The genome of the elusive giant squid has been sequenced, providing insight into its anatomical and evolutionary secrets.
Lurking in the murky depths and measuring up to 43ft – the size of some boats – it’s easy to understand the fascination the giant squid has inspired in seamen and the general public alike. Despite being found across the world, giant squid sightings are incredibly rare, and little is known about them.
Now, scientists have analyzed the genome of the enigmatic giant squid in the hope that it provides clues to the squid’s evolutionary history, intending to answer the giant question: Just how did it get so big?
The team was led by the University of Copenhagen (Denmark) and included Caroline Albertin (Marine Biological Laboratory, MA, USA) who was part of the team that first sequenced the genome of a cephalopod in 2015.
As a giant squid has never been caught alive, DNA was extracted from the remains of a dead squid. Due to the impossible nature of obtaining fresh organ samples, RNA was extracted from three other live-caught cephalopod species. The RNA samples were utilized to sequence the proteomes of all the species to inform genome annotation.
As expected, it was discovered that the giant squid’s genome is large, approximately 2.7 billion DNA base pairs. However, the researchers were surprised to find only single copies of Hox and Wnt genes – vital developmental genes present in most animals – demonstrating that the giant squid’s genome did not evolve through whole-genome duplication.
“While cephalopods have many complex and elaborate features, they are thought to have evolved independently of the vertebrates. By comparing their genomes, we can ask, Are cephalopods and vertebrates built the same way or are they built differently?” Albertin commented.
Protocadherins are a family of genes thought to be involved in vertebrate brain development. Approximately 135 protocadherin genes were identified in the giant squid genome, these aren’t commonly found in invertebrates; however, in 2015 over 100 protocadherins were identified in the octopus genome. Octopi are renowned for their intelligence, suggesting a key role for protocadherins in the development of a complex brain.
This study has provided a unique insight into the evolutionary path of the giant squid and other cephalopods, it sets the stage for further research into this legendary creature.
“Having this giant squid genome is an important node in helping us understand what makes a cephalopod a cephalopod. And it also can help us understand how new and novel genes arise in evolution and development,” concluded Albertin.