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Cell Vol 51 1071 1078 December 24 1987 Copyright 0 1987 by Cell Press A Cell That Dies during W ild Type C elegans Development Can Function as a Neuron in a ted 3 Mutant Leon Avery and H Robert Horvitz Department of Biology Massachusetts institute of Technology Cambridge Massachusetts 02139 Summary Mutations in the C elegans gene ted 3 prevent almost all programmed cell deaths so that in a ted 3 mutant there are many extra cells We show that the pharyngeal neuron M4 is essential for feeding in wild type worms but in a ted 3 mutant one of the extra cells probably MSpaaaaap the sister of M4 can sometimes take over Ml s function The function of MSpaaaaap unlike that of M4 is variable and subnormal One possible explanation is that its fate being hidden by death and not subject to selection has drifted randomly during evolution We suggest that such cells may play roles in the evolution of cell lineage analogous to those played by pseudogenes in the evolution of genomes Introduction During multicellular development many cells are born only to die later without serving any obvious function This phenomenon referred to as programmed cell death occurs in organisms as diverse as bacterih Kaiser et al 1979 nematodes Sulston and Horvitz 1977 Sulston et al 1983 insects Taylor and Truman 1974 and vertebrates Hamburger and Oppenheim 1982 Cell death is especially common in the development of nervous systems In the hermaphrodite of the nematode Caenorhabditis elegans for instance 82 107 131 of the programmed cell deaths that occur are in lineages that lead only to neurons and glia although only 37 358 959 of the nuclei in a mature hermaphrodite are neurons or glia Sulston and Horvitz 1977 Sulston et al 1983 One possible explanation for the ubiquity of programmed cell death is that the cells that die are evolutionary relics These cells may have once served some function that later became unnecessary or even deleterious Cell death would thus eliminate unnecessary cells It might also

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