NEW YORK (GenomeWeb News) – Genetic patterns in snail populations across Western Europe and the UK — described in a new PLOS ONE study — point to a possible human migration from the Eastern Pyrenees or south of France to Ireland during the middle Stone Ages.
A pair of researchers based at the University of Nottingham looked at the genetic relationships between more than 100 populations of modern-day banded wood snails (Cepaea nemoralis) from locales in Western Europe and the UK.
Based on multiple mitochondrial gene markers from nearly 1,000 snails, they determined that snails in Ireland and the Eastern Pyrenees region bordering northern Spain and southern France belong to the same mitochondrial lineage.
Because that lineage is distinct from those found in intervening parts of Europe, investigators suspect some of the Irish snails' ancestors may have reached the island by hitchhiking with humans who made their way from the Eastern Pyrenees by sea.
Based on the age of the oldest snail fossils in Ireland, that migration appears to roughly coincide with the timing of Ireland's colonization, though the study's senior author biologist Angus Davison is quick to point out that there may have been humans already living in the area by the time the snail-bearing voyagers arrived.
"This is evidence of a group of humans — not necessarily the first to land in Ireland — arguably moving from one place to the other," Davison, a University of Nottingham biologist, told GenomeWeb Daily News.
Though speculative, the findings seem to jibe with physical features found in some C. nemoralis snails, Davison and co-author Adele Grindon, a graduate student in his lab, noted. Whereas C. nemoralis snails found in most of Europe have relatively small, black-lipped shells, snails with much more massive, white-lipped shells have been described in both Ireland and the Pyrenees.
In an effort to untangle C. nemoralis' origins in Ireland — and to delve into their own evolutionary biology studies — Davison and Grindon used Sanger sequencing to assess cytochrome oxidase subunit I and 16S ribosomal RNA genes from 880 C. nemoralis snails belonging to 111 populations scattered across western Europe.
Using sequences at these two mitochondrial markers, the team classified the snails into seven main mitochondrial lineages, designated A through G. Across most of mainland western Europe, snails from the A lineage were most common, the analysis suggested, with D lineage snails not far behind.
In contrast, almost three-quarters of snails from Ireland carried mitochondrial sequences from the C lineage, which also turned up in snails from the central and eastern parts of the Pyrenees area.
With the exception of a few isolated snails in Wales, southwestern France, and the Isle of Man, representatives from the C lineage weren't found at sampling sites between Ireland and the Pyrenees, suggesting the far-flung snails are not related as a result of snail movement across mainland Europe. Rather, researchers speculated that the snails may have been carted across the sea by humans traveling from southern Europe to Ireland.
There's no way of knowing for sure whether that transport was accidental or intentional. But as Davison and Grindon point out in their PLOS ONE paper, archeological studies suggest snails have been used as a food source for populations in the Mediterranean and parts of the Pyrenees for thousands of years.
Coupled with Cepaea snail shell fossils from Ireland going back 8,000 years or more, that evidence of ancient snail-eating hints that snails could have hitched a ride on an ancient voyage to Ireland from southern Europe, perhaps via the Garonne River, which has links to the Atlantic Ocean and abuts the eastern Pyrenees region where C lineage snails currently reside.
"When we look at human genetics of people currently inhabiting Ireland or these islands, you can begin to work out where they came from," Davison said. "But what you're looking at there … is the result of, potentially, many different events."
"If our speculation is correct," he noted, "rather than seeing kind of an average picture, what you're beginning to see is an individual migration event."
Davison and Grindon cautioned that there are alternative explanations to their migration theory that still need to be ruled out. For instance, it's possible that snails from the C lineage moved between southern France and Ireland mainly by land before becoming extinct in most of mainland Europe.
The duo also noted that analyses of additional mitochondrial and nuclear markers may somewhat alter the relationships that can be discerned between snail populations.
"What we'd like to do is use other genetic markers to firm up the data," Davison said.
"But of course when you use other genetic markers you do sometimes get different patterns. So that might be the case," he added. "That will be something to investigate in the future."
His team is already using genomic approaches such as restriction site-associated DNA sequencing, or RAD-seq, to find genetic markers that can be used for ongoing efforts to link snail traits to specific genetic features.
In a study published in Molecular Ecology this month, for instance, Davison and co-authors reported on potential markers for shell color and banding patterns in C. nemoralis snails that were found with RAD-seq.
Davison and his colleagues have also taken a crack at sequencing genome of at least one snail species, though genome assembly has been hindered by the size and repeat-rich nature of snail genomes in general.