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10s of millions - but I am one in a million
About the same size as a human genome
The common ancestor goes way back - but still the same genes may determine the left and right sides of human and snail bodies.
I should be sequenced because...: we can find out find out why about one in a million snails like myself has a left-coiling shell.
Although most snails have shells that coil to the right, or clockwise, when viewed from above, around one in a million garden snails is a ‘lefty’, so that the shell coils anticlockwise. In 2016, a lefty garden snail was found in a garden in South West London, by a retired scientist from the Natural History Museum. This snail came to be known as Jeremy, after Jeremy Corbyn, left wing leader of the Labour party and keen gardener.
Jeremy the snail with offspring (and ant)
My scientists are proposing to use genome mapping methods to identify the genes that may be responsible for making rare lefty snails. The first step would be to decode the DNA that makes a garden snail – the prize that is being awarded by 25 Genomes.
To understand whether Jeremy’s rare left-coiling is inherited, then we have to follow the appearance of the ‘lefty’ trait over several snail generations. As Jeremy cannot mate with normal snails, the University of Nottingham launched a citizen science campaign to find a mate for Jeremy, using the hashtag #snaillove.
Jeremy the snail became an internet “shellebrity“, the most famous snail in the world (Jeremy’s twitter is here; read more on Jeremy’s story here and here). Within weeks, two further lefty garden snails were found, one in a garden in Ipswich by a snail enthusiast, called Lefty, and another found by a snail farmer and restaurateur in Mallorca, Spain, called Tomeu – that’s me! To much dismay, I mated with Lefty first, and went on to produce more than 170 offspring. Jeremy was left on the shel-f!
Jeremy died in October, before my scientist could preserve the DNA, but in a final twist, fathered offspring with me just before his death. In a rapid about turn, Dr Davison now plans to use my DNA, to decode the garden snail genome. This map would then be used to help find any genes that made Jeremy (and other snails) left-coiling.
Two week old baby snails – Jeremy as the father and Tomeu as the mother. Which way do their shells coil? Lefties or righties?
Following the citizen science campaign to find a mate for Jeremy, 10 other lefty snails have been found, 8 of them from Spain. Is that because the snails are common there, or because Spaniards love to eat them? Either way, good job I got out while I could!
In 2016, a team of scientists, led by Dr Angus Davison at the University of Nottingham, discovered a cell scaffolding or cytoskeletal gene that determines the direction of shell coiling, or chirality, in pond snails. Find out more here and here. By acting in the first few hours of life, variants in this gene cause the adult snails to have clockwise or anticlockwise coiling shells. Collaborating scientists in the USA discovered that the same gene may set up the left and right sides of a frog’s body. As frogs are vertebrates and so quite closely related to ourselves, then similar genes may also be involved in setting up our own asymmetry (e.g. heart to the left). Other undiscovered genes (perhaps in Jeremy) are probably involved in determining the asymmetry of snail and vertebrate bodies.
In humans, approximately one in ten thousand people is mirror imaged internally (e.g. heart to the left, instead of right), a rare condition called situs inversus. Celebrities with this condition include Enrique Iglesias and Donny Osmond. See here.
The directional coiling or chirality of the shell is also reflected in the asymmetry of the snail body. Very rare lefty snails with anticlockwise coiling shells are usually unable to mate with normal clockwise-coiling snails, because the genitals are on the ‘wrong’ side of their head. As lefty snails can not mate and do not produce offspring, then the gene variants that make them different tend not to spread.
Most land snails are hermaphrodites, so mate as male and female at the same time. Even though snails are hermaphrodite, the sexes are still separate within an individual snail e.g. a female gives birth to eggs which were fertilised by another snail acting as a male.
Although some snails are able to self-fertilise, most prefer to outcross if possible. Some snails stab love-darts into each other when mating. The darts are sharp, fluted crystalline structures, and vary according to the species. Find out more here.
The darts are not directly involved in fertilisation, and come from a separate organ to the genitals. It is believed that the mucus on the dart increases the chance that the snail being stabbed will use the sperm it receives to fertilise its eggs. Even though dart-stabbing is potentially harmful to the recipient, there is a benefit to the mating snails if it gains greater paternity (fathers more offspring). Love darts probably arose from sexual conflict – when males and females have different evolutionary interests.