Morphological defenses against predation in zooplankton result from targeted cell division and growth and provide unique opportunities for the integrated study of ecology and cell biology. This study examined the cellular basis of the various predator-induced head shapes (neckteeth, spines, and helmets) in the cladoceran genus Daphnia. Several lines of evidence suggest that polyploid cells serve as developmental control centers to govern head shape. First, polyploid cells are present in the cephalic epidermis of Daphnia and their distribution seems to be linked to changes in head shape. For example, a strong correspondence exists between the position of polyploid cells and the presence of neckteeth. Second, variation in the number of polyploid cells seems linked to helmet size across members of the subgenus Ctenoduphniu and perhaps within Hyuloduphniu mendotue. Third, only among those species capable of producing spines or helmets were the DNA contents of epidermal polyploid cells routinely higher in the cephalic than in thoracic regions. Finally, mitotic activity in the cephalic epidermis of H. mendotue was concentrated in regions surrounding polyploid cells, suggesting that these cells serve as active developmental fields. This effect may be produced through the release of a mitogen, whose subsequent diffusion results in a chemical concentration gradient, with division rates of diploid cells varying in relationship to their position in this gradient. For nearly a century, limnologists have been intrigued b
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