Ring to the Linear

Abstract

The morphogenesis of diatom valves proceeds from the center, following one of two specific patterns. In centrics, there is a ring-shaped annulus, and pennates have a linear sternum. Pennate diatoms are evolutionarily derived from centric diatoms; however, the transition in valve morphogenesis pattern type from ring-shaped to linear remains unclear. Among pennates, raphid diatom valve formation has been examined by many researchers. However, investigation of araphid diatoms is limited. Herein, we observed sequential valve development in two araphid diatoms using electron microscopy. Fragilaria mesolepta has a narrow linear sternum in its mature valve. However, a single silicified chain comprising several rings appeared in earlier developmental stages. The rings comprised a series of spherules with an indistinct periphery ~55–75 nm in diameter with a core of ~30–40 nm. This chain was associated with laterally developing short ribs (virgae), which were aligned radially at one pole with bifurcation. The chain was short in the early stage but gradually elongated in the middle stage and fused to form a linear sternum. The mature valve of Staurosira binodis has a sternum expanded into a narrow spindle shape. In the early stage, two thin threads arranged as facing arcs were observed. They formed a continuous loop at one pole, were fused into a single linear structure at the other pole and had laterally developing virgae. As valve formation proceeded, the pair of threads were united and transformed into a sternum. The early structures of these diatoms suggest homology with the annulus of centrics and also indicate two potential pathways associated with the evolution of the sternum from annuli-possessing ancestors. Pattern center deformation during the ontogenetic process indicated that the siliceous structure in the early stages is not strictly fixed but retains plasticity within the silica deposition vesicle.