Insights from sharks: evolutionary and developmental models of fin development.

The shark and its embryology have recently returned to the spotlight as a model animal in the quest to determine the origins of paired appendages during vertebrate evolution. As the most basal living gnathostomes, sharks and other extant chondrichthyans are ideal models to elucidate the developmental mechanisms utilised in mesoderm-derived primitive fin morphologies. Chondrichthyans occupy a phylogenetic position and possess morphological structures that can answer major questions on the origin of the body plan of vertebrates. This review will outline the past, present, and future use of shark species as a model system with particular emphasis on the recent studies that have utilised comparative molecular embryology of chondrichthyan species to examine the question of the origin of the paired fins. We will also examine the problems and pitfalls of utilising chondrichthyans and the barriers that remain to their utilisation in the modern era of developmental biology.     

Ancient origins of human developmental plasticity.

Animals have the ability to alter development, physiology, growth, and behavior in response to different environmental conditions. These responses represent critical assessments of both external and internal factors. For example, the timing of metamorphosis, hatching, or birth depends on the trade-offs between growth opportunity and mortality risk in the developmental habitat. Physiological sensors compute these trade-offs as a function of energy balance and environmental stress, and effectors initiate physiological, developmental, and behavioral responses to these determinations. The neuroendocrine stress axis provides a means for animals to integrate information from multiple sources and to respond accordingly. Considerable evidence now supports the view that the secretion of hormones critical to development (corticosteroid and thyroid hormones) is controlled by a common neuroendocrine stress pathway involving corticotropin-releasing factor (CRF) and related peptides. CRF produced in the hypothalamus stimulates the biosynthesis and secretion of both thyroid and corticosteroid hormones, leading to accelerated tadpole metamorphosis. Similarly, in mammals CRF of fetal and placental origin has been shown to influence the timing of birth. Studies in several experimental animal models and in humans show that early life experience can have long-term phenotypic consequences. Furthermore, there is evidence that phenotypic expression is strongly influenced by the actions of stress hormones produced during development. The integrated neuroendocrine response to stress, and its role in timing critical life history transitions and establishing long-term phenotypic expression, arose early in the evolution of vertebrates.     

Embryonic development of goldfish (Carassius auratus): A model for the study of evolutionary change in developmental mechanisms by artificial selection

Background: Highly divergent morphology among the different goldfish strains (Carassius auratus) may make it a suitable model for investigating how artificial selection has altered developmental mechanisms. Here we describe the embryological development of the common goldfish (the single fin Wakin), which retains the ancestral morphology of this species. Results: We divided goldfish embryonic development into seven periods consisting of 34 stages, using previously reported developmental indices of zebrafish and goldfish. Although several differences were identified in terms of their yolk size, epiboly process, pigmentation patterns, and development rate, our results indicate that the embryonic features of these two teleost species are highly similar in their overall morphology from the zygote to hatching stage. Conclusions: These results provide an opportunity for further study of the evolutionary relationship between domestication and development, through applying well‐established zebrafish molecular biological resources to goldfish embryos. Developmental Dynamics 242:1262–1283, 2013. © 2013 Wiley Periodicals, Inc.     

The developmental origins of musicality

The study of musical abilities and activities in infancy has the potential to shed light on musical biases or dispositions that are rooted in nature rather than nurture. The available evidence indicates that infants are sensitive to a number of sound features that are fundamental to music across cultures. Their discrimination of pitch and timing differences and their perception of equivalence classes are similar, in many respects, to those of listeners who have had many years of exposure to music. Whether these perceptual skills are unique to human listeners is not known. What is unique is the intense human interest in music, which is evident from the early days of life. Also unique is the importance of music in social contexts. Current ideas about musical timing and interpersonal synchrony are considered here, along with proposals for future research.     

Craniofacial development: New views on old problems

Several clinical syndromes, including the DiGeorge syndrome, are characterized by clusters of developmental defects of the heart and great vessels with structures derived from the embryonic pharyngeal apparatus including thymus and parathyroids. The connective tissue derivatives of neural crest are necessary for the normal development of these structures, and there is new experimental evidence that depletion of neural crest causes defects similar to these clinical syndromes. Therefore it is proposed that many of these syndromes are due to inappropriate development of neural crest. The implications of this hypothesis include the predictions (1) that asplenia and certain other anomalies have the same etiology, and (2) that it is possible to observe the effects of teratogenic agents upon a cellular population (neural crest) at the time when it is being altered, rather than waiting until definitive organs may be examined.     

