It's in the Genes

The genetic revolution has allowed us to learn an incredible amount about DNA; what it does, how it works, how it forms, why it gets damaged. This in turn has helped us to delve even deeper into understanding how animals evolve and the role that their genes play in their fight for survival. Whilst we still have a lot to learn about shark genetics, every new finding has been more mind-blowing than the next. So how big are shark genomes? What do all the different genes do? And how has their DNA evolved to make sharks so perfectly adapted?

A brown banded bamboo shark's genome is larger than a human's, but smaller than that of a great white (Image Credit: SergeUWPhoto / Shutterstock)

The Long and Short of It

Not all sharks have the same amount of DNA. Each species is unique; ranging in weight from from 2.86 to 17.05 pg. That is less than 0.00000000002 grams! But generally speaking sharks actually have pretty large genomes. For example, a whale shark's (Rhincodon typus) genome is 3.44Gbp long; meaning it is is made up of more than 3.4 million base pairs of DNA nucleotides (Hara et al, 2018; Marra et al, 2019; Sáez et al, 2023).

There is a huge range in the size of the genome [SHOWN AS BLUE BARS] (measured as millions of base pairs) between different animals (Image Credit: Palazzo Gregory / WikimediaCommons)

This is large compared to many other animals. In fact shark genomes are often longer than those of mammals, birds and reptiles. For example, coming in at 4.63Gbp long, the great white's (Carcharodon carcharias) genome is nearly twice as large as that of a human being (Marra et al, 2019; Sáez et al, 2023).

However, it might surprise you to learn that the genomes of many relatively simple organisms (like worms) are actually much bigger than many more complex animals, like sharks. Sharks also do not boast the largest underwater genomes, as they are beaten out by lungfishes (Dipnoi) and salamanders (Urodela) (Marra et al, 2019; Sáez et al, 2023).  

The cloudy catshark has the largest genome of any shark studied so far (Image: Hara et al, 2018)

Shark's genomes are, however, generally much larger than those of their close relatives, the skates, rays and chimaeras. The average is driven up by the exceptionally large genome sizes found in the bullhead sharks (order Heterodontiformes), angel sharks (Squatiniformes) and dogfish (Squaliformes).

Genomes are also generally larger in sharks that live in cold environments. But the largest genome for all sharks belongs to the cloudy catshark (Scyliorhinus torazame), which comes in at a whopping 6.7 Gbp (Marra et al, 2019; Sáez et al, 2023).

Since their split from the chimaeras some 380 millions years ago, sharks' and batoids' genomes significantly expanded, so genome size is generally large across all the different taxonomic orders of sharks and rays alive today [SHOWN AS DIFFERENT COLOURED BARS] (Sáez et al, 2023)

Size Doesn't Matter

The size of the genome really doesn't matter as much as you might imagine... A larger genome does not automatically mean an animal is more complex or better adapted and a smaller genome is not necessarily a weakness. This is because not all DNA actually does anything. In fact, the vast majority of all of our genomes is just junk DNA that doesn't really have a function, but because it doesn't cause any harm to the animal (or human!), there is no pressure for the DNA to be eradicated, so it just hangs about (Marra et al, 2019).

The brown banded bamboo shark has a genome 3.38 Gbp long (Image: Hara et al, 2018)

Only a very small amount of many modern genomes have a function. As these bits code for a protein they are known as "coding exons". Within these functioning parts there is huge variation between species in what the different genes do and how they benefit the animal (Marra et al, 2019).

Shark's genomes [BARS] are made up of large proportions of repeat sequences and non-coding DNA [SHOWN AS ALL THE COLOURS EXCEPTING RED]. Only small amounts are actually coding regions that make proteins [SHOWN AS RED] (Hara et al, 2018).

The Long and Short of It

The rest of the genetic material is made up of repetitive sections called LTR elements (Long Terminal Repeats), and "transposable elements" called  SINEs (Short interspersed nuclear elements) and LINEs (Long interspersed nuclear elements), that don't actually code for a protein at all (Hara et al, 2018; Marra et al, 2019; Sáez et al, 2023).

LTR Elements are lengths of viral DNA that have been inserted into the host's DNA by a virus. This is very common and if you have ever had the flu or common cold, it is very likely that your genome includes bits of their LTR elements (Hara et al, 2018; Marra et al, 2019).

