The intranuclear microtubule organizing centre in the plasmodium of the myxomycete Physarum polycephalum

Physarum possesses two different microtubule cytoskeletons. In amoebae, cytoplasmic and mitotic microtubules are nucleated by a typical centrosome. In contrast, it has been reported that plasmodia have an intranuclear spindle organizing centre (SPOC) devoid of centrioles. We present genetic evidence suggesting that the SPOC located in the centrosome is very similar to the intranuclear plasmodial SPOC. The immunostaining properties of a new monoclonal antibody against Physarum centrosome has been used to compare these different MTOCs. Moreover, a dense plasmodial microtubule network was present in interphase plasmodia and absent in plasmodia undergoing mitosis. MTOCs responsible for the nucleation of the cytoplasmic microtubule network and intranuclear SPOCs were located in two different compartments of the plasmodium.