The C. elegans male tail is being studied as a model to understand how genes specify the form of multicellular animals. Morphogenesis of the specialized male copulatory organ takes place in the last larval stages during male development. Genetic analysis is facilitated because the structure is not necessary for male viability or for strain propagation. Analysis of developmental mutants, isolated in several functional and morphological screens, has begun to reveal how fates of cells are determined in the cell lineages, and (...) how the specification of cell fates affects the morphology of the structure. Cytological studies in wild type and in mutants have been used to study the mechanism of pattern formation in the tail peripheral nervous system. The ultimate goal is to define the entire pathway leading to the male copulatory organ. (shrink)

Two decades ago, Rolf Landauer (1991) argued that “information is physical” and ought to have a role in the scientific analysis of reality comparable to that of matter, energy, space and time. This would also help to bridge the gap between biology and mathematics and physics. Although it can be argued that we are living in the ‘golden age’ of biology, both because of the great challenges posed by medicine and the environment and the significant advances that have been made—especially (...) in genetics and molecular and cell biology—we feel that information as an essential aspect of life has been neglected, or at least misunderstood. We therefore summon Maxwell’s Demon and its distant relative the ratchet, and apply these to biology. (shrink)

One of the most remarkable recent findings in developmental biology has been the colinear and homologous relationships shared between the Drosophila HOM‐C and vertebrate Hox homeobox gene complexes. These relationships pose the question of the functional significance of colinearity and its molecular basis. While there was much initial resistance to the validity of this comparison, it now appears the Hox/HOM homology reflects a broad degree of evolutionary conservation which has reawakened interest in comparative embryology and evolution.The evolutionary conservation of protein (...) motifs in many gene families (including those for growth factors, secreted and membrane bound signalling factors, adhesion molecules, cytoplasmic receptor kinases, nuclear receptors and transcription factors) has lead to speculation on the extent to which these homology relationships represent common developmental processes and underlying molecular mechanisms. Structural identities in a protein may indicate the biochemical/molecular function that a protein plays in cellular and developmental processes, without reflecting a conserved role in a cascade of developmental events. However, the analysis of genes encoding transcription factors has provided evidence suggesting that there are gene complexes in arthropods and vertebrates which are true homologues and which may share common roles in the specification of regional identity along embryonic A‐P axis. These genes comprise the Box/HOM‐C homeotic complexes. This review will detail some of the evidence for this proposed relationship and will speculate on the functional implications. (shrink)

When two populations of cells within a tissue mass differ from one another in magnitude or type of intercellular adhesions, a boundary can form within the tissue, across which cells will fail to mix. This phenomenon may occur regardless of the identity of the molecules that mediate cell adhesion. If, in addition, a choice between the two adhesive states is regulated by a molecule the concentration of which is periodic in space, or in time, then alternating bands of non‐mixing tissue, (...) or segments, can form. But temporal or spatial periodicities in concentration will tend to arise for any molecule that is positively autoregu‐latory. It is therefore proposed that segmentation is a ‘generic’ property of metazoan organisms, and that metamerism would be expected to have emerged numerous times during evolution. A simple model of segmentation, based solely on differential adhesion and periodic regulation of adhesion, can account for segment properties as disparate as those seen in long and short germ band insects, and for diverse experimental results on boundary regeneration in the chick hind brain and the insect cuticle. It is suggested that the complex, multicom‐ponent segment‐forming systems found in contemporary organisms (e.g., Drosophila) are the products of evolutionary recruitment of molecular cues such as homeobox gene products, that increase the reliability and stability of metameric patterns originally templated by generic self‐organizing properties of tissues. (shrink)

A wide range of anatomical features are shared by all vertebrates, but absent in our closest invertebrate relatives. The origin of vertebrate embryogenesis must have involved the evolution of new regulatory pathways to control the development of new features, but how did this occur? Mutations affecting regulatory genes, including those containing homeobox sequences, may have been important: for example, perhaps gene duplications allowed recruitment of genes to new roles. Here I ask whether comparative data on the genomic organization and expression (...) patterns of homeobox genes support this hypothesis. I propose a model in which duplications of particular homeobox genes, followed by the acquisition of gene‐specific secondary expression domains, allowed the evolution of the neural crest, extensive organogenesis and craniofacial morphogenesis. Specific details of the model are amenable to testing by extension of this comparative approach to molecular embryology. (shrink)