Number Of Heterozygous Individuals Research Articles

Efficiency of using SNP markers in the MSTN gene in the selection of the Pushkin breed chickens

In the poultry industry, indicators reflecting the growth rate of young stock and the exterior characteristics of chickens are important benchmarks for breeding. Traditional selection based on phenotypic evaluation is characterized by low efficiency with a low character inheritance ratio and is difficult to apply in small groups of animals and birds bred in bioresource collections. The use of molecular genetic markers associated with economically important traits makes it possible to carry out early selection of birds. This entails an increase in the profitability of the poultry industry. Recently, single nucleotide polymorphisms (SNPs) have served as convenient markers for selection purposes. For five generations (P1–P5), an experimental selection of hens of the Pushkin breed was carried out for live weight. It was based on selection for single nucleotide polymorphism rs313744840 in the MSTN gene. As a result, a significant increase in the frequency of allele A in this gene, from 0.11 to 0.50, took place. The association of SNP markers with meat qualities in the experimental group led to changes in the exterior profile of an adult bird at 330 days of age. The individuals with the AA and AG genotypes had the greatest live weight and longest body. As a result of selection, the bird on average became larger due to an increase in the number of heterozygous individuals with long bodies and large chest girths. The depth of the chest and the width of the pelvis increased due to an increase in the frequency of allele A in the experimental population. A tendency towards an increase in these indicators with the substitution of G with A in the genotype was found. Saturation of the population with desirable alleles led to an increase in the average live weight of the chickens. Analysis of the exterior parameters of adult birds showed that this growth is achieved by increasing the depth and volume of the bird body, and not by increasing the length of the limbs. Thus, marker selection carried out for five generations in the experimental population of Pushkin breed chickens to increase body weight has reliably (p < 0.001) changed the exterior profile of adult birds.

GENETIC ANALYSIS GERMAN SHEPHERD BREED DOGS USING MICROSATELLITE DNA MARKERS

Modern methods of breeding dogs are based on getting of stable phenotypic uniformity by using close inbreeding or breeding by one line, but such strategy leads to a loss of genetic diversity, and as a result there are genetic defects in breeds which have no external manifestations or manifest in adulthood of dogs and are transmitted from generation to generation. Therefore, to prevent use of dogs with genetic abnormalities in breeding, and to develop standards for a breed and make an accurate pedigree, must carry out the genetic evaluation of animals. One of the modern tools for dogs’ genetic evaluation is DNA-testing using microsatellite loci permitting to match the parental couple effectively, identify (to certify) animals, undertake a comprehensive assessment for heterozygous and homozygous genotypes in populations, permitted for use in the selection process, and illustrate clearly the impact of artificial selection on the genetic characteristics of breeds. The study was conducted in Ukrainian Laboratory of Quality and Safety of Agricultural Products in Department of Molecular Biology Research. For the genetic analysis 42 German Shepherd dogs, used for breeding in kennels of Ukrainian Kennel Union (UKU), were selected. The material for the research was DNA isolated from dogs’ buccal epithelium cells and blood. Genomic DNA was extracted using a standard set of reagents for DNA isolation. Level of theoretically expected heterozygosity (Hexp) varied between 0.385 (PEZ1) to 0.835 (PEZ8). On average theoretically expected heterozygosity with coefficient of 0.657 had not significant advantage over value of actual heterozygosity (0.629), it also shows that the status of the sample of dogs is close to balance. The same is observed in actual and expected heterozygosity for PEZ 6 (0.629) and PEZ 8 (0.657) loci, which also shows the balance. For FHC2010 loci actual heterozygosity is higher than expected, indicating increasing the number of heterozygous individuals. For FHC2054 locus, by contrast, theoretically expected heterozygosity (0.670) dominates the actual (0.429), indicating the lack of heterozygous genotypes in this micropopulation. The value of PIC (polymorphism information content) of the analysed loci ranged from 0.325 to 0.740 with average value 0.574. PEZ6, PEZ8, FHC 2010 and FHC 2054 loci optimally meets their suitability for genetic certification of genotypes because their frequency varies from 0.587 to 0.740. The reduced average index of polymorphism for PEZ1 locus with coefficient of 0.325 confirmed the insufficient level of its polymorphism for full genetic evaluation of the micropopulation of German Shepherd dogs (PIC < 0.500), as confirmed by Chinese researcher J.-H. Ye, according to his data PIC value for PEZ1 locus was 0,320, which correlates with our results. And PIC value for PEZ8 locus was 0.740 in our studies, whereas according to J.-H. Ye – 0,720, which, by contrast, indicates high polymorphism and confirms the effectiveness of its use in genotyping of dogs. Probability of exclusion of accidental allele coincidence (PE), which is 0.675 on average, indicates a lack of the number and informativeness of the selected microsatellite markers for German Shepherd as in this case a combined probability (CPE) of accidental allele coincidence is 0.933886 or 93.3%. The chosen microsatellite loci to study the genetic structure of the German Shepherd dog population, show a sufficiently high informativeness of chosen system of molecular genetic DNA markers. However, there is the need for using additional microsatellite markers which will increase the combined probability of accidental allele coincidence (CPE) from 93.3% to 99.9%. The analysis of heterozygosity is important in studying the dynamics of genetic processes in populations, because heterozygosity has an effect on many factors, including mutations, selection, non-random mating, genetic drift, etc., so continuous monitoring of genetic diversity is required for their timely identification and development of measures to improve breeding work on biodiversity in different dog breeds.

