  The development of multicellular organisms proceeds through a series of
morphogenetic and cell-state transitions, transforming homogeneous zygotes into
complex adults by a process of self-organization. Many of these transitions are
achieved by spontaneous symmetry breaking mechanisms, allowing cells and
tissues to acquire pattern and polarity by virtue of local interactions without
an upstream supply of information. The combined work of theory and experiment
has elucidated how these systems break symmetry during developmental
transitions. Given such transitions are multiple and their temporal ordering is
crucial, an equally important question is how these developmental transitions
are coordinated in time. Using a minimal mass-conserved substrate-depletion
model for symmetry breaking as our case study, we elucidate mechanisms by which
cells and tissues can couple reaction-diffusion driven symmetry breaking to the
timing of developmental transitions, arguing that the dependence of patterning
mode on system size may be a generic principle by which developing organisms
measure time. By analyzing different regimes of our model, simulated on growing
domains, we elaborate three distinct behaviours, allowing for clock-, timer-,
or switch-like dynamics. By relating these behaviours to experimentally
documented case studies of developmental timing, we provide a minimal
conceptual framework to interrogate how developing organisms coordinate
developmental transitions.
