Morphometric allometry of representatives of three naviculoid genera throughout their life cycle

Abstract

The frustule architecture of diatoms and the nature of the vegetative cell division phase of their life cycle constrain cell size and shape. For decades, diatomists have observed that size diminution is accompanied by valve shape changes. However, allometric shape changes have rarely been assessed using quantitative statistical tools. In the present study, we employed geometric morphometrics to examine the shape dynamics of raphid diatom frustules. An investigation was carried out to explore whether shape characteristics, such as circularity or asymmetry, and variation of valve outline, increase with decreasing cell size. Four monoclonal strains (Luticola dismutica strain 1, L. dismutica strain 2, Navicula cryptocephala, and Sellaphora pupula) were cultivated under stable conditions for two years in order to capture the complete range of cell sizes from initial to sexually competent cells. Shape changes and the pattern of shape change relative to size were quantified using geometric morphometrics. A quantitative shape analysis revealed similar allometric trends among the different strains and genera. With decreasing cell size, circularity of the valve outlines increased, that is, the complexity of the valves decreased. However, shape variation of valves within the populations increased with decreasing cell size. The levels of asymmetry did not change consistently throughout the size diminution phase. In two out of four strains, horizontal (dorsiventral) asymmetry was significantly lower than vertical (heteropolar) and transversal (sigmoid) asymmetries. The increasing morphological variation in clonal strains was likely caused by an accumulation of structural deviations during morphogenesis. In this respect, this is a specific example of the structural inheritance of morphological characteristics, which is naturally related to the peculiar vegetative life cycle of the diatoms.