A new method for examining the complexity and relationships of "timers" in developing systems.

Simple methods are developed for analyzing the rate-limiting pathways, or "developmental timers," for consecutive stages in a developing system. Two conditions are first defined for short and long timing to a developmental stage. Shifts are then performed at time intervals from short to long and long to short conditions. The total time to the stage (time under first condition plus time under second condition) is scored and plotted as a function of the time of shift, resulting in two plots, one for shifts from the short to long condition, and the other for shifts from the long to short condition. Each plot is then analyzed for the number of components, slopes of components, absolute times of origins and termini of components, and discontinuities between components. This information is then used (1) to distinguish between single- and multiple-component timers, (2) to assess the sensitivity of each timer component to the change in the environmental condition employed in the method, including reversibility, (3) to test for the addition of a new timer component under long conditions, and (4) to test for an identity change of a timer component between short and long conditions. These interpretations in turn provide a minimum estimate of the complexity of the rate-limiting pathway to a developmental stage, temporally define major transition points between timer components, and provide some insight into the nature of timer components. By characterizing the rate-limiting pathway from the origin of a developmental program for each consecutive stage in that program, distinctions can also be made between single, parallel, sequential, and branching timer relationships. From these interpretations, a detailed temporal "map" of the rate-limiting program can be generated for any developmental system in which consecutive stages can be reproducibly monitored with time.