Plasmodium formation in Physarum polycephalum: cytological events and their timing relative to commitment

Amoebae of "selfing" strains of Physarum polycephalum transform into multinucleate plasmodia without change in ploidy. The transformation occurs when cultures of amoebae reach sufficiently high cell densities or when cultures of low densities are exposed through filters to dense "inducing" cultures of amoebae. We have induced selfing amoebae to transform individually and in populations in order to study cytological changes and their timing relative to the irreversible commitment of cells to plasmodium formation. An increase in cell size was observed to precede commitment, and the first nuclear mitosis, which gives rise to a binucleate cell, was found to follow commitment.