Development and ultrastructure of the marine,parasitic oomycete,Lagenisma coscinodisci Drebes (Lagenidiales)

The thalli of Lagenisma coscinodisci are coenocytic. In young thalli each nucleus is associated with two procentrioles and four dictyosomes. Before nuclear division two pairs of procentrioles are found on two opposite poles each lying in a pocket of the nuclear membranes. The spindle microtubules radiate from these poles; the nuclear membrane remains completely closed in all divisional stages. The thallus transforms holocarpically into a zoosporangium. The differentiation begins after the breakdown of the host with the formation of a cell wall and with the development of a discharge tube. During these processes peripheral dilated tubules are observed which are in close, local contact with small infoldings of the plasmalemma. The discharge tube develops independent of the point of entry of the infection tube and enlarges by tip growth. There are several light and electron microscopical observations which suggest that the last nuclear divisions in zoosporangia of old cultures are meiotic.They give rise to sexually determined zoomeiospores.