Eukaryotes, like animals, fungi, plants, and you, carry nuclear DNA in within the nucleus of the cell. The amount of nuclear DNA varies from organism to organism, and some cells have to physically contain a lot more DNA than others. The effects of DNA capacity have been studied in plants and animals, but Dora Čertnerová and Pavel Škaloud have examined how genome size variation affects unicellular algae.
It might seem odd to examine a unicellular organism now, and that biologists would have started small before working up to multicellular organisms. Čertnerová and Škaloud point to a gap in data, which they say is caused by the challenges of working with single-cell species. They say that often biologists work with a specific strain of an organism, and so filter out a lot of the diversity in a species, before they start work. As a result, little has been done in within-species variation in genome size in unicellular organisms.
To tackle this problem, Čertnerová and Škaloud chose to study Synura petersenii. S. petersenii is a common freshwater alga found around the world. It has a distinctive appearance thank to the siliceous scales it grows over the cell surface. It comes in quite a lot of slightly different forms and it has been suggested that instead of being one species, it is in fact a species complex of very similar species. Indeed some recent work has distinguished quite a few different species from the complex already.
The botanists aimed to describe the intraspecific variability of genome size in an alga, and then follow-up with more questions. Are there phenotypic or physiological consequences to genome size? And does genome vary with ecogeographical location?
They gathered over a hundred strains of S. petersenii from over sixty locations in the northern hemisphere, and looked to see how much nuclear DNA the algae had. They found that, depending on which cell you examined, some algae could have over twice the nuclear DNA of other algae, of the same species. What causes the differences?
First, scientists are human, and tools have limits, so could this be a result of error? “[O]wing to the robust FCM protocol, consistent methodology and generally high precision of our analyses, we are convinced that the error of measurement have not substantially contributed to the genome size variation. Taking into account the two-fold difference between the lowest and highest genome size estimates, alternating life cycle stages or whole genome doubling (polyploidization) events would seem as likely explanations,” write the authors in their article. Though they also note a flaw in that explanation.
“However, none of these mechanisms can be the sole source of the diversity observed in Synura as there were no discrete genome size categories that would reflect the inherent ploidy shifts. Another argument against the alternating life cycle stages is that strains re-analysed after weeks (or two years) exhibited more or less stable genome size estimates.”
They argue that another explanation would be the existence of a lot of transposable elements (TEs). These are parts of the genome that can move around with in it. Duplications would add chunks to the genome, and Čertnerová and Škaloud cannot rule out duplications of whole chromosomes.
Another explanation they suggest is cryptic diversity, and that S. petersenii still describes a number of species. The problem in isolating a single species could be due to how the alga lives.
“Synura petersenii is a colonial species and it is generally unknown whether the colonies are composed of genetically identical cells or may combine multiple genotypes (strains). Since the cultures for this study were established from one colony of cells each and always had uniform genome size, we hypothesize that strains of different genome size coexist at a locality in well-separated colonies,” write Čertnerová and Škaloud.
“Our results cannot rule out the scenario that various genome size categories in S. petersenii are coupled with reproductive barriers and thus reflect cryptic diversity within the taxon.”
The scientists found the difference in genome size had consequences for the alga. Cells with more DNA were larger, but grew more slowly. The correlation was not as tight as found in other species. This, the authors say, may be due to the tests being done within one species – or that S. petersenii is capable of responding to different temperatures and nutrient concentrations.
The authors also found not obvious ecogeographical pattern to genome distribution. Indeed, they found there were locations where algae with different-sized genomes were co-existing. It’s not yet clear what these differences indicate say Čertnerová and Škaloud. “Whether these strains are associated with reproductive barriers (suggesting cryptic diversity within S. petersenii) remained unresolved, though prevailing clonal reproduction of the species could substantially contribute to the maintenance of local genome size diversity even in their absence.”
Cover image: Canva.