The neural crest, a multipotent embryonic cell type, originates at the border between neural and nonneural ectoderm. After neural tube closure, these cells undergo an epithelial-mesenchymal transition, migrate to precise, often distant locations, and differentiate into diverse derivatives. Analyses of expression and function of signaling and transcription factors in higher vertebrates has led to the proposal that a neural crest gene regulatory network (NC-GRN) orchestrates neural crest formation. Here, we interrogate the NC-GRN in the lamprey, taking advantage of its slow development and basal phylogenetic position to resolve early inductive events, 1 regulatory step at the time. To establish regulatory relationships at the neural plate border, we assess relative expression of 6 neural crest network genes and effects of individually perturbing each on the remaining 5. The results refine an upstream portion of the NC-GRN and reveal unexpected order and linkages therein; e.g., lamprey AP-2 appears to function early as a neural plate border rather than a neural crest specifier and in a pathway linked to MsxA but independent of ZicA. These findings provide an ancestral framework for performing comparative tests in higher vertebrates in which network linkages may be more difficult to resolve because of their rapid development.neural plate border ͉ transcription factor ͉ agnathan
The naked mole rat (Heterocephalus glaber) is a subterranean rodent whose eyes are thought to be visually nonfunctional and as such is an ideal animal with which to pursue questions in evolutionary developmental biology. This report is the first in-depth study on the development and morphology of the naked mole rat eye. Using standard histological analysis and scanning and transmission electron microscopy, we describe the structural features of the eye. We further report on the morphological changes that accompany the development of this eye from neonate to adult and compare them with those that occur during mouse eye development. We observed numerous abnormalities in the shape and cellular arrangement of the structures of the anterior chamber, with notable malformations of the lens. Cell proliferation and cell death assays were conducted to investigate the possible causes of lens malformation. We found that neither of these processes appeared abnormal, indicating that they were not responsible for the lens phenotype of the mole rat. In order to investigate the process of lens differentiation, we analyzed the expression of ␥-crystallins using Western blots and immunocytochemistry. At birth, levels of ␥-crystallin appear normal, but soon thereafter, the ␥-crystallin expression is terminated. Absence of detectable ␥-crystallins in adults suggests that there is a gradual degradation and loss of these proteins. The evolutionary factors that could be responsible for the eye morphology of the naked mole rat are discussed. A model for abnormal lens differentiation and the role it plays in the morphogenesis of the rest of the eye in the naked mole rats is proposed.
Sea lampreys (Petromyzon marinus) are cyclostomes, the most basal extant group of vertebrates, and are thought to have existed largely unchanged for more than 500 million years. They are aquatic, eel-shaped animals that spend a major part of their life as filter-feeding larvae called ammocoetes, inhabiting many freshwater bodies in the northern hemisphere. After metamorphosis, sea lampreys migrate to the ocean (or to the Great Lakes), where they feed on the blood and bodily fluids of salmonid fish and ultimately return to freshwater streams and rivers to spawn and die. The unique evolutionary position of lampreys and the relative ease of obtaining mature adults and embryos make this animal an ideal model for investigations into early vertebrate evolution. Studies of features shared between lampreys and jawed vertebrates, but distinct from those in nonvertebrate chordates, have provided information on the origin and evolution of hallmark vertebrate characteristics such as the neural crest, ectodermal placodes, and jaw. In addition, studies of features that are unique to lampreys (e.g., the variable lymphocyte receptor-mediated immune system) provide insights into mechanisms of parallel evolution (e.g., the adaptive immune system). With the establishment of techniques for the extended maintenance and spawning of lampreys in the laboratory, the sequencing of the lamprey genome, and the adaptation and optimization of many established molecular biology and histochemistry techniques for use in this species, P. marinus is poised to become an evolutionary developmental model of choice.
Lampreys are one of the most basal animals in which many of the true vertebrate characteristics (e.g., neural crest, placodes, segmented brain, skull, paired sensory organs, pharyngeal skeleton) are present. Studying the molecular and developmental mechanisms responsible for the formation of these structures in lamprey and higher vertebrates can provide insight into how these vertebrate characteristics evolved. The relative ease of obtaining mature adults and embryos makes this animal an ideal model for investigations into early vertebrate evolution. In addition, studies of features that are unique to lampreys can provide insights into mechanisms of parallel evolution. This protocol describes how to produce lamprey embryos by collecting sperm and eggs from mature lampreys, performing fertilization, and culturing the embryos through to the desired developmental stage.
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