The nature of the genetic control of pattern formation in development is a major unsolved problem. The analysis of transdetermination phenomena in Drosophila by S. Kauffman has raised the possibility that the controlling principle is not of the obvious ‘one controlling gene-one pattern element’ type, but involves combinatorial coding, with a given pattern element specified by a combination of binary circuit settings. In attempting to decipher this ‘epigenetic code’ (an area where formidable technical difficulties obstruct the biochemical approach), the abstract coding theory approach, so useful in solving the genetic code, could again be of value. A chance to apply it arises from the existence in certain annelids (segmented worms) of simple regularities in their patterns of metamerism. Annelid segments often are subdivided into annuli: the number of annuli per segment varies in a simple way from one segment to the next. If a given segment contains n annuli, the following segment will contain either n annuli (repetition) or n ± 1 annuli (progressive modification). Similar patterns are sometimes seen in segmentation itself. It seems quite possible that these simple rules reflect some deep principle of pattern coding. The theory offered here involves two interlocking parts. Firstly, the coding system itself is based on a type of code known to electronic circuit designers as Gray codes. The essential feature of this type of code is that coded ‘outcomes’ (e.g., segments, or other pattern elements) which appear consecutively must, to ensure accuracy, be coded by combinations which differ in the assignment of only one circuit. The logic of this constraint is discussed. Secondly, as to the ‘translation’ of the code: the annulation pattern of each segment is held to be ‘read off’ from a subset of the circuits in a ‘one circuit-one annulus’ fashion. This part of the scheme is justified on the grounds of its extreme simplicity. Together, the two postulates account for the empirical pattern features. © 1979, International Society of Differentiation. All rights reserved.
