For every rule, there are exceptions. The same is true for how organisms organize their genomes. From how DNA is packaged in the nucleus to what genetic code is used for translating proteins, notable exceptions are found all across the tree of life. Most organisms use the standard genetic code. A few, such as ciliates, use a different code. Most organisms condense their DNA using histone proteins, packing strands of DNA into structural units called nucleosomes. A few don’t, the most notable exception being dinoflagellates, which, instead of using histones, supercoil their DNA into liquid-crystal condensates. The typical mammalian genome is between 1.6–6.3 billion nucleotides in length. For other types of organisms, such as the carnivorous plant genus Genlisea, their genomes are ultra-short with variable lengths due to a loss in non-coding regions.

These exceptions teach us what the rules are. With the advent of mass genetic sequencing capabilities and major sequencing initiatives such as the Earth Biogenome Project, we are learning more about the different types of genomic organization. Many of these defy our expectations, offering new insights about the rules for genomic organization, as well the selective pressures affecting genomic evolution.

Dinoflagellates: small organisms, big genomes

Dinoflagellates are marine algae which often make the news due to the phenomenon known as a “red tide.” A red tide occurs when dinoflagellates and other marine microorganisms reach high enough numbers to change the color of the water to a dark red color. It is toxic to humans, fish, mussels, and more. But despite this toxicity, dinoflagellates are also critical for the survival of any water environment and help with the growth of reef-building coral. 

Dinoflagellates, which have genomes that are up to 80 times larger than the human genome, are an excellent organism for understanding the different ways of organizing DNA inside of the nucleus. Instead of nucleosomes, which are found in most eukaryotes, dinoflagellate DNA is supercoiled into liquid crystalline chromosomes (LCCs). These LCCs are permanently condensed DNA fixtures of the nucleus and even remain while dinoflagellate cells divide. Researchers are still working to uncover the mysteries of how these structures evolved, how they are created and maintained in the living dinoflagellate, and how LCCs impact the dinoflagellate life cycle.