Prising open the scallop genome

In Alfred Hitchcock’s 1963 horror-thriller, The Birds, flocks of deranged creatures gather, watch and inexplicably, violently attack residents of a costal Californian town. The film is based on a Daphne du Maurier novelette of the same name, but also reportedly draws on a similar, though less violent, real life event. In 1961 in Capitola, California, locals described how ‘hordes of seabirds were dive-bombing their homes, crashing into cars and spewing half-digested anchovies onto lawns.’¹

The likely cause of the birds’ strange behaviour was revealed by research published in 2011 – a chemical called domoic acid². Produced by algae, it accumulates in fish and shellfish with seemingly no ill effect. But the chemical is toxic to birds and mammals higher up the food chain. In people, domoic acid causes ‘Amnesic Shellfish Poisoning’ which initially results in vomiting and gastrointestinal symptoms. In more severe cases, it can cause disorientation, permanent loss of short-term memory, seizures, loss of control of movements and sometimes death³.

Domoic acid was blamed for a poisoning in Canada in 1987 where over 100 people were affected after eating mussels, two of whom died⁴. It is also thought to be responsible for incidences of marine animals behaving strangely – it was recently reported that a sea lion attacked a teenager, which was described as ‘extremely unusual’ behaviour for the creatures⁵.

Tackling toxins

As filter feeders, scallops feed on planktonic algae that produce domoic acid. Whilst apparently healthy, they can accumulate high levels of the toxin in their tissues. Fortunately, levels of domoic acid in shellfish are now monitored by public health authorities and fisheries to protect human consumers.

As part of the Sanger Institute’s 25 Genomes Project, the king scallop, Pecten maximus, had its genome sequenced. The genome will contribute to the Darwin Tree of Life project, a mission to sequence all 60,000 species of animals, plants, fungi and protists across the British Isles.

Published in Giga Science⁶, the scallop genome sequencing was led by researchers at the Sanger Institute and the Natural History Museum in London.

The research team revealed that specific genetic mutations within the sodium channel gene, Neuron Navigator 1 (Nav1) could be the key to its resistance to domoic acid’s affects.

“By comparing the Nav1 gene sequence from king scallops to the equivalent gene sequence in other species, we were able to infer the small changes in gene sequence that cause large changes in animal biology and behaviour. Changes in this gene may affect the toxicity of domoic acid, although there are likely to be other genes that also play key roles. Domoic acid is of particular interest since it mimics the neurotransmitter glutamate, which has been linked to Alzheimer’s and Parkinson’s Disease. Discovering how the scallop’s genome provides immunity has the potential to lead to new insights for human health” says Dr Nathan Kenny, a postdoctoral fellow who led the genome research at the Natural History Museum, now based at Oxford Brookes University.

Didn’t see that coming…

Dr Suzanne Williams, Head of the Invertebrate Division at the Natural History Museum, nominated the scallop for genome sequencing. As well as their immunity to toxins, she is interested in their eyes and their colouring.

“The evolutionary history of colour and vision has been closely intertwined over the last 500 million years, leading to a dazzling diversity of colours and visual systems in Mollusca,” says Suzanne.

“Surprisingly, given the range of colours and patterns in this group, nearly all molluscs are thought to be colour blind and most have poor visual acuity. Yet they showcase the greatest diversity of eye types in the animal kingdom.”

Scallops, unlike many bivalve molluscs, have eyes⁷. Up to 200 bright blue eyes peer out along the mantle between their two shells, looking for approaching predators. Scallops can rapidly and erratically swim short distances if needed, clamping their shells together to move by jet propulsion.

Scallop eyes are unique in the animal kingdom. The most common type of eye, like our own, uses a lens to focus light on a retina. The scallop eye uses an array of mirrors, like a telescope, to focus light onto two retinas. The structure is not found in any other animal.

“Having the genome sequence will underpin further research into this rare and fascinating feature,” says Suzanne.

Suzanne is also interested in the colour of scallop shells. “Colour can be used to attract, warn, camouflage, mimic, or even control temperature. My studies have focussed mainly on the identification of shell pigments and the molecular pathways responsible for their production in molluscs. I am particularly interested in heritable colour differences, where alternate colour morphs occur within a single species and are genetically controlled,” she says.

The power of genomics

Researchers producing genome sequences of the scallop along with other British species, which will be openly available to the research community, are part of the global effort to sequence the genomes of all animals, plants, fungi and protists on Earth.

Dr Kerstin Howe, Senior Scientific Manager at the Sanger Institute said, “The scallop assembly is of very high quality and the most contiguous of all published bivalve genome assemblies so far. It will be a very useful resource for investigations of evolutionary genomics in molluscs, growing scallops as food, population genetics, the biology of neurotoxins, and the evolution of novelties such as eyes and colouration, for many years to come.”