Transcriptional flexibility and inflexibility in differentiation and responsiveness. Upper panel: flexible system. Gene cohorts 1 and 3 are developmentally regulated, and either highly expressed or not expressed; examples of these types of gene products are the trypanosome surface antigens, VSG in the bloodstream form (red) and procyclin in the insect stage (green). The vast majority of genes fall into cohort 2, where, for example, either small or large changes to transcription could result from alterations to the environment (light and dark blue), or a more continually altering transcriptional profile is present that may seek to track changing conditions (purple). This behavior may propagate from one life stage to the next (light and dark blue) or be lost (purple) resulting in altered transcriptional flexibility for genes between life stages. Such a profile is found in higher eukaryotes, including humans and yeast, and probably also many protists, including E. gracilis. Lower panel: inflexible system. In this model gene cohorts 1 and 3 behave as before, but transcription of the genes in cohort 2 remains unchanged. The relative levels of mRNAs from the genes in this cohort may remain constant following differentiation (light blue) or be significantly altered (dark blue and purple). Such a profile is observed here for T. brucei and has been reported previously for P. falciparum, and is potentially a result of a parasitic life style where the host is responsible for provision of a homeostatic environment.