A theory of developmental change in quantitative phenotypes applied to cognitive development

A model is presented for the changes in familial resemblance as a function of age. The model allows for separate developmental components of genetic and environmental effects and for the influence of earlier phenotypic values on current measurements. Genetic and environmental effects may be specific to occasions or constant over time. Expected covariances are derived within individuals and between relatives measured at different ages. Parameter substitution shows that models with different assumptions about the mechanism of development yield different predictions for temporal changes in family resemblance. The application of the model is illustrated by the analysis of published longitudinal data on cognitive development. The data suggest that the continuity of cognitive performance over time and the increase in heritability with age reflect the cumulative long-term effects of a single set of genes expressed throughout development. The quality of the shared environment changes from family to family over time but appears to exercise a long-term effect on cognitive development.This research is supported by Grants AG-04954 and HL-31010 from the National Institutes of Health. We thank Dr. N. G. Martin for his helpful criticism.     

The functional and anatomical organization of marsupial neocortex: evidence for parallel evolution across mammals

Marsupials are a diverse group of mammals that occupy a large range of habitats and have evolved a wide array of unique adaptations. Although they are as diverse as placental mammals, our understanding of marsupial brain organization is more limited. Like placental mammals, marsupials have striking similarities in neocortical organization, such as a constellation of cortical fields including S1, S2, V1, V2, and A1, that are functionally, architectonically, and connectionally distinct. In this review, we describe the general lifestyle and morphological characteristics of all marsupials and the organization of somatosensory, motor, visual, and auditory cortex. For each sensory system, we compare the functional organization and the corticocortical and thalamocortical connections of the neocortex across species. Differences between placental and marsupial species are discussed and the theories on neocortical evolution that have been derived from studying marsupials, particularly the idea of a sensorimotor amalgam, are evaluated. Overall, marsupials inhabit a variety of niches and assume many different lifestyles. For example, marsupials occupy terrestrial, arboreal, burrowing, and aquatic environments; some animals are highly social while others are solitary; different species are carnivorous, herbivorous, or omnivorous. For each of these adaptations, marsupials have evolved an array of morphological, behavioral, and cortical specializations that are strikingly similar to those observed in placental mammals occupying similar habitats, which indicate that there are constraints imposed on evolving nervous systems that result in recurrent solutions to similar environmental challenges.     

Functionally distinct NK-cell subsets: developmental origins and biological implications

NK cells were initially identified based on their capacity to destroy susceptible target cells via granule-mediated cytotoxicity. Subsequently, NK-cell cytokine production (IFN-gamma, TNF-alpha) was shown to be critical in restricting pathogen infection, defining a non-cytotoxic role for NK cells in host defense. Recently, specialized NK-cell subsets with biased effector functions have been described in mice and man. This overview will consider the developmental origins and biological implications of this NK-cell diversification.     

The functional development of Leydig cells in a marsupial

Leydig cells are the major source of androgen in the male mammal. We describe here for the first time the development of the Leydig cell in a macropodid marsupial, the tammar wallaby, Macropus eugenii. Leydig cells are first recognized morphologically 2 days after birth with the appearance of lipid droplets in the cytoplasm of certain interstitial cells. Lipid content closely matches the steroid content of the developing testis and marks the maturation of the steroid synthesis pathway in the tammar testis. Morphologically mature Leydig cells, marked by distinct mitochondria with tubular cristae and an extensive anastomosing network of smooth endoplasmic reticulum, are developed by day 10 after birth - the time of peak testosterone content in perinatal tammar testes. The volume percentage of each cell type in the testis does not change over time so the growth of each cellular component keeps pace with growth of the whole testis. There was no morphological or quantitative evidence of a change from one population of Leydig cells to another in the tammar testis as has been reported in several other species including the rat, mouse and human. Maturation of the testis is also marked by the development of tight junctions between the cell membranes of adjacent Sertoli cells. These appear around day 30 after birth and coincide with the onset of mitotic arrest in male germ cells. Overall, the development of the Leydig cell in the tammar wallaby follows a similar pattern to that seen in other mammals, although the start of Leydig cell differentiation is, like many other organ systems in marsupials, post natal, not fetal and there appears to be only a single population of Leydig cells.