The ultrastructure of the plasmodial protist Leucodictyon marinum Grell

Abstract

Leucodictyon marinum Grell, obtained from tide pools near the Shimoda Marine Research Center (Japan), is a meroplasmodium with flattened cell bodies, enclosed in loricae, and a common reticulopodial network. The cell bodies have a single, excentrically located nucleus which contains a central nucleolus, several heterochromatic areas and a filamentous rodlet of unknown function. Plastids are lacking. The mitochondria have tubular cristae. Extrusomes, resembling discobolocysts, do occur, but are rarely met with. Many vacuoles with a homogeneous content indicate that the final part of digestion takes place in the cell bodies. The reticulopodia are slender threads, but show dilatations, caused by vesicles which transport food to the cell bodies. The reticulopodia contain microtubules. There are two types of cytokinesis: Equal division leads to enlargement of the meroplasmodium, unequal division to zoosporo-genesis. The development of the two flagella of the zoospore is described. The flagellar root system consists only of a short fibre connecting both basal bodies and some microtubules running to the plasmalemma. The difficulty to classify the recently investigated species Reticulosphaera socialis, Chlorarachnion reptans and Leucodictyon marinum is discussed. If the "reticulo-meroplasmodial complex" is a synapomorphy, indicating their affinity, they may derive from an early offshoot of the ancestral Heterokontophyta and can scarcely be affiliated to one of the established classes.