ABSTRACT
Recent studies of gene expression in higher organisms highlight the possible significance of change in control sequences for the evolution of new phenotypes in which co-ordinated activity of a number of gene-action systems is required. Three examples are briefly reviewed: (1) a set of genes involved in the control of transcriptional activity in embryogenesis and under stress in adult life; (2) a gene complex involved in a co-ordinated behavioural activity in a mollusc; and (3) the genes involved in segmental organisation and compartmentalised gene function in determination and differentiation during insect development. These three examples illustrate the significance of regulatory sequences in allowing for the evolution of gene function to cover more than one rôle, and also show how the time of activation of a set of genes and their relative productivity may be co-ordinated.
In seeking to explain the basis of major evolutionary divergence of form and function, it is necessary to analyse the mode of action of genes which have an especially significant rôle in mediating morphological and/or functional (including behavioural) organisation. Such genes include those affecting transcriptional patterns at developmental stages crucial for morphogenesis (as in early cleavage and during metamorphosis in holometabolous insects), and those influencing alternative states of cells compartmentalised in cellular fields.
The basic processes involved in body organisation of the segmented animal phyla may have been highly conserved through evolution. Homoeotic mutations take on new significance as showing how compart-mentalisation of gene action allows major change in phenotype without concomitant disruption of body function. It has been suggested that rapid evolutionary change may have been favoured in primitive Metazoa by a higher degree of focusing of gene action at the compartmental level than is observed in more advanced groups. The contention in this chapter is that gene action is focused at the compartmental level in advanced as well as in 190primitive groups. What has changed in the course of evolution of at least the segmented Metazoa is the imposition of tagmatal and other higher order functional groupings of body structures on a primitive, compartmentalised body plan, resulting in an apparent slowing down of the rate of appearance of ‘large-scale’ morphological variations in later geological time.
