Acrasin

Abstract: Publisher Summary This chapter discusses the biochemical and physiological aspects of differentiation in the fungi. Differentiation in fungi is generally associated with the end of the period of active growth. Fungi are easily cultivated in the laboratory, the response of many is synchronous, and in most cases morphogenesis can be controlled by adjustment of certain physical or chemical factors in the environment. The presence of a rigid cell wall determines to a large extent the cellular form of fungi and, by its very nature, renders these organisms amenable to investigations of the molecular basis of their form. The chapter discusses Acrasiales that are often termed as “communal slime moulds.” The slime moulds undergo a transition from the unicellular, free-living conditions found in many microorganisms to the multicellular integrated conditions associated with higher organisms. The component myxanioebae retain their individuality by not fusing and cooperate as members of an organized community until sporulation occurs. Taxonomically, the Myxomycetes are considered to show certain affinities to fungi. The mechanism, which enables cells of the slime mould to make the transition from a unicellular state to the multicellular condition involves a chemotactic response. The aggregation process is mediated by a chemical substance, acrasin that can be destroyed by an extracellular enzyme—acrasinase. Cell adhesion is regarded as necessary in promoting morphogenetic interaction. The adhesion of the cells is specific and involves cell recognition and sorting out. There are changes in the cell surface such as the appearances of a new lipoprotein antigen in the cell membrane. In slime moulds, morphogenesis is initiated in part by some degree of nutritional deficiency. The availability of the appropriate energy source during the early stages of morphogenesis will inhibit or reverse morphogenesis completely.

Abstract: Experiments designed to give some insight into the nature of polarity were performed on the amoeboid slime mold Dictyostelium discoideum. The separate myxamoebae which stream together to form aggregations of myxamoebae show an external polarity and the resulting cell mass has a clear over-all antero-posteriority. In both cases it was shown that the internal parts do not reflect the external polarity. It is known from previous experiments that the aggregating myxamoebae orient in a concentration gradient of a chemical substance tentatively called acrasin, and that acrasin is found in later stages of development. By reversing the gradient of acrasin, separate myxamoebae did not back up but re-oriented towards the point of high concentration. However, it was shown by various experiments that the external acrasin gradient cannot always determine the direction of the polarity, for in the cell masses the polar movement occurs when there is no external gradient of acrasin, or in some cases away from the region o...