Symmetric and Asymmetric Components of Shape Variation in the Diatom Genus Frustulia (Bacillariophyta)

Abstract

Irregularities in cell division can produce asymmetry in symmetric structures, such as outlines of diatom cells, which can reflect genetic, environmental, or random variability in developmental processes. This study examined 12 phylogenetic lineages of the diatom genus Frustulia using landmark-based geometric morphometrics to assess the variation between cell segments separated by apical and transapical axes. Although asymmetric variation within cells differed in some lineages, these irregularities most likely did not reflect the evolutionary history of the lineages. The intraclonal phenotypic plasticity of diatom frustules was induced rather by nongenetic factors, i.e., inherited valve abnormalities, constraints of siliceous cell walls, and random developmental instability during morphogenesis. The positive correlations between the symmetric and asymmetric components of shape variation indicated that the morphogenesis of diatom cell walls affected irregularities within cells and variability among the symmetrized cells to a similar extent.

Geometric Morphometrics of Bilateral Asymmetry in Eunotia bilunaris (Eunotiales, Bacillariophyceae) as a Tool for the Quantitative Assessment of Teratogenic Deviations in Frustule Shapes

“…The most significant shape trend in our data, which was captured by PC1 as illustrating more than 80% of the total symmetric and asymmetric variation, was clearly related to the allometric shape-to-size relationship in which cells progressively shrink during a series of vegetative divisions while also changing shape symmetrically in both halves of the frustule. This phenomenon of shape allometry associated with a diminution series during the vegetative cycle has been illustrated in pennate diatoms by several examples [19,21,23,53,54]. In E. bilunaris, it mainly involves a gradual shortening of the cells, while largely maintaining their relatively stable width [52,55].…”

“…When investigating diatoms with different types of valvar symmetry, the design of the asymmetry analysis would have to be adjusted to their respective symmetry groups. For example, three perpendicular geometric components of shape asymmetry can be decomposed in configurations with biradial symmetry that are typical for most araphid diatoms and several raphid genera, such as Frustulia, Luticola, Navicula, and Achnanthidium [19,20,23,57]. In addition, teratogenic shape asymmetry in genera with heteropolar valves, such as Gomphonema, Licmophora, and Didymosphenia [61], needs to be quantified based on the reflection of configurations across the apical axis.…”

“…The pennate diatoms, which usually dominate the freshwater phytobenthos, are usually characterised by different types of two-dimensional biradial and bilateral symmetry in the valvar view of their frustules. For example, biradial symmetry, characterised by two perpendicular axes of symmetry dividing the valve into four ideally identical segments, is typical for the genera Frustulia, Navicula, Luticola, and Achnanthidium [20][21][22][23]. However, even in these biradial genera, their ideally symmetric arrangement is usually broken by slight systematic asymmetries of valve morphology, such as the structure of the raphe system or Voigt discontinuities [24][25][26].…”

Biomimetic Diatom Biosilica and Its Potential for Biomedical Applications and Prospects: A Review

“…Diatom cell walls, known as frustules, represent a remarkable example of threedimensional (3D) structures found in nature [19]. Diatoms are classified in two groups based on frustule symmetry: centric diatoms that exhibit radial symmetry with respect to an axis that passes through the cell center, and pennate diatoms that present bipolar symmetry with a longitudinal axis that runs parallel to the plane of symmetry [20]. Although they differ in shape, diatom frustules are commonly bipartite structures formed by two overlapping valves referred to as thecae.…”