Morphological evolution of silica scales in the freshwater genus Synura (Stramenopiles)

Abstract

A high degree of morphological variability is expressed between the ornately sculptured siliceous scales formed by species in the chrysophycean genus, Synura. In this study, we aimed to uncover the general principles and trends underlying the evolution of scale morphology in this genus. We assessed the relationships among thirty extant Synura species using a robust molecular analysis that included six genes, coupled with morphological characterization of the species‐specific scales. The analysis was further enriched with addition of morphological information from fossil specimens and by including the unique modern species, Synura punctulosa. We inferred the phylogenetic position of the morphologically unique S. punctulosa, to be an ancient Synura lineage related to S. splendida in the section Curtispinae. Some morphological traits, including development of a keel or a labyrinth ribbing pattern on the scale, appeared once in evolution, whereas other structures, such as a hexagonal meshwork pattern, originated independently several times over geologic time. We further uncovered numerous construction principles governing scale morphology and evolution, as follows: (i) scale roundness and pore diameter decreased during evolution; (ii) elongated scales became strengthened by a higher number of struts or ribs; (iii) as a consequence of scale biogenesis, scales with spines possessed smaller basal holes than scales with a keel and; and (iv) the keel area was proportional to scale area, indicating its potential value in strengthening the scale against breakage.

The downsizing of gigantic scales and large cells in the genus Mallomonas (Synurales, Chrysophyceae)

“…Smaller elliptical-shaped scales would be more efficient in covering a curving ellipsoidal cell surface than larger and square-shaped scales, and allow for a closer fitting cell covering. Jadrná et al 26 recently reported that scales of the closely related synurophyte genus, Synura , have also become smaller and more elongate over geologic time, complementing the observations for Mallomonas . Taken together, these findings support the idea that the evolutionary trend for synurophyte organisms has been towards smaller, elliptical scales.…”

Elucidating the phylogeny and taxonomic position of the genus Spiniferomonas Takahashi (Chrysophyceae)

“…Some of these evolutionary trends in changes in spine complexity and transitions between the two genera were originally suggested by Nicholls (1984), but without knowledge of their phy logeny. Such similar trends in scale ultrastructure and size changes are only partially known for Mallomonas and Synura silica-scaled taxa (Čertnerová et al 2019;Jadrná et al 2021;Siver 2022). Therefore, molecular characterization of other Spiniferomonas and Chrysosphaerella species is very important as it may provide a key to better understanding the evolution of silicascaled chrysophytes and their armour in general.…”

The effect of patterned structures on the mechanical resistance of microscopic silica scales

“…The scales of Synurales taxa therefore clearly play an important role for their carriers. Many hypotheses have been proposed to explain the potential benefits of a silica cell covering, including conserving resources, increasing light diffraction, reducing dan gerous UV radiation, or impeding grazing, parasitoid or viral attack (Finkel & Kotrc 2010;Jadrná et al 2021 and references herein). Though a mechanical defense against predation is the most mentioned function of silica structures in chrysophytes, the results of laboratory experiments are quite confusing since in some cases predators preferred the scale-covered over naked cells or colonies of the same prey species (Sandgren & Walton 1995).…”