Valve and seta (spine) morphogenesis in the centric diatom Chaetoceros peruvianus Brightwell

Summary

In Chaetoceros peruvianus, the two very long, delicately tapered setae (spine-like processes) of the “lower” valve curve downwards gently until they are often almost parallel, while those emerging from the “upper” valve curve sharply downwards until oriented almost in the same direction as the setae of the lower valve. This curvature creates a deep pit between the bases of the upper valve’s setae, where they emerge from the valve. In live cells, extension of setae is rapid and very sensitive to disturbance. After cleavage the new silica deposition vesicle (SDV) appears in the centre of the furrow and expands outwards over it. A distinct microtubule centre (MC) appears directly on top of the SDV. Microtubules (MTs) from the MC ensheath the nucleus, and others fan out over the SDV and plasmalemma. A little later, the MC in the lower daughter cell moves off the SDV, and its MTs now appear to mould the plasmalemma/ SDV into the deep pit between the base of the setae. In the upper daughter cell, the MC remains on the SDV. Initiation of setae is first observed as protuberances of bare cytoplasm growing from the sides of the daughter cells, through gaps in the parental valve. Many MTs initially line the plasmalemma of these protuberances as they grow outwards and the SDV also expands over the new surface. As the setae get longer, a unique complex of three organelles appears. Just behind the naked cytoplasm at the tip of the seta, a thin flat layer of fibrous material lines the plasmalemma. This, the first of the complex, is called the “thin band”. Immediately behind this is the second, a much thicker, denser fibrous band, the “thick band”. At the front edge of the SDV, 5–6 “finger-like outgrowths” of silicified wall grow forwards. These are interconnected by the elements of the thick band which thus apparently dictate the polygonal profile of the seta.