Electron microscopic and microprobe analysis of calciuminduced differentiation of the white mutant (LU887 × LU897) strain of Physarum polycephalum

Abstract

Differentiation of the white mutant (LU887 × LU897) strain of Physarum polycephalum leading to spherule formation can be induced by CaCl 2 if the concentration in the nutrient medium is increased by 5mM prior to the transfer to a non-nutrient salts medium. All stages previously reported for the typical (M 3cVII) strain of Physarum polycephalum from microplasmodia to spherules are seen but the mutant lacks the synchrony that the replacement technique induces in the typical strain. X-ray microanalyses locate calcium and phosphorus in granules in mitochondria and in the cytoplasm of specimens fixed without osmium. Mitochondria accumulate calcium-containing granules during early differentiation and appear to be essentially without granules in mature spherules. Mobilization of mitochondrial calcium is implicated in the initiation of differentiation. A longitudinally striated cytoplasmic inclusion is abundant in microplasmodia grown in media that have not been supplemented with additional calcium and is seen more rarely during calcium-induced spherulation. Whether or not this inclusion represents cytoplasmic contractile elements is unknown. The calcium-treated mutant strain, previously considered non-differentiating, may prove to be a good alternate model for the study of factors influencing differentiation. It was employed earlier as a control in studies of strains that readily spherulate in response to routine procedures.