Hoang+page


 * Polymorphism in //Cepaea nemoralis// in Monk’s Riseborough **

Alina Miedzik, Sarah Hoang*, Sevda Dogan, Mahilini Kandasamy, Makka Habre, Mathurra Karunalingam , Martinez Varon and Tasmiya Wahed.

INTRODUCTION // Cepaea nemoralis // is a western European land snail which prefers warmer climates and is found on dunes in cultivated areas. This species is a convenient model organism as it is not widely dispersed, therefore its genetic patterns are localised within a particular area. Their shell patterns reflect their genetic makeup hence costly genome sequencing can be avoided. In addition, it is __less unethical[double negative]__ to study //C. nemoralis// __although [sounds as though you are contradicting the previous statement]__ the findings from studying this species can be applied to other organisms including humans.

// C. nemoralis // have a pink, yellow or brown background colour with a number of bands varying from zero to five. This is __evidence of polymorphism [it is polymorphism according to the following definition... it is evidence if you define polymorphism at the level the genotype]__, an occurrence of two or more phenotypes within a population. The genes coding for these phenotypes are clustered together on a chromosome and are collectively known as a supergene. Previous studies have shown that yellow coloured snails are found in grassland and __prefer [prefer?]__ lower temperatures, whereas dark coloured snails are found in woodlands where the temperature is __higher [evidence?]__. The different background colours allow the snails to camouflage in order to avoid predation; this characteristic could explain the phenotypic frequencies observed in various habitats.

The purpose of this study is to determine whether genetic drift, selection or gene flow accounts for the polymorphism in //C. nemoralis //. Large samples were collected from two different locations at high and low altitudes in Pulpit Hill Reserve in Monk’s Riseborough. All the snails’ shell sampled were found in shrubs and scored according to their background colour and presence of bands.

A Chi square test was used to analyse the data collected in order to determine if there was a significant difference between the phenotypic frequencies at both locations. If there is no significant difference, __this would indicate that sampling error is unlikely to account for the observations [no no, completely the wrong way round!].__ However, it may be explained by genetic drift, selection or gene flow.

Selection is a non-random process where an allele becomes fixed or lost over many generations. According to the different selection pressures, certain phenotypes would be observed at different locations. For example, only pink snails would be found at high altitude whereas, yellow snails would be present at low altitudes. Selection can be distinguished from genetic drift, a change in allele frequency due to chance. So if genetic drift is acting on //C. nemoralis,// the phenotypic frequency on both locations would be random. In this case, both pink and yellow snails would be found at each location .The variation within //C. nemoralis// phenotypic frequency could also be explained by gene flow, a migration of genes from one population to the other within the same species. For example, if a large number of brown snails were found on many sites within one location and a small quantity of pink snails was observed in a few, this could be attributed to gene flow.

Despite the statistical analysis, a definite conclusion cannot be drawn about which of the three evolutionary processes has affected the allele frequency, as the population history was not known before the samples were collected. Furthermore, many factors such as predation, temperature or human activity have not been taken into account. However, findings from this study could be used to gain a better understanding and an insight into polymorphism in humans and other organisms.

Word Count: 544 words

REFERENCES Jones, J.S., Leitch, B.H., Rawlings, P. 1977. Polymorphism in //Cepaea// : //A Problem// with Too Many Solutions? Ann Rev Ecol Syst. **8 ** , 109-143

DISCUSSION

After analysis of the raw data gathered, it was brought to our attention that the yellow banded snail is the most abundant phenotype in both altitudes, followed by banded pink snails. We believe that the fact that there was more yellow than pink shells in each shrub may be due to selection. The yellow shell colour is selected for in the shrub habitat. This feature allows snails to blend in with the environment, so they are less visible to the predators such as song thrushes.

A Chi square test indicated that there was no significant difference in allele frequency at high and low altitudes for sites two, three, four and six. The most likely explanation for lack of difference in allele frequency in site  six is sampling error, as a sample size of 29 was too small <span style="color: #7030a0; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">, <span style="font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">but <span style="color: #000000; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;"> for site two, three and four, there was not actual difference in allele frequency. Site one and five shows <span style="font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">there is a significant <span style="color: #000000; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;"> difference in allele frequency which may be due to genetic drift, selection or gene flow.

<span style="color: #000000; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">Not a single population was found to be distinct from other populations at both altitudes. In addition the population history is unknown so we do not believe that gene flow has a strong impact on the distributions of allele frequency. <span style="font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">Genetic drift is unlikely to account for the differences in allele frequency, as it would act equally on all phenotypes. If genetic drift has an effect, the number of yellow snails would be random at both altitudes. However, natural selection cannot be eliminated <span style="color: #000000; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">as other variables such as weather, <span style="font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">soil composition <span style="color: #000000; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">, predation and human activities were not measured.

<span style="font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">Part of our findings support the existing work of Jones //et al// (1977), although we did not investigate grassland specifically. In both studies, yellow snails were the most abundant in lower altitudes, however in ours they were also found in high altitude. In his study Jones attributed this particular distribution to predation as yellow shells camouflage against black backgrounds. Likewise we did not find many of brown snails as our sampling design excluded woodlands.Our study contradicts some of the findings of Jones //et al// (1977), he suggests that the polymorphism of //C. nemoralis// is due to genetic drift, whereas our study suggest that the differences in allele frequency is more likely caused by selection. However our findings supports Lewontin (1974), these conflicting conclusions indicate the challenges in determining which evolutionary processes act on the allele frequency of //C. nemoralis.//

<span style="color: #000000; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">We cannot solely rely on our data as we experienced some difficulties distinguishing between the three shell colours. For instance, some appeared white and were excluded from our sampling, as its original colour may have faded away due to many environmental factors.

<span style="color: #000000; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">The experimental design could be improved by increasing the number of sites sampled in each location, maintaining a consistent sample sizesand to ensure there is an adequate number in each category to avoid grouping in the Chisquare test. In addition, the experiment should be repeated with controlled environmental factors and also during another season to see if a different pattern is observed.

The polymorphism of snails has been studied since the beginning of the 20th century (Lang, 1904, 1908). Undeniably, their polymorphism is a complex example of Mendelian genetics and our understanding of their inheritance patterns may be related to humans. For instance, our explanation for the variation in allele frequency observed in //C. nemoralis// at different altitude could be used to determine the relative importance of genotype and the environment on human phenotypes.

<span style="color: #000000; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">A complete explanation of polymorphism in //C. nemoralis// cannot be derived from this study as it is not clear which evolutionary processes have acted on the allele frequency. Nonetheless this is an incremental step forward to further investigate polymorphism in //C. nemoralis.//

<span style="color: #000000; font-family: 'Calibri','sans-serif'; font-size: 18.6667px;">Word count: 630