Fottea X:X | DOI: 10.5507/fot.2025.008

Mallomonas intermedia: A case study of speciation and evolutionary dynamics of protists

Pavel Škaloud1, *, Magda Škaloudová1, Petr Knotek1, Iva Jadrná1, Martin Pusztai1, 2
1 Charles University, Faculty of Science, Department of Botany, CZ-12800 Praha, Czech Republic;
* Corresponding author e-mail: skaloud@natur.cuni.cz
2 Institute for Nanomaterials, Advanced Technologies and Innovation, Technical University of Liberec, Studentská 1402/2, CZ-46117 Liberec, Czech Republic

A total of 81 strains morphologically corresponding to the circumscription of Mallomonas intermedia were isolated from 27 freshwater localities across Europe and North America. Molecular genetic analyses revealed two distinct lineages with a strict geographical pattern, which diverged approximately 8 million years ago. According to comparative morphological analyses, we concluded that European populations corresponded to M. intermedia, whereas North American populations were described as a new species, M. retimedia sp. nov. The most notable morphological difference between the species is in the number of pores enclosed in the mesh formed on the siliceous scales. While in M. intermedia the mesh typically surrounds only one pore, meshes of M. retimedia usually encircle 3-4 pores. By incorporating fossil evidence into our time-calibrated phylogeny, we propose that the ancestor of M. intermedia and six related species likely lacked a thick secondary layer but had a distinct transverse rib, present in extant M. galeiformis, M. intermedia and M. retimedia species. The evolutionary history of this group further reveals that parallel ribs on the siliceous scales have evolved independently at least three times during the Miocene epoch, suggesting an adaptive significance, although their exact function remains unclear.

Keywords: Chrysophyceae, evolution, microfossils, morphology, silica scales, speciation, Synurales

Received: May 18, 2025; Revised: June 24, 2025; Accepted: June 25, 2025; Prepublished online: September 3, 2025 

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