SnailsRus

__ INVESTIGATION INTO THE ACTION OF GENETIC DRIFT, GENE FLOW AND SELECTION ON THE FREQUENCY OF SHELL COLOUR POLYMORPHISM AND BANDING PATTERN OF //CEPAEA NUMERALS// __

Matthew Kelly*, Venus Ahmed, Athithya Athavan, Umeeka Hansora, Mumina Javed, Yasser Tahir __Aims__ The purpose of the experiment was to understand and be able to implement a strategy to __record__ __ [we can't do that as the processes can't be observed directly - we must infer] __ the action of genetic drift, gene flow and selection on the frequency of shell colour polymorphism and banding pattern of the snail species known as //Cepaea nemoralis//. At the same time, __whether the snail was living or deceased and whether it was juvenile or adult was recorded__ __ [not an aim, more a method] __ to obtain an understanding of whether the above three factors had __more effect on adult or juvenile snails__ __ [logic of how you could do this is unclear] __ and as a result, i__f the mortality rates differs____ [ how you would you know, from the ratio of snails that died as pre-adults to adult? Say ] __ between the low and high sites, this __may show that the effect of selection may be evident.__[e__xplain better]__ Over all the purpose of the experiment is to show that genetic drift, gene flow and natural selection have a large role to play in the polymorphism of the //Cepaea nemoralis//. __ [no - it is to investigate which of the processes are operatin] __

__Background__ The genus Cepea has many species; one of them being //Cepaea nemoralis//. //C. nemoralis// is a western European species hence found quite abundantly in the United Kingdom, The species is colloquially referred to as the Brown Lipped Snail; is a banded wood snail which possesses a variety of shell colours. It is a macroscopic Gastropod that particularly displays pink, brown and yellow glossy shells. It has 2 predominant morphs and they are; banding pattern and colour. The size of an adult //C.nemoralis// usually ranges between 20-25mm.This specie is a hermaphrodite and is the most common snail that exists in much of Western Europe e.g. Germany, Austria and Spain. Jones et al (2009) also found that they are present on dunes and cultivated areas and 2100m in the Pyrenees. Recently, they have been introduced to North America. The dark brown snails usually live in woodlands and the yellow snails in meadows and grasslands. In addition, they are nocturnal. Banded wood snails can also live at very high population densities amongst many regions of Western Europe. They possess brown lips and exhibit a glossy, smooth and thin shell. There are up to 5 spiral dark bands on its shell and the //C. nemoralis// shell is much thinner compared to its sibling: //Cepaea hortensis// (which is white lipped). They are visually polymorphic thus are much used as model organisms in projects on heredity; genetics; evolution and ecology. Arnason et al (1975) concluded from their research into //Cepaea Hortensis// that perhaps the banding on snails may be dependent on their habitat and thus protecting them from the potential threat of predators. They found that snails on the ice caps of Iceland had no or hardly any banding whatsoever. From previous studies into this area, it has become evident that //C. Nemoralis// can habituate many different types of land, each of which have their own individual adaptations to survive. For example, Currey et al (1963) found that samples taken from woodland populations have, in general, fewer yellow and unbanned shells, as opposed to those taken from populations in ‘open’ habitats, implying that, in England at least, selection by visual predators is paramount in the determination of gene frequencies in //C. Nemoralis//. Rosin et al (2011) also found evidence of avian predation, with a particular preference for brown-lipped banded snails, perhaps explaining why such a low amount of brown snails were found in our samples.

__Method__ In this current experiment the sample area was split into two locations, High elevation and Low elevation, each elevation was then spit into three sites – left, middle, right. 30 snails from each site were recorded to give an adequate number to use statistical analyses on. The colour of snail shells, the number of bands on the snails, their age adult or juvenile and their their mortality were recorded. The distinction between these two locations was made by roughly estimating the half way point of the hill form the low point to the fence surrounding the sample area near the top of the hill. The same snail was not be counted twice as the researchers moved in a grid type pattern to ensure no repeat readings occurred.

__Hypothesis__ Doing this experiment will allow us to find if there is a statistically significant difference in shell colour and banding at different altitudes. This difference will be tested with a chi squared test.

__Null Hypothesis__ The null hypothesis is that the change in altitude has no effect on the shell colour and banding and they will all have similar frequencies in each site.

__Statistical method__ <span style="font-family: Georgia,serif; font-size: 13px;">If the data is found to be significantly divergent from the null hypothesis (5% chance of being due to the null hypothesis) then it will be rejected unless no other explanation can be found. If there is no distinguishable difference then any variation is likely due to genetic drift or gene flow. If there is a statistically significant difference between environments and phenotypes then selection likely to be the factor.

