As a biologist, I’m constantly amazed by the diversity of life on our planet. One aspect that never fails to fascinate me is the incredible variation in genome sizes across different species. You might think that more complex organisms will have larger genomes as if larger genomes means more information which equates to more traits. Sure, bacteria have smaller genomes than cells with nuclei (eukaryotes) and most single celled eukaryotes have smaller genomes than multicellular organisms, but you might be surprised how often a seemingly simple organism has genomes many times the size of highly complex organisms. In my recent video I explore some of the largest genome and talk about what they might mean with respect to the nature of genetic material. If you’re interested in the full story, check out the video below.
Here are a few of the items I discuss in that video:
The Lungfish: A Record-Breaking Animal Genome
One of the most striking examples of genome size variation comes from the lungfish. Recent research has revealed that the South American lungfish (Lepidosiren paradoxa) has a genome of about 91 billion base pairs. To put that in perspective, it’s nearly 30 times larger than the human genome!
What’s particularly interesting is that despite this enormous genome, lungfish certainly don’t appear to be 30 times more complex than humans. In fact, they have a similar number of protein-coding genes (around 19,000-21,000) compared to humans (23,000-25,000). So what’s filling up all that extra space? It turns out that about 90% of the lungfish genome consists of repetitive sequences, many of which are transposable elements – genetic sequences that can copy and paste themselves throughout the genome.
Plants Push the Limits: The Fern with the Largest Known Genome
If you thought the lungfish genome was big, then what about the fern Tmesipteris obliqua? This unassuming plant holds the current record for the largest known eukaryotic genome, weighing in at a whopping 160 billion base pairs! That’s about 50 times larger than the human genome.
What’s even more mind-boggling is that a closely related species, T. oblonga, has a genome of “only” 147 billion base pairs. That’s a difference of 13 billion base pairs – equivalent to about 4 human genomes – between two very similar-looking ferns!
Diatoms: When Genome Size Matters for Survival
Diatoms are microscopic algae found in oceans worldwide. There are possibly 100,000 species of diatoms alive on Earth right now. Research has shown that diatom species can vary up to 50-fold in genome size. Interestingly, this variation correlates with their habitat. Diatoms living in colder waters tend to have larger genomes, while those in warmer waters have smaller ones. This pattern likely relates to the speed at which cells can divide – in warmer waters, faster division can happen, and a smaller genome allows for quicker replication. In cold water, cell division occurs more slowly. As a result it is hypothesized genome size can be much larger and not be a detriment to the dividing cell even if the larger size of the genome doesn’t add any new features and is just extra DNA that the cell is carrying around.
The Implications: Junk DNA and Evolutionary Pressures
These extreme examples of genome size variation raise some intriguing questions about the nature of DNA and evolution. If organisms with similar levels of complexity can have such vastly different genome sizes, what’s all that extra DNA doing?
The answer, in many cases, seems to be “not much.” A large portion of these enormous genomes consists of repetitive sequences that don’t code for proteins or have any known regulatory function. This is often referred to as “junk DNA,” although that term is somewhat controversial.
The presence of this non-functional DNA challenges some ideas about genetic material always being under strong selective pressure. It seems that in many cases, organisms can tolerate large amounts of extra DNA without significant negative consequences. However, as we saw with the diatoms, there are situations where genome size does matter and can be subject to selective pressures.
Exploring these genome size extremes reminds us of the complexity and diversity of life. It’s clear that there’s no simple relationship between genome size and organismal complexity. Instead, genome size seems to be influenced by a variety of factors, including the activity of transposable elements, the strength of selection pressures, and perhaps just random chance.
For me, these findings underscore the importance of studying a wide range of organisms to truly understand genomic evolution. They also highlight how much we still have to learn about the genomes of the myriad species that share our planet. Every genome sequenced has the potential to surprise us and reshape our understanding of genomes and their functions, or lack thereof.
Naturalis Historia 