A whale shark's genome is quite large at 3.44 Gbp long (Image Credit: DJ Mattaar / Shutterstock)

Transposable elements are lengths of genetic material that can move around to different locations in the animal's genome, so they are sometimes called jumping genes. They can be short (SINEs) or they can be very long (LINEs). In complex animals like human beings, transposable elements make up about 34% of the whole genome. In great white sharks, 59% of the genome is made up of these jumping genes (Hara et al, 2018; Marra et al, 2019; Sáez et al, 2023).

Whole genomes can also accidentally completely double themselves, so the animal ends up with two copies of the exact same DNA. This is known as "polyploidy" and can make for truly huge genomes. For example, ploidy events have pumped the genomes of the New Caledonian fork fern (Tmesipteris oblanceolate) up to some 160 billion base pairs! Scientists suspect that a ploidy event, where the whole genome was duplicated, occurred in the ancient shark lineage several hundred million years ago (Hara et al, 2018; Marra et al, 2019; Sáez et al, 2023).

As much as 58.55% of a white shark's 4.63Gbp long genome is made up of repeat DNA sequences - this is much higher than many other types of animals, including us (Marra et al, 2019)

Gene Therapy

For all organisms DNA is constantly at risk of being damaged, as it is bombarded by UV rays, harmful radiation or chemical carcinogens. What's more, every time it is copied (for growth and repair), there is the potential for mistakes to be made and the DNA to be detrimentally altered. Therefore, in many animals, a lot of genes are actually responsible for ensuring genome integrity (Marra et al, 2019).

Scientists are still investigating the genetic mechanisms behind sharks' remarkable wound (Image Credits: TOP Josh Schellenberg, BOTTOM John Moore, Source: Black, 2023)

What is especially intriguing about sharks is that a large proportion of their genome is devoted to wound healing (to learn more check out Time Heals All Wounds) and genetic stability. In whale sharks and white sharks as much as a third of the coding sections of the genome are devoted to protecting and healing the DNA (Marra et al, 2019; Black, 2023).

The secrets behind how sharks are able to repair their genomes could have implications beyond just marine biology...

Sharks are known to only get cancers relatively rarely (to learn more head over to Myth Busted: Sharks DO Get Cancer). As cancers are caused by unregulated growth of cells as a result of damage to the DNA, the high proportion of genes devoted to protecting and healing the genome is likely a factor which keeps the rate of cancers so low in sharks (Marra et al, 2019). 

It is possible that genes responsible for DNA stability and repair help Greenland sharks to reach hundreds of years of age without severe physical or mental deterioration (Image Credit: Hemming1952 / WikimediaCommons)

Scientists are also very curious about how this affects ageing, as genetic damage is thought to play a huge role senescence. Sharks, however, can live very long lives, only suffering minimal mental and physical deterioration. Greenland sharks (Somniosus microcaphlaus), for example, may live as long as 600 years (see The 150 Year Old Virgin for more information). They are the longest living vertebrate on the planet and scientists suspect that the stability of their genome probably aids in their long-term health (Marra et al, 2019).

As our technology advances and we continue to learn more and more about thir genetics, who knows what other amazing secrets we will discover within sharks' genomes.

A large proportion of a white shark's genome is devoted to maintaining the integrity of the DNA (Image Credit: Alessandro De Maddalena / Shutterstock)

References

Black C (2023). Resilience in the depths: First example of fin regeneration in a silky shark (Carcharhinus falciformis) following traumatic injury. Journal of Marine Sciences.

Access online. 

Hara Y, Yamaguchi K, Onimaru K, Kadota M, Koyanagi M, Keeley SD, Tatsumi K,

Tanaka K, Motone F, Kageyama Y, Nozu R, Adachi N, Nishimura O, Nakagawa R, anegashima C, Kiyatake I, Matsumoto R, Murakumo K, Nishida K, Terakita A, Kuratani S, Sato K, Hyodo S & Kuraku, S (2018). Shark genomes provide insights into elasmobranch evolution and the origin of vertebrates. Nature Ecology & Evolution, 2:11. Access online.

Marra NJ, Stanhope MJ, Jue NK, Wang M, Sun Q, Pavinski Bitar P, Richards CP, Komissarov A, Rayko M, Kliver S, Stanhope BJ, Winkler C, O’Brien SJ, Antunes A, Jorgensen S & Shivji MS (2019). White shark genome reveals ancient elasmobranch adaptations associated with wound healing and the maintenance of genome stability. Proceedings of the National Academy of Sciences, 116:10. Access online.

Sáez MT, Hofreiter M & Straube N (2023). A pluralistic view on the evolutionary forces shaping genome size diversity in sharks. Access online.

By Sophie A Maycock for SharkSpeak