Giant squid, pretty big genome

The giant squid – the subject of many a dramatic tale – is actually as mysterious as it is big. 

Rarely sighted, it has never been caught and kept alive, so relatively little has been known about its biology. 

We now know more. A research team led by Rute da Fonseca from the University of Copenhagen, Denmark, has sequenced the genome of Architeuthis dux and published the results in the journal GigaScience.

The first thing to note is that the genome is big: with an estimated 2.7 billion DNA base pairs, it’s about 90% the size of the human genome.

But, says co-author Caroline Albertin from the Marine Biological Laboratory, US, in terms of their genes “we found the giant squid look a lot like other animals. This means we can study these truly bizarre animals to learn more about ourselves”. 

In 2015, Albertin led the team that sequenced the first genome of a cephalopod – the group that includes squid, octopus, cuttlefish, and nautilus.

For the new study, she analysed several ancient, well-known gene families in the giant squid, drawing comparisons with the four other cephalopod species that have been sequenced and with the human genome.

She found that important developmental genes in almost all animals were present in single copies only in the giant squid genome. 

That means the giant squid did not get so big through whole-genome duplication, a strategy that evolution took long ago to increase the size of vertebrates. Hopefully, more probing of the genome will explain how it did.

“While cephalopods have many complex and elaborate features, they are thought to have evolved independently of the vertebrates,” Albertin says. “By comparing their genomes, we can ask, ‘Are cephalopods and vertebrates built the same way or are they built differently?'” 

Albertin also identified more than 100 genes in the protocadherin family – typically not found in abundance in invertebrates – in the giant squid genome.

“Protocadherins are thought to be important in wiring up a complicated brain correctly,” she says. 

“They were thought they were a vertebrate innovation, so we were really surprised when we found more than 100 of them in the octopus genome [in 2015]. That seemed like a smoking gun to how you make a complicated brain. And we have found a similar expansion of protocadherins in the giant squid, as well.”