labeled PGCs were observed in the developing gonads of fish for at least 3 mo after injection. We also found that FD-500 could be used to visualize PGCs in other sturgeon species. To the best of our knowledge, this report is the first to demonstrate in any animal species that PGCs can be visualized in vivo for a long period by the injection of a simple reagent.

The origin and migration of primordial germ cells in sturgeons

Primordial germ cells (PGCs) arise elsewhere in the embryo and migrate into developing gonadal ridges during embryonic development. In several model animals, formation and migration patterns of PGCs have been studied, and it is known that these patterns vary. Sturgeons (genus Acipenser) have great potential for comparative and evolutionary studies of development. Sturgeons belong to the super class Actinoptergii, and their developmental pattern is similar to that of amphibians, although their phylogenetic position is an out-group to teleost fishes. Here, we reveal an injection technique for sturgeon eggs allowing visualization of germplasm and PGCs. Using this technique, we demonstrate that the PGCs are generated at the vegetal pole of the egg and they migrate on the yolky cell mass toward the gonadal ridge. We also provide evidence showing that PGCs are specified by inheritance of maternally supplied germplasm. Furthermore, we demonstrate that the migratory mechanism is well-conserved between sturgeon and other remotely related teleosts, such as goldfish, by a single PGCs transplantation (SPT) assay. The mode of PGCs specification in sturgeon is similar to that of anurans, but the migration pattern resembles that of teleosts.

The corn snake yolk sac becomes a solid tissue filled with blood vessels and yolk-rich endodermal cells

The amniote egg was a key innovation in vertebrate evolution because it supports an independent existence in terrestrial environments. The egg is provisioned with yolk, and development depends on the yolk sac for the mobilization of nutrients. We have examined the yolk sac of the corn snake Pantherophis guttatus by the dissection of living eggs. In contrast to the familiar fluid-filled sac of birds, the corn snake yolk sac invades the yolk mass to become a solid tissue. There is extensive proliferation of yolk-filled endodermal cells, which associate with a meshwork of blood vessels. These novel attributes of the yolk sac of corn snakes compared with birds suggest new pathways for the evolution of the amniote egg.

Metamorphosis in a frog that does not have a tadpole

The evolutionary removal of the tadpole from the frog life history is a very successful strategy, particularly in the tropics. These direct developers form limbs and a frog-like head early in embryogenesis, and they have reduced or lost tadpole-specific structures, like gills, a long, coiled intestine, and tadpole teeth and jaws. Despite the apparently continuous development to the frog morphology, the direct developer, Eleutherodactylus coqui, undergoes a cryptic metamorphosis requiring thyroid hormone. As in Xenopus laevis, there is a stimulation by corticotrophin-releasing factor (CRF) and an upregulation of thyroid hormone receptor β (thrb). In addition to changes in skin and muscle, thyroid hormone stimulates yolk utilization for froglet growth from a novel tissue, the nutritional endoderm. The activities of CRF and corticosterone (CORT) in metamorphosis may provide the basis for the multiple evolutionary origins of direct development in anuran amphibians. Potential roles for maternally supplied thyroid hormone and its receptor and for deiodinases in regulating tissue sensitivity to thyroid hormone should be the subjects of future investigations.

Developmental diversity of amphibians

The current model amphibian, Xenopus laevis, develops rapidly in water to a tadpole which metamorphoses into a frog. Many amphibians deviate from the X. laevis developmental pattern. Among other adaptations, their embryos develop in foam nests on land or in pouches on their mother's back or on a leaf guarded by a parent. The diversity of developmental patterns includes multinucleated oogenesis, lack of RNA localization, huge non-pigmented eggs, and asynchronous, irregular early cleavages. Variations in patterns of gastrulation highlight the modularity of this critical developmental period. Many species have eliminated the larva or tadpole and directly develop to the adult. The wealth of developmental diversity among amphibians coupled with the wealth of mechanistic information from X. laevis permit comparisons that provide deeper insights into developmental processes.

Internalizing the vegetal cell mass before and during amphibian gastrulation: vegetal rotation and related movements

The movement of the prospective mesoderm and endoderm to the interior of the amphibian embryo starts in the vegetal cell mass well before the onset of overt gastrulation. By an animally directed movement of cells, the vegetal mass constricts its outer part and expands its inner region including the blastocoel floor, in a process of pregastrulation emboly. Further internalization of the vegetal region has been studied in the Xenopus embryo. At the onset of gastrulation, vegetal rotation sets in at the periphery of the vegetal cell mass, first dorsally and then spreading laterally and ventrally. It consists of an intense inward surging of cells due to active cell rearrangements that can be observed in explants of the vegetal cell mass. In its course, the blastocoel floor expands further and becomes apposed to the blastocoel roof. The boundary between apposed floor and roof forms Brachet's cleft. Another effect of vegetal rotation is the downward and inward movement of the mesodermal marginal zone, constituting the first phase of involution. Together, the upward and outward movement of the peripheral vegetal mass and the downward and inward translocation of the marginal zone lead to an apparent rotation of the whole peripheral region of the gastrula. Vegetal rotation continues to contribute to endoderm internalization to near the end of gastrulation.

Early development of Ensatina eschscholtzii: an amphibian with a large, yolky egg

BACKGROUND

Comparative analyses between amphibians, concentrating on the cellular mechanisms of morphogenesis, reveal a large variability in the early developmental processes that were thought to be conserved during evolution. Increased egg size is one factor that could have a strong effect on early developmental processes such as cleavage pattern and gastrulation. Salamanders of the family Plethodontidae are particularly appropriate for such comparative studies because the species have eggs of varying size, including very large yolky eggs.

RESULTS

In this paper, we describe for the first time the early development (from fertilization through neurulation) of the plethodontid salamander Ensatina eschscholtzii. This species has one of the largest eggs known for an amphibian, with a mean ± SD diameter of 6 ± 0.43 mm (range 5.3-6.9; n = 17 eggs). Cleavage is meroblastic until approximately the 16-cell stage (fourth or fifth cleavage). At the beginning of gastrulation, the blastocoel roof is one cell thick, and the dorsal lip of the blastopore forms below the equator of the embryo. The ventral lip of the blastopore forms closer to the vegetal pole, and relatively little involution occurs during gastrulation. Cell migration is visible through the transparent blastocoel roof of the gastrula. At the end of gastrulation, a small archenteron spreading dorsally from the blastopore represents the relatively small and superficial area of the egg where early embryonic axis formation occurs. The resulting pattern is similar to the embryonic disk described for one species of anuran.

CONCLUSIONS

Comparisons with the early development of other species of amphibians suggest that an evolutionary increase in egg size can result in predictable changes in the patterns and rate of early development, but mainly within an evolutionary lineage.