GAS

=Group members=

Rob Blackler Jordan Hanbidge Jake Cush Ned Berry Jenar Thiyagarajah

Introduction
// Cepaea nemoralis // (common name Grove snail or Brown-lipped snail) is a species of small terrestrial pulmonate mollusc, belonging to the //Helicidae// family. This attractive snail is a successful coloniser and is found widespread across vast parts of Europe. Its aesthetic appeal has even seen it purposefully introduced to North America (Whitson, 2005). //Cepaea nemoralis// is found in a variety of habitats including woods and shrubs in plains and highlands, gardens, roadsides and even dunes. Like most terrestrial snails //nemoralis// is a hermaphrodite and must mate in order to produce fertilised eggs. It has a relatively slow development; usually taking up to three years to reach full, potentially breeding adulthood (Silvertown, 2011). It can be distinguished from its closest relative // Cepaea hortensis // (White-lipped snail) by the darkened apertural lip at the base of the shell. This particular species of gastropod exhibits a huge degree of variation in shell colour and banding. The presence of a range of phenotypes in a randomly mating population is known as genetic polymorphism and //Cepaea nemoralis// is a classic example that has been the subject of much research over the years. In the early 1900’s it was thought that this case of polymorphism could be explained by the rules of Mendelian genetics (Lang, 1904, 1908). However, simple laws of segregation cannot explain all the occurrences of variation. The development of techniques for the molecular detection of polymorphism has caused pronounced disagreement between scientists. The variation can be attributed to selective environmental pressures on the snails but also the natural drifting of genes through a population. The shells can range in colour; pink, brown and yellow, and can be banded with up to 5 bands. Research has shown that a set of alleles control ground colour, number of bands and type of band, forming a five-loci linkage group. Two additional loci modify the five-banded phenotype (reviewed by Murray, 1975, Cook 2007). This shows a huge degree of variation within a single population but what is it that has caused and maintained these phenotypic differences? Predation is likely to play a role. The Song thrush is the Grove snails’ principal nemesis and has a ‘searching image’ that can govern the way it predates on the snails, but to what extent is this relevant? The elements and parameters that make up the structure of the habitat can play a role in phenotypic variation. Are there any correlations that can help explain the phenotypic variety, is it a case of genetic drift, or is the variation between closely located populations kept low by gene flow? The snails’ ability to reserve the sperm it collects would be expected to help the latter process. To find out what is dictating the varying frequency of the alleles that control shell colour and banding, samples were taken from Pulpit Hill, Lower Cadsen, Buckinghamshire, a study area that exhibits a good range of //nemoralis’// typical habitat; grassland, shrub area and woodland at a range of altitudes.

Hypothesis
The frequency of a particular polymorphism in //Capaea nemoralis// will be higher in the scrub/bushes, which suggests that the higher frequencies of a certain polymorphism are a result of selection

Null hypothesis
The frequency of polymorphisms will not be higher in any given region (i.e. grassland or scrub), which suggests that genetic drift is affecting the entire population

Method
We plan on doing two horizontal transects of the filed site at different heights, sampling two vegetation sites. Furthermore, we will work in collaboration with one other group and pool our results, giving us a slightly greater quantity of data to work with. Because gene flow generates associations between sub-populations, the method must ensure that the observations are independent. The samples must be at least 20m apart, as this is how far a snail can travel in a single generation. Prior to the collection and identification of snails, the group will have a common predetermined criteria that determines the classification of each individual (for example, colour and banding patterns).



Strengths

 * By combining our data with that of another group we will have a greater range of data available for analysis
 * Using a transect method reduces biased sampling
 * By sampling at two different altitudes, we will be able to partly determine whether this variable is linked to ploymorphisms in //Capaea nemoralis//
 * Our two samples will be independent of each other due to distance between sampling sites

Weaknesses

 * The transects only cover a specific area and the sample size is still quite small
 * The small sampling area does not give an accurate representation of the meta-population