title: The ultrastructure of Reticulosphaera socialis grell (Heterokontophyta).
abstract: Reticulosphaera socialis GRELL, a protist from the Caribbean coast of Mexico, has a life cycle consisting of a benthic and a pelagic phase. The benthic phase is the dominating part. It is represented by the meroplasmodium, a cellular association which consists of the spherical cell bodies and a common reticulopodial network. Each cell body is surrounded by an ampulla-like wall, whose narrow part also protects the reticulopodial root connecting the cell body with the remaining network. A single nucleus with nucleolus is situated in the middle of the cell body. A centriole is attached to the nuclear envelope. In addition to the usual organelles (mitochondria, Golgi complexes, plastids), the cytoplasm includes many vesicles and a variable number of bacteria. The latter are probably parasites. The reticulopodial strands serve for capturing and digesting diatoms. Apart from the phagocytized food included in digestive vacuoles, the reticulopodia contain only mitochondria, bacteria and vesicles of different size. Bundles of microtubules extend in a longitudinal direction. The meroplasmodium grows by binary fission of its cell bodies. Mitosis is characterized by single centrioles at opposite poles of the spindle and partial dissolution of the nuclear envelope. At cytokinesis, which begins with an unequal division or budding, one of the daughter nuclei passes from the major part into the minor part. Later on, an equalization occurs, in which the minor part becomes supplied with cellular material by its reticulopodial root. When the diatoms become sparse and light intensity is sufficient, some of the cell bodies form clusters of yellow cells which carry out photosynthesis. Their plastids, also observable in life, show the ultrastructure typical for heterokont algae. The lamellae consist of three thylakoids. Girdle lamellae are lacking. Each plastid is surrounded by two membranes, but remnants of two additional membranes may also be present. Under moderate illumination the meroplasmodium disintegrates into single cells, the so-called transitional cells. They resemble amoebae and transform into the pelagic cells which float in the sea water. Although the pelagic cells look like small heliozoans, their ultrastructural analysis has shown that they have two flagella, inserted at an oblique angle to each other. Both flagella possess a "transitional helix". Whether they are of different length could not be clarified. It is certain, however, that they are heteromorphic: One flagellum bears a basal swelling, associated with a single layer of osmiophilic droplets (eyespot) in a nearby plastid. With respect to the root fiber system, it is of interest that a rhizoplast is lacking. The pelagic cell has only two plastids, associated with the nucleus. Except at the edge of the eyespot, both plastids have girdle lamellae. They show remnants of a periplastidial compartment. The flagellar and photoreceptor apparatus support the assumption that Reticulosphaera socialis has to be classified with the heterokont algae (Heterokontophyta). The color of the photosynthesizing cells and the lack of a rhizoplast during mitosis of the cell bodies and in the root fiber system of the pelagic cells suggest an affiliation to the Xanthophyceae (Tribophyceae).
authors.affiliation: Lehrstuhl Spezielle Zoologie, Universität Tübingen, Tübingen, FRG.
authors.forename: K G
authors.initials: KG
authors.lastname: Grell
authors.forename: A
authors.initials: A
authors.lastname: Heini
authors.forename: S
authors.initials: S
authors.lastname: Schüller