The necessity of mitochondrial genome DNA for normal development of Dictyostelium cells

Most unexpectedly, there is now increasing evidence that mitochondria have novel and crucial functions in the regulatory machinery of the growth/differentiation transition, cell-type determination, cellular movement and pattern formation. Here we created p(Delta) cells with a reduced amount (about 1/4) of mitochondrial DNA (mtDNA) from Dictyostelium discoideum Ax-2 cells, by exposing Ax-2 cells to ca. 30 mu/ml of ethidium bromide (EtBr) in axenic growth medium. Importantly, the p(Delta) cells exhibited a series of fascinating behaviors: when they were starved, they showed a marked delay of differentiation and stopped their development at the slug stage, thus failing to construct fruiting bodies. Moreover, cell patterning and cell-type proportioning were found to be greatly modified in slugs (referred to as p(Delta) slugs) derived from p(Delta) cells. That is, prestalk differentiation was significantly enhanced in p(Delta) slugs, while prespore differentiation was markedly inhibited. In addition, the clear anterior prestalk/posterior prespore pattern was considerably disturbed in p(Delta) slugs, presumably because of incomplete sorting between the two types of differentiated cells. After the assay of phototaxis, p(Delta) slugs also exhibited highly disordered movement towards the light source. Taken together, these results suggest that mtDNA might have important multiple functions in a variety of cellular processes during Dictyostelium development.