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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 extantSynuraspecies 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 uniqueS. punctulosa, to be an ancientSynuralineage related toS. splendidain the sectionCurtispinae. 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.
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The downsizing of gigantic scales and large cells in the genus Mallomonas (Synurales, Chrysophyceae)
Abstract Mallomonasis the largest and most speciose genus within the Synurales, a monophyletic clade of siliceous scale-bearing organisms within the class Chrysophyceae. The genus consists of unicellular, motile, photosynthetic organisms found in freshwater localities worldwide.Mallomonasdiverged from other synurophytes during the lower Cretaceous at approximately 130 Ma. Recent discoveries of fossil species were used to examine shifts in scale and cell size over geologic time. On average, scales of fossil species were 2.5 times larger than those produced by modern species. However, a smaller subset of extinct fossil taxa lacking modern analogs had scales over four times larger than modern species, and the largest recorded specimens were six times larger. Data from modern species were further used to develop a model relating scale size to cell size, and applied to the fossil specimens. Based on the model, the mean size of fossil cells was almost twice as long and 50% wider compared to modern species, and cells of taxa lacking modern analogs close to three times as large. These large cells, covered with robust siliceous scales, were likely slow swimmers requiring significant energy to maintain their position in the water column, and possibly prone to increased predation.
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- Award ID(s):
- 1940070
- PAR ID:
- 10364387
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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