Heterotopy and heterochrony in the developmental programs of bilateria

The modern concept of heterochrony and heterotopy are tightly connected with discovery of the molecular basis of morphogenesis. These phenomena primarily result from changes in the control of genes important for development. The methods of molecular genetics applied to the classical questions of evolutionary biology have revealed direct relationships of microevolutionary and macroevolutionary events (Smith, 2003). There is no surprise that Barbara Mak-Klintok in 1989 said “… If I could control the time of gene expression, I could cause a fertilized snail egg to develop into an elephant. Their biochemistries are not all that different; it’s simply a matter of timing” (cited after West-Ebernard, 2003). During the past ten years, this courageous statement ceased to be a joke. A new field of scientific exploration, evolutionary developmental biology (EvoDevo from Evolution and Development), appeared, developing and introducing new axiomatics. Some statements of this new science are as follows: (1) Morphological evolution is manifested at the level of developmental programs. (2) Developmental processes are controlled by a set of factors belonging to a limited number of evolutionarily conservative gene families. (3) Regulatory gene sites controlling development are arranged in modules, each containing cis-sites for attachment of the transcriptional regulators and allowing particular genes to be switched on (or off) in a certain place and time, depending on a local set of activators and repressors. (4) Summation of “incoming” signals results in initiation or suppression of a regulatory gene within cell population or individual cells.