THE CELLULAR BASIS OF DIVERGENT HEAD MORPHOLOGIES IN DAPHNIA

Morphological defenses against predation in zooplankton result from ta rgeted cell division and growth and provide unique opportunities for t he integrated study of ecology and cell biology. This study examined t he cellular basis of the various predator-induced head shapes (necktee th, spines, and helmets) in the cladoceran genus Daphnia. Several line s of evidence suggest that polyploid cells serve as developmental cont rol centers to govern head shape. First, polyploid cells are present i n 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 ne ckteeth. Second, variation in the number of polyploid cells seems link ed to helmet size across members of the subgenus Ctenodaphnia and perh aps within Halodaphnia mendotae. Third, only among those species capab le of producing spines or helmets were the DNA contents of epidermal p olyploid cells routinely higher in the cephalic than in thoracic regio ns. Finally, mitotic activity in the cephalic epidermis of H. mendotae was concentrated in regions surrounding polyploid cells, suggesting t hat these cells serve as active developmental fields. This effect may be produced through the release of a mitogen, whose subsequent diffusi on results in a chemical concentration gradient, with division rates o f diploid cells varying in relationship to their position in this grad ient.