The arrangement of scales and bristles on <i>Mallomonas</i> (Chrysophyceae): a proposed mechanism for the formation of the cell covering

Abstract

The siliceous cell covering of Mallomonas consists of a series of imbricated scales and bristles arranged in a very precise pattern. Scales on a given cell are aligned in spiral rows with their longitudinal axes parallel, perpendicular, or at an oblique angle to the longitudinal axis of the cell. The scales within each spiral row are overlapped in a posterior to anterior manner, whereas the spiral rows themselves are overlapped in an anterior to posterior fashion. The result is that an individual scale is overlapped by the scale positioned posterior to it in the same spiral row and by the scale(s) in the spiral row anterior to it. A mechanism is proposed to explain the formation of the cell covering whereby the scales and bristles are released from the cell sequentially at a given site and simultaneously moved over the outside of the cell membrane to their final positions. The force used to release a scale is coupled with that needed to slide the scales into place. The V-rib and (or) proximal rim provide a means by which each scale is properly spaced and ultimately guided into position. The proposed mechanism is contrasted with an alternative model where the siliceous components are moved around and orientated within the cell before being released into their final positions.