<span style="font-family: Georgia,serif; font-size: 13px;"> <span style="font-family: Georgia,serif; font-size: 13px;">__Analysis of the results__ From the results found we can now deduce that out hypothesis is correct. From the analysis of the chi square tests it was found that overall there was a significant difference in //Capea nemoralis// shell colour (pink, yellow or brown) and the banding patterns at different altitudes. Figure 1 shows brown and pink coloured snails were more abundant at high altitudes whereas yellow coloured snails were more abundant in lower altitudes. There were 28% more yellow coloured snails in the lower site compared to the brown and pink snails. <span style="font-family: Georgia,serif; font-size: 13px;"> Figure 2 shows that there was a much larger amount of brown and pink coloured snails with less than 3 bands in the high site compared to the yellow snails with the same number of bands. <span style="font-family: Georgia,serif; font-size: 13px;"> There were only 30% of yellow snails that possessed 4-6 bands in the high site but, there were 68% of yellow snails that had 4-6 bands in the low site, as opposed to only 28% of brown and pink snails in the same altitude. <span style="font-family: Georgia,serif; font-size: 13px;"> Analysis of the chi square results show that there was a significant difference in shell colour in the low site, x² = 9.03, which exceeded the critical value of 7.88 (p = 0.005). <span style="font-family: Georgia,serif; font-size: 13px;"> But, there was no significant difference found in shell colour at the high site, x² = 0.78, which did not exceed the critical value of 3.84 (p = 0.05). There was a significant difference in snail banding number in low areas as x² = 9.03, which exceeded the critical value of 7.88, (p = 0.005). There is a significant difference in snail banding number in high areas as x² = 6.75, which did exceed the critical value of 6.64, (p=0.01).

<span style="font-family: Georgia,serif; font-size: 13px;"> <span style="font-family: Georgia,serif; font-size: 13px;">__Validity__ Experiment may not be valid as it should because of the use of only one habitat to study, therefore our results may only apply to the given population and cannot be generalised to a larger one and therefore, this may affect our studies’ external validity. <span style="font-family: Georgia,serif; font-size: 13px;"> All variables were carefully recorded and collated so that a chi-squared test could be carried out, the chi-squared test allowed us to use a degree of confidence in accepting or rejecting the null hypothesis. As the chi-square is a parametric test, it won’t work as well with small amounts of data, meaning that data inputted into it would always be of a high validity. <span style="font-family: Georgia,serif; font-size: 13px;"> Confounding variables may have arisen such as the subjectivity of peoples’ perceptions of the colours, particularly between the yellow and then browns. However, only a few brown snails were found, meaning that this problem may not have caused such a huge problem. <span style="font-family: Georgia,serif; font-size: 13px;"> We found at least 60 samples from each of the three sites, thus totalling 180 samples in total, a large number that would mean that the effects of any other extraneous variables may become exempt and not affect our final results. <span style="font-family: Georgia,serif; font-size: 13px;">

<span style="font-family: Georgia,serif; font-size: 13px;"> __How do your findings relate to previous findings?__ There is no evidence to suggest that darker coloured snails will cool down faster in terms of body temperature than lighter coloured snails in sunny areas (reference to Leith) therefore colour of snails is found not to significantly distributed across all areas. The snail species C.vindobonensis which have fully pigmented shells have increased body temperature in the sun than snails with reduced pigmentation on their shells. Previous studies have shown that pink and unbanded shells are more frequently found in wooded areas, whereas yellow and five banded snails are favoured in open areas. (Lamotte, M. 1969) however in some areas this was not found to be the case, such as in the Pyrenean valleys. <span style="font-family: Georgia,serif; font-size: 13px; line-height: 24px;">

<span style="font-family: Georgia,serif; font-size: 13px; line-height: 24px;"> __Improvements and potential problems__ Can be improved by comparing to other Cepaea species in the same environment. <span style="font-family: Georgia,serif; font-size: 13px; line-height: 24px;"> Only a few brown snails were found, thus we could exempt this variable altogether in future research, and just choose to briefly mention it later or use a scale to measure the intensity of a colour as being either pink or brown.

<span style="font-family: Georgia,serif; font-size: 13px;">__References__ <span style="font-family: Georgia,serif; font-size: 8px;">J.S.Jones, B.H. Leith, P.Rawlings, (2009), ‘Polymorphism in Cepaea: A problem with too many solutions?’ in Annual Review of Ecology and Systematics, vol.8 (1977), pp.109-143. Accessed: 01/11/2009

<span style="font-family: Georgia,serif; font-size: 8px;">Arnason E, Grant P. R (1975) Climatic selection in C. Hortensis at the northern limit of its range in Iceland. Evolution. 30 499-505.

<span style="font-family: Georgia,serif; font-size: 8px;">Currey J. D, Arnold W & Carter M. A (1963) Further examples of variation of populations of Cepaea Nemoralis with habitat. Evolution

<span style="font-family: Georgia,serif; font-size: 8px;">Rosin Z. M, Olborska P, Surmacki A & Tryjanowski P (2011) Differences in predatory pressure on terrestrial snails by birds and mammals. Journal of Bioscience. 36 691-699.

<span style="font-family: Georgia,serif; font-size: 8px;">Cain, A. J., King, J. M. B., Sheppard, P. M. 1960. New data on the genetics of polymorphism in the snail Cepaea nemoralis L. Genetics 45:393-411

<span style="font-family: Georgia,serif; font-size: 8px;">Cook, L. M. 1967. The genetics of Cepaea nemoralis. Heredity 22:397-410

<span style="font-family: Georgia,serif; font-size: 8px;">‍Lamotte, M. 1969. Relations entre deux couples de caracteres dependant dedeux locus etraitement lies: les caracteres jaune/rose et avec bandes/sans bandes dans les populations naturelles de Cepaea nemoralis (Mollusques Heli- cides) du Sud de l'Aquitaine. C. R. Acad. Sci. Ser. D 268:2476-79‍