When we first placed Pseudoscourfieldia marina under the microscope, it felt like we were looking at two different organisms. One was a small, round, non-motile cell; the other was an elegant swimmer covered with delicate scales and propelled by a pair of flagella. For decades, these contrasting appearances led many to treat them as separate species. Our work reveals a different story: they are two morphotypes of the same species, different life‑cycle phases expressed through strikingly different forms.

This blog shares the path that led us to this conclusion: the questions that motivated us, the surprises we met in the data, and why we think P. marina can become a model for understanding how genomes produce such diverse cell architectures and life histories.

The question that wouldn’t go away

Our starting point was simple: How can a single species look so different? In plant and algal biology, morphology often reflects a combination of genetic potential and developmental context. Yet in unicellular organisms, we tend to expect a tighter link between genotype and phenotype. P. marina refused to fit the mold.

Two strains became our focal points:

• CCMP1203: coccoid, non-flagellated.
• UIO007: flagellated and covered with extracellular scales.

If both were truly P. marina, what genomic and regulatory differences could account for such divergence?

From genomes to expression: setting up the comparison

We sequenced and compared the nuclear genomes and transcriptomes of the two strains. Technically, that meant building high-quality assemblies for both, phasing alleles where possible, and then using RNA-seq to capture which genes are “on” or “off” under comparable growth conditions. We also examined repeat content, transposable elements, and the overall collinearity of chromosomes, looking for structural differences that might underpin the biology we observed.

Two early generalizations emerged:

1. Genomes conserved, expression diverged. The two strains share a highly conserved chromosomal architecture and gene repertoire. Yet thousands of single-copy orthologs were differentially expressed.
2. Different ploidy states. CCMP1203 behaves like a diploid genome, whereas UIO007 is haploid with extremely low heterozygosity.

What powers a swimmer? Flagella in focus

We curated a set of 274 flagella-associated genes that cover the expected components of a 9+2 axoneme, dynein arms, radial spokes, nexin–dynein regulatory complexes, and the intraflagellar transport machinery. The genes were present in both strains, and their protein sequences were tightly conserved. The difference lay in expression: the vast majority were upregulated in UIO007, consistent with its motile phenotype.

How to build a coat of scales

The flagellated cells of Pseudoscourfieldia marina are covered with tiny scales that form a complex outer layer. These scales are assembled inside the cell and are rich in specialized sugars, including Kdo, a carbohydrate previously suspected to play a key structural role. Our analyses show that the genes needed to produce, transport, and assemble this sugar are strongly activated in the flagellated morphotype. In particular, P. marina has an unusually large and diverse set of enzymes dedicated to linking sugars into complex structures, many of which are switched on only in scaled cells. Together, these findings suggest that scale formation relies on a highly specialized molecular toolkit that enables this single‑celled alga to construct an elaborate extracellular coat.

Outlier chromosomes: small worlds with big consequences

Both genomes harbor outlier chromosomes with lower GC content, higher repeat density, and a bias toward carbohydrate-related genes and methyltransferases. In UIO007, the outlier chromosome carries numerous intact LTR retrotransposons that are actively transcribed. In other green microalgae, analogous chromosomes have been implicated in rapid evolution and viral interactions. While we refrain from over-interpreting, the convergence of features points to evolutionary hotspots that may help cells respond to environmental or biotic pressures.

A life cycle hiding in plain sight

The pieces began to align around a heteromorphic haplodiplontic life cycle:

• The diploid CCMP1203 shows higher expression of several meiosis-associated genes.
• The haploid UIO007 upregulates HAP2/GCS1, associated with gamete fusion, while KAR5/GEX1 shows a pattern consistent with nuclear fusion roles.
• We also identified TALE-class homeodomain transcription factor homologs that likely participate in phase transitions.

In other words, the coccoid and the flagellated morphotypes map naturally onto diploid and haploid phases, respectively, two stages of one life cycle that emphasize different cellular architectures and ecological functions.

What surprised us most

Two things. First, just how conserved the genomic “hardware” is between morphotypes, given how different the “machines” look. Second, the depth of specialization in the sugar-assembly toolkit. These scales aren’t ornament; they appear to be the product of dedicated, regulated pathways with parallels to cell-wall biology in plants, yet tuned to the needs of a tiny, motile alga.

Why it matters

Understanding how ploidy, gene regulation, and cell-surface engineering intersect in a unicellular organism offers a compact window onto questions that also animate plant and fungal biology: How do genomes re-deploy conserved parts lists to generate new morphologies? How do cells balance robustness and evolvability? And how might specialized chromosomes contribute to adaptation, including interactions with viruses in the marine environment?

The road ahead

We see several promising directions for future work that will build on the genomic and transcriptomic framework established here.

First, completing the life cycle in the laboratory remains a major objective. Identifying the environmental cues that trigger transitions between haploid and diploid phases will be essential to fully validate P. marina as a model for haplodiplontic life cycles in unicellular green algae.

Second, epigenetic regulation is likely to play a central role in morphotype-specific gene expression. Profiling the epigenomic landscapes of the two morphotypes could provide key insights into how developmental states are stabilized and switched in single-celled eukaryotes.

Finally, an in-depth characterization of the polysaccharides that compose the cell wall and extracellular scales is now a critical next step. Detailed glycobiological studies will be essential to link gene expression patterns to scale composition and function, and to clarify the roles of the many upregulated carbohydrate-active enzymes identified in the flagellated morphotype.

A personal note

This project thrived on collaboration, culturing, microscopy, long-read sequencing, transcriptomics, and comparative genomics, and on many conversations where a puzzling expression pattern or a stray domain prediction sent us back to the whiteboard. The moment it clicked, that two dramatically different cells could be phases of one organism, was both satisfying and humbling. Nature often solves problems with regulation rather than reinvention.

We are grateful to our colleagues, facilities, and funders who supported the work, and to the community of algal biologists whose foundational insights guided our hypotheses and analyses.