Genetic programming of development: a model

Life history and bioeconomy of the house mouse

1. More is known about the western European house mouse, Mus (musculus) domesticus than any other non-human mammal. If laboratory and field information is combined, an extremely valuable understanding of the species' bioeconomy could be obtained. 2. The seven stages of mouse life-history are surveyed (up to birth, nest life, sex life, social structure, population statics and stability, senescence, and death), and the interactions between the changing phenotype and the environment are described. 3. These interactions can be used to build up a model of the opportunities and compromises which result in the fitness of individual mice. It is not yet possible to quantify such a model, but this should in principle be achievable.

A model for development and evolution of complex morphological structures

How 'complex' or composite morphological structures like the mammalian craniomandibular region arise during development and how they are altered during evolution are two major unresolved questions in biology. Herein, we have described a model for the development and evolution of complex morphological structures. The model assumes that natural selection acts upon an array of phenotypes generated by variation in a variety of underlying genetic and epigenetic controlling factors. Selection refines the integration of the various morphogenetic components during ontogeny in order to produce a functioning structure and to adapt the organisms to differing patterns of environmental heterogeneity. The model was applied to the development and evolution of the mammalian mandible (which is used as a paradigm of complex morphological structures). The embryology of the mandible was examined in detail in order to identify the fundamental developmental units which are necessary to assemble the final morphological structure. The model is quite general since equivalent units exist for the development of many other biological structures. This model could be applied to many other developing morphological structures as well as other groups of organisms. For example, it can be applied to cell parameters during Drosophila development (Atchley, 1987). The model as discussed in this paper assumes that morphological changes in the mandible result from evolutionary changes in its underlying developmental units. The developmental units relate to characteristics of cellular condensations which are produced from the differentiation of embryonic neural crest cells. The developmental units include: the number of stem cells in preskeletal condensations (n), the time of initiation of condensation formation (t), the fraction of cells that is mitotically active within a condensation (f), the rate of division of these cells (r), and their rate of cell death (d). These units and their derivative structures are discussed in terms of types of tissue differentiation (chondrogenesis, osteogenesis, primary/secondary osteogenesis, intramembranous/endochondral ossification) and growth properties of major morphological regions of the mandible. Variation in these five units provides the developmental basis for ontogenetic and phylogenetic modification of mandibular morphology. We have discussed how these developmental units are influenced by (a) the cell lineage from which they arise, (b) epithelial-mesenchymal (inductive tissue) interactions, (c) regulation of cell differentiation, and (d) extrinsic factors such as muscles, teeth and hormones. Evidence was provided that variation in mandibular morphology is heritable, subject to modification by natural selection, and that divergence among different genetic stocks has apparently occurred through changes in these developmental units and their derivative structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification and mapping of a mouse gene influencing cerebellar folial pattern

Little is known about the role of inheritance in the pattern of cerebral and cerebellar cortical folding, or the causes of individual variation. Inouye and Oda (J. Comp. Neurol., 190 (1980) 357-362) found that the cerebellar folial pattern varies markedly between inbred strains of mice and between individuals in a closed (non-inbred) colony, but shows little variation between individuals within a given inbred strain. They concluded that strain-specific variation in cerebellar folial pattern is under genetic control. In the present study, the folial pattern was examined in crosses between the C57BL/6J and DBA/2J inbred strains of mice, which differ in the presence or absence of a single cerebellar fissure, and in recombinant inbred strains derived from a cross between the same strains. We found that variation is discrete, that neither phenotype is dominant, and that the strain difference is due predominantly to allelic differences at a single locus (Cfp-1) on chromosome 4. Incomplete penetrance of the simpler pattern suggests that this genetic locus interacts in a probabilistic manner with epigenetic mechanisms involved in morphogenesis of the cerebellum.

Development of the dentition in Alligator mississippiensis: upper jaw dental and craniofacial development in embryos, hatchlings, and young juveniles, with a comparison to lower jaw development

Development of the upper dentition in Alligator mississippiensis was investigated using a close series of accurately staged and aged embryos, hatchlings, and young juveniles up to 11 days posthatching, as well as some young and old adult specimens. Studies from scanning electron microscopy, light microscopy, acetate and computer reconstructions, radiography and macroscopy were combined to elucidate the details of embryonic dental development, tooth initiation pattern, dentitional growth, and erupted functional dentition. The results were compared with those from the lower jaw and related to the development of other craniofacial structures. Approximately 17 early teeth in each jaw half develop as surface teeth, of which 13 project for 1 to 12 days before sinking into the mesenchyme. The first three teeth initiate directly from the oral epithelium at Ferguson stages 14-15 (days 15-19 after egg laying), before there is any local trace of dental lamina formation. All other teeth develop from a dental prolamina or lamina; and with progressive lamina development, submerged teeth initiate from the aboral end leading to the formation of replacement teeth. All teeth form dentin matrix, but 12 early teeth do not form enamel. Approximately 20 embryonic teeth are resorbed, 6 are transitional, and 42 function for longer periods after hatching. The embryonic tooth initiation pattern (illustrated by defining a tooth position formula) does not support the previous models of Odontostichi, Zahnreihen, and Tooth Families, each of which postulates perfect regularity. Up to three interstitial tooth positions develop between sites of primary tooth initiation, and families with up to five generations at hatching are at first arbitrarily defined.

Pharmacological concepts and developmental toxicology

This communication provides evidence to support the concept that developmental toxicants (teratogens) produce their effect by either interfering with or enhancing the time-dependent signal-response mechanisms within the embryo. Essential to this hypothesis is the need to show that an observed effect is a function of the administered dose, that there is a positive correlation between the observed effect and pharmacokinetic parameters and that there is evidence for the existence of a specific receptor for the toxicant. While extensive effort is required for ultimate validation of this concept, it serves to emphasize the value of applying known pharmacological principles in defining a mechanistic framework for the biological activity of developmental toxicants.

The development of cell lineages: a sequential model

The concept of cell lineage and the empirical characterization of specific lineages provide valuable insight into the problems of developmental biology. Of central interest is the decision-making process that results in the diversification of cell lines. Studies of the haemopoietic system, in which stem cells can be committed to one of at least six pathways of differentiation, have suggested that the restriction of differentiation potentials is a progressive and stochastic process. We have recently proposed an alternative model which hypothesizes that lineage potentials during haemopoiesis are expressed individually and in a predetermined sequence as progenitor cells mature. The model first arises from experimental studies which show that both normal myeloid progenitor cells and a human promyeloid cell line, which are able to differentiate towards either neutrophils or monocytes, express these potentials sequentially in culture. The close linear relationship between other haemopoietic progenitor cells is inferred from collective data from studies of bipotent progenitor cells and of haemopoietic proliferative disorders. If the development of haemopoietic cell lineages shows a tendency to follow a particular program, such a mechanism is likely to operate throughout development. In this paper we consider the evidence in favour of programmed events within progenitor cells implementing diversification, and the implications of predetermined and restricted pathways of embryonic development.