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A phenomenon discovered in Drosophila by Garcia-Bellido during investigations of the distribution of genetically marked clones of cells during the development of imaginal discs (q.v.). When such clones are analyzed it is found that they do not randomly overlap any area of the disc, but are confined to compartments and never cross the borders between compartments. A compartment contains all the descendants of a small number of founder cells called a polyclone. As development proceeds, large compartments are progressively split into smaller ones. The founder cells are related to each other by position, not ancestry; the progeny of the founder cells form the compartment under consideration, and no other cells contribute to it. The developmental prepattern (q.v.) of each compartment is controlled by a selector gene. When selector genes mutate, the cells in a compartment may develop a pattern of cell types appropriate for another compartment. The homeotic mutants (q.v.) are examples of mutated selector genes. See Chronology, 1973, Garcia-Bellido et al.; 1975, Morata and Lawrence; engrailed.

Subjects: Genetics and Genomics.

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