Is there selection in favour of heterozygotes in families with merosin-deficient congenital muscular dystrophy?

Merosin-deficient congenital muscular dystrophy is an autosomal recessive neuromuscular disorder caused by partial or total absence of laminin-2 (merosin) in the skeletal muscle. Affected children have severe weakness, hypotonia at birth, high creatine kinase (CK) levels (more than 10 times normal) and are not able to walk or stand unsupported. Linkage and mutation analysis demonstrated that the gene encoding for the laminin-alpha2 chain, mapped on chromosome 6q22-23, is invariably responsible for this form of congenital muscular dystrophy. We investigated the pattern of inheritance of the haplotypes associated with the mutated allele in 29 informative merosin-deficient families, using tightly linked informative polymorphic microsatellite markers. This allowed us to identify heterozygous individuals from normal homozygotes, who are clinically, pathologically and biochemically indistinguishable. By linkage analysis, we found a statistically significant increase in the number of heterozygous individuals carrying either the paternal or the maternal haplotypes associated with the mutated allele. This could suggest a selection in favour of the alleles carrying mutations at the laminin alpha2-chain locus.

Minimal selection requirements for the correlation between heterozygosity and growth, and for the deficiency of heterozygotes, in oyster population

AbstractWe examine several models that may account for the observation that in populations of marine molluscs in general, and of the American oyster (Crassostrea virginica) in particular, the growth of an individual is related to its degree of heterozygosity and, also, that the number of heterozygous individuals in the population is less than expected on the assumption of random mating and no selection. We classify these models into nonselective, selective, and mixed models. We conclude that mixed models are the most likely to apply to real populations, but cannot exclude selective models. Nonselective models appear least likely. Current evidence favors a model that assumes that heterozygotes enjoy a fitness advantage as adults, primarily because of their faster growth, and that the lower numbers of heterozygotes in the population result from some form of nonrandom fertilization. One possible source of nonrandom fertilization is variation in the time of spawning of individuals due to differences in body size.

GROWTH RATE IN OYSTERS: AN OVERDOMINANT PHENOTYPE AND ITS POSSIBLE EXPLANATIONS.

It is a well established phenomenon that homozygosity induced either by inbreeding or by especially designed experiments (such as, e.g., the technique of Sved and Ayala [1970] for comparing overall performance of chromosomal heterozygotes) generally results in low fitness. Because in all such cases a large part of the genome is made homozygous, the reduction in fitness can be equally well attributed to either reduced heterozygosity at heterotic loci, or to homozygosity for detrimental recessive genes. The problem of distinguishing between the two explanations is one of major importance in population genetics and is thoroughly discussed by Lewontin (1974). Another approach to studying the effect of heterozygosity is to correlate heterozygosity with a quantifiable character in individuals drawn randomly from a population. This approach does not rely on inbreeding, and although some inbreeding may occur in the population, if reduced homozygote performance is observed, it may well be attributed to other factors. Chromosomal inversions and electrophoretic variants provide suitable means for such an approach, the latter having the additional advantage of allowing one to study the effect of homozygosity or heterozygosity at a rather small segment of the genome. It appears, however, that most researchers have preferred to search for balancing selection in natural populations by comparing the observed levels of heterozygosity in the populations with that expected under the assumption of no selection (e.g., Marshall and Allard, 1970; Richmond and Powell, 1970; Allard et al., 1972). Only rarely has the individual's state in regard to homozygosity or heterozygosity been compared with its score for a quantitative character. Most of these studies have shown that heterozygotes perform differently than homozygotes (Koehn et al., 1973; Chaisson et al., 1976; Schaal and Levin, 1976; Stalker, 1976; Mitton, 1978). In a previous paper (Singh and Zouros, 1978) we reported the main findings from a study of a cohort of oysters. The cohort consisted of individuals of same age, drawn as spat (settling larvae) from a natural population, and grown under uniform conditions. We observed that the number of heterozygous individuals in the population was much lower than predicted from Hardy-Weinberg equilibrium. We also observed that there was a correlation between an individual's weight and the number of loci at which the individual was heterozygous. The first observation is common for populations of marine molluscs (for references see Singh and Zouros, 1978, and Skibinski et al., 1978). The second observation is less common in the literature and has an obvious bearing on the question of adaptive significance of enzyme heterozygosity. Here we report the findings of a second study. Our main objective was to see whether the patterns observed in the first study are reproducible. We have employed more loci and analyzed a much larger sample. This allowed us to firmly establish the correlation between heterozygosity and growth rate. It also provided us with a sizeable body of informati6n on which to evaluate certain hypotheses about the nature of the correlation, and ' Present address: Department of Zoology, University of Western Ontario, London, Ontario, Canada N6A 5B7.