A Study of Two Morphological Variants of the Diatom Fragilaria crotonensis Kitton using Electron Microscopy

Two forms of Fragilaria crotonensis Kitton, one with flaredends to the cells, the other with ends that taper to a narrowrod-like tip, were previously distinguished by light microscopyand by their contrasting relationships with the parasitic chytridsRhizophydium fragilariae Canter and one referred to as species3. Examples of the above diatom types, taken from a number ofgeographically widely separated lakes, have been examined byelectron microscopy. Additional morphological differences aredescribed. The ocellus in flared-type cells consists of 4–5rows of pores while in rod-type cells fewer (2–3) rowsof pores are present. Valve thickenings on the mantle at theapex of rod-type cells are rarely visible whereas in flared-typecells they are usually very prominent. A rimoportula was presentin both cell types. In flared cells its external aperture liesimmediately behind the apex usually in line with the striaeand the internal aperture is often orientated with its longaxis oblique to that of the diatom cell. The rimoportula inrod-type cells opens at the centre of the valve face and theinternal aperture is orientated at 90° to the apical axisof the valve. Comparison made with type material suggests thatthe rod-form should be recognized as F. crotonensis var. crotonensisand the flared-form as var. prolongata Grunow ex van Heurck. Fragilaria crotonensis, diatom, systematics, chytrid infection, rimoportula     

Studies on British Chytrids. XXX. On Podochytrium cornutum Sparrow

What is considered to be Podochytrium cornutum Sparrow is recorded from the plankton of Crosemere, Cheshire, where it parasitizes the diatom Stephanodiscus rotula (Kütz) Hendey. In the main essentials of its sporangial development and its rhizoidal system the Crosemere material agrees well with that of the type described from U.S.A. Observations concerning dehiscence and the structure of living zoospores, previously unknown, have been made and resting spores are described for the first time.     

Fungal Parasites of the Phytoplankton. II (Studies on British Chytrids, XII)

This paper brings together the species of Chytrid fungi recordedfrom large bodies of water in the British Isles. Four new speciesare described.     

EFFECT OF SUBLETHAL DOSES OF CADMIUM, INORGANIC MERCURY AND METHYLMERCURY ON THE CELL MORPHOLOGY OF AN INSECT CELL LINE (AEDES ALBOPICTUS, C6/36)

The effect of CdCl₂(44 μ ), HgCl₂(3.7 μ ), and MeHgCl (2 μ ) on the morphology ofAedes albopictusC6/36 cells was studied at the light microscopical level. Treatment times and metal concentrations were in the sublethal range as determined by a fluorometric dye exclusion test. The three metal species had profound effects on the cell morphology. MeHgCl treatment induced the development of a large number of short, actin-supported, tangled filopodia. Both CdCl₂and HgCl₂induced long extensions. Pretreatment with colchicine but not with cyto-chalasin B prevented formation of these extensions which suggests that they were supported by microtubules. This was confirmed by immunostaining for microtubules. The extensions were relatively stable towards colchicine post-treatment. To authors’ knowledge, this effect has not yet been described for heavy metals. The similarity with 20-hydroxyecdysone-treated cells and the occurrence of cytoplasmic feet in insect cells is discussed.