The embryonic origin of periodic color patterns.

The periodic color motifs such as the spots or stripes that adorn the coat of vertebrates have served as emblematic systems in empirical and theoretical studies of pattern formation, because they vary extensively between taxa but often have conserved orientation and are highly reproducible within species. Two major patterning theories have been proposed, namely instructional signaling, in which positional information is encoded as a program, and self-organization, in which position is spontaneously acquired within the developing tissue. We review here recent empirical evidence that supports both theories in vertebrates: with the advent of new molecular techniques and functional approaches, researchers nowadays take advantage of natural populations of mammals, birds and fish species, closely-related to model organisms and varying in periodic patterns. As a whole, results strongly suggest that instruction and self-organization act in combination in space and time. The orientation and reproducibility of periodic patterns relies on initial foundations provided by early developmental landmarks while their periodicity and natural variation are shaped by late-acting self-organizing processes.