This chapter provides an overview of the genetic systems regulating gene expression during development. Regulatory systems composed of two or more interacting elements are known. One of the elements is always at the locus of the structural gene whose expression is controlled by the system. The other element or elements of the system are located elsewhere in the chromosome complement. Each system of controlling elements is essentially autonomous in that the functioning of the elements of one systemdoes not interfere with the functioning of those of another system. Evidence that the Spm system can preset a gene locus at one stage in development to give a particular pattern of gene expression at a later stage was obtained from studies of several different gene loci that came under the control of the system.The review of modes of regulation of gene action by the Spm system illustrates the breadth of accomplishment of a system composed of only two genetic elements. Such systems provide a means of inducing extensive plasticity of expression of the genome. The presence of controlling elements may also be suspected merely on the basis of viewing plants in their native habitats. During development, all components of the genome must undergo sequential stages of programming, which must continue until the final stage of differentiation of a cell. The controlling elements associated with the genes may be the components that are concerned with all these varied expressions of gene regulation.
