When mitosis is not completed during a cell cycle, the resulting somatic nuclei will be packed with multiple copies of the haploid genome. These naturally occurring polyploid cells are present in most animals and plants (even in some ciliate protozoans!), but they can still be overlooked and the biological importance and physiological function of this phenomenon are not well understood. One organism that displays extensive endopolyploidy is the freshwater crustacean, Daphnia. With hundreds of polyploid cells dispersed throughout its body at levels levels between 4 and 2048C (C is the amount of DNA in a haploid chromosome set), this diminutive creature will be used to illustrate the some of the patterns that have been observed in the literature. Recent work in my lab has been aimed at elucidating the link between endopolyploidy and the epigenetic regulation of helmet formation. In response to predation pressure, many species of Daphnia form head structures and elongated tailspines. The presence of these inducible defenses may be a result of signaling by polyploid cells. Our strategy for identifying the genetic basis of these polyphenisms will be described.
Dr. Margaret Beaton
Mount Allison University, Canada