An analysis of the observed heterozygosity of Lake Trout populations from three lakes: Devil, Eagle, and Loughborough, inferred from microsatellite genotypes. Abstract: This study was undertaken in order to compare the heterozygosity of three Lake Trout populations at various loci. Samples of twenty-five Lake Trout were collected from three lakes: Devil, Eagle and Loughborough, all three of which are situated north of Kingston, Ontario. An autoradiograph was used to analyze the genotypes of the individuals at six different loci of microsatellites, which are repeat sequences in the DNA that are neutral and do not code for proteins.
This data was used to compare the genetic diversity of the three different trout populations. Numerical values for observed heterozygosity (Ho) were then generated using the data and the Doh heterozygosity calculator. The results have indicated that the mean heterozygosity in respect of Devil Lake trout was significantly greater than that of the trout in Eagle Lake (p=2. 89E-7) as well as that of Loughborough Lake (p=1. 44E-19). Furthermore, the mean heterozygosity for Eagle Lake trout was significantly greater than that of Loughborough Lake (p=2. 2E-6). This may be due to the fact that natural selection acts as a force to cause inbreeding to eliminate harmful genes causing low heterozygosity in a population. In addition, human and natural effects occurring in the lakes, for example, fishing and water temperature may cause differences in heterozygosity. Understanding and using these findings may help with sustaining fish populations. Introduction: Heterozygosity is the measure of the genetic variation in a population at a particular gene locus.
Genetic variation within a population is important in maintaining or increasing the fitness of members in the population and ultimately the survival of the species. Fitness describes the capability of an individual species of a certain genotype to reproduce, and is usually equal to the proportion of the individual’s genes in all the genes of the next generation. A positive correlation was found between the heterozygosity at the loci and the fitness (survival and maturation) of the fish, suggesting that heterozygosity is advantageous (Pujolar et al. 005). A heterozygote advantage describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. An individual’s fitness is manifested through its phenotype, and the phenotype may be affected by both genes and environmental characteristics. One such characteristic that was observed to possibly have an effect on levels of heterozygosity in a population was the area in which the population lives. In an experiment conducted by Rowe et al. 1999) the heterozygosity of various populations of Natterjack Toads (Bufo calamita) found in several areas were compared, ultimately discovering a lower heterozygosity in a population that is isolated from others. Volckaert and Zouros (1989) conducted a study to measure genetic diversity levels in scallops (Placopecten magellanicus) and discovered levels of heterozygosity to be highest as age increased. Ferguson (1990) found similar information that affects diversity among rainbow trout (Oncorhynchus mykiss) and concluded that heterozygosity levels were proven to have a direct relationship between the sex, size and age of the fish.
Using six microsatellite loci from 25 Lake Trout from all three lakes, observed heterozygosity values that act as an indicator for genetic diversity, will be obtained and analyzed. This data can be further used by analyzing and providing additional information about the influences of certain characteristics on population genetics. Results: Figure 1 illustrates that the lake with the greatest observed heterozygosity is Devil Lake. It was determined that the observed heterozygosity of Devil Lake is significantly greater than the observed heterozygosity of Loughborough Lake (p=1. 4E-19). The sample size for all 3 lakes was 25 Lake Trout. Figure 1. The graph illustrates the mean observed heterozygosity of the three lakes. The error bars represent standard deviation. Discussion: The conducted experiment involving heterozygosity of Lake Trout from Devil, Eagle and Loughborough Lake shows that there are significant differences between the three lakes. Devil Lake had the highest mean heterozygosity within its population, Eagle Lake heterozygosity was found to be in the middle and Loughborough Lake with the lowest.
It was determined that the observed heterozygosity of Devil Lake was significantly greater than the observed heterozygosity of Loughborough Lake (p=1. 44E-19). The difference in the data set’s outcomes may be explained by a number of factors, such as natural selection, fishing and restocking the lake, and lake temperatures. All these factors may cause diversity in heterozygosity. The goal of an organism is to reproduce and pass their genes on to the next generation allowing the species to survive.
The passing on of genetic material can be achieved through inbreeding or outbreeding. Inbreeding is the breeding amongst family or self; outbreeding is the breeding with members of the same species that are not closely related. It may be believed that inbreeding is not good for a population with such opinions being based on having seen the result of inbreeding in humans. Inbreeding as well as outbreeding, however, has both advantages and disadvantages. One advantage of inbreeding is its ability to depress the expression of recessive alleles (Ellstrand and Elam 1993).
In a population with a damaging recessive allele, an individual may not seek to mate with anyone who potentially carries or expresses that allele. In this example the population might inbreed to decrease the heterozygosity in an attempt to remove the harmful gene. Mating within the family- when it is apparent that the family does not carry the detrimental allele, is more ideal in an evolutional prospective than putting the survival of that population at risk.
In regards to Ellstrand and Elam’s study, this situation could occur in the Lake Trout from Loughborough causing the Lake Trout to have a lower mean heterozygosity. This Lake Trout population could be purging undesired alleles from its gene pool. One can conclude that not only does genetics have an effect on heterozygosity, but humans do as well. Another factor that may cause a loss of genetic diversity is fishing pressures. Smith et al (1990) suggested that fishing activities which concentrate on spawning populations differentially remove the older and more heterozygous individuals from the virgin stock.
Previously stated, levels of heterozygosity are higher as age increases (Volckaert and Zouros 1989). Due to fishing, the amount of Lake Trout may decrease and there would be less fish. To fix the amounts of fish in the lakes, humans restock the lakes with hatchery fish (fish that are grown by humans and released into the wild). Evans et al. (1991) found that the human harvested fish tend to have lower genetic variation and actually decrease the fitness and survival of the native species. Loughborough Lake has the biggest population but the lowest heterozygosity.
Compared to Eagle Lake and Devil Lake, most people from the Loughborough Lake area receive their income from fishing (Ontario Ministry of Natural Resources 1970). Excessive fishing depletes the amount of fish and creates the perceived need to continually restock the lake with fish. The practice of restocking the lake with hatchery fish may result in the large population of Lake Trout which would in turn decrease the heterozygosity of Loughborough Lake. There are other factors that may contribute to increase levels of heterozygosity in fish.
One such characteristic that may increase levels of heterozygosity in fish is fluctuations in water temperature. Zimmerman and Richmond (1981) found that highly variable thermal regions demand for greater fitness. The fittest of fish are more heterozygous because they are able to survive in different temperatures. In Zimmerman and Richmond’s experiment, the greatest temperature fluctuation was 7°C, with the highest heterozygosity level of 49%. This trend may prove that the greater the temperature fluctuation, the greater the heterozygosity of a population living within the waters.
The temperature fluctuations of the three lakes are: Devil Lake at 31°F, Eagle Lake at 21°F, and Loughborough Lake at 7°F (Ontario Ministry of Natural Resources 1970). These numbers correlate with the data by showing that Devil Lake with the highest temperature fluctuation has the greatest heterozygosity, whereas Loughborough Lake with the lowest temperature fluctuation has the lowest heterozygosity. The mean heterozygosity of Lake Trout from Devil Lake was significantly greater than that of trout from Eagle Lake, which was greater than that of Loughborough Lake.
Potential reasons for genetic diversity may be caused by natural selection acting as a force to cause inbreeding to eliminate harmful genes, fishing in the lakes which then require the lakes to be restocked with hatchery fish, and thermal fluctuations that cause differences in heterozygosity. Further research and experiments specifically looking in depth at effects that causes genetic diversity should provide greater insight into why the heterozygosity in populations varies. Literature Cited: Ellstrand N. , Elam R. 1993.
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