ORIGINAL RESEARCH article
Front. Ecol. Evol.
Sec. Phylogenetics, Phylogenomics, and Systematics
This article is part of the Research TopicExploring the Hidden Diversity of Eukaryotic MicroorganismsView all articles
Genomic and Morphological Evidence Supports Rhopalodia californica sp. nov. as a Divergent Lineage Within Rhopalodiales
Provisionally accepted
Aimee Caye G. Chang1,2*
Heidi Abresch3
Mailor W. W. Amaral2
Kyle Keepers2Megan Greenwood2
Jingchun Li2
Sarah E. Hamsher4
Scott R. Miller3
J. Patrick Kociolek2*- 1University of Santo Tomas, Manila, Philippines
- 2University of Colorado Boulder, Boulder, United States
- 3University of Montana, Missoula, United States
- 4Grand Valley State University, Allendale, United States
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The diatom Order Rhopalodiales comprises canal-bearing taxa with nitrogen-fixing cyanobacterial endosymbionts, but its genus-level systematics remains unresolved due to the paraphyly of Rhopalodia. We characterized two Northern California strains of Rhopalodia californica sp. nov. using valve morphology, complete organellar and spheroid body (SB) genomes, and multi-gene phylogenetics. Chloroplast (133,208-133,222 bp), mitochondrial (35,955-36,097 bp), and SB (3.064 Mbp) genomes were all structurally conserved and identical in gene content. Phylogenetic analysis of nuclear (18S, 28S), chloroplast (rbcL, psbC), and mitochondrial (cob) genes placed R. californica with Epithemia sp. GazosCreekCA in a strongly supported ‘californica’ clade, distinct from other clades of Epithemia and Rhopalodia species. Genome-wide nucleotide similarity was high within the clade (>97.6%) but markedly lower (<95%) when compared to other Rhopalodiales, a pattern that aligns with distinctions observed in the SB, chloroplast, and mitochondrial datasets. Evolutionary rate was fastest for the mitochondrion, followed by the SB and then the chloroplast. Synteny analysis revealed that gene order and structure were highly conserved between the two R. californica strains and remained well conserved even in comparison with Epithemia sp. GazosCreekCA. These results demonstrate that the ‘californica’ group is a genetically and morphologically cohesive lineage within Rhopalodia sensu lato, supporting its recognition in future taxonomic studies.
Keywords: Diatoms, Mitogenome, Plastome, spheroid body, Symbiosis
Received: 20 Dec 2025; Accepted: 16 Feb 2026.
Copyright: © 2026 Chang, Abresch, Amaral, Keepers, Greenwood, Li, Hamsher, Miller and Kociolek. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence:
Aimee Caye G. Chang
J. Patrick Kociolek
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