Morphogenetic Forces in Diatom Cell Wall Formation

Abstract

Diatoms are armored with a rigid cell wall which consists of two thecae overlapping in the girdle region and resembling a bipartite box (Fig. 1). Cell division occurs by binary fission parallel to the valvar plane. Each daughter cell inherits one part of the mother cell wall, which in both cells becomes the epitheca, while the hypotheca is newly created (Figs. 2, 7, 8). This causes a decrease in size for some of the daughter cells during successive divisions (Fig. 2). At a certain stage this is compensated by sexual reproduction, resulting in a large auxospore, within which the initial valves are laid down (Fig. 3). The initial valves are rounded and do not exhibit the surface pattern characteristics of the species. The species-specific surface form of the vegetative valves is created with the first division (Fig. 3). Each individual part of the wall is composed of an inorganic opaline silica component that is enveloped by an organic coat (ref. in Schmid et al. 1981). The enormous variety and complexity of valve shapes and particularly their patterns induces the question of “how” and “why” they are formed in such a remarkable way; and what determines their outline, their topography, and their pattern? With respect to overall frustule pattern, why is there always a geometric center from which it fans out, forming a ring, or central area, in Centrales, while in Pennales it lies along an axial rib, which in a number of genera is penetrated by a slit, the raphe (Fig. 4)?