Inbreeding in New Hampshire, U.S.
FU Berlin
Digitale Dissertation
Kathrin Irgang :
Comparison of the development of inbreeding and homozygosity based on genetic markers in a closed New Hampshire population
Vergleich von Inzucht- und Homozygotieentwicklung anhand molekulargenetischer Marker in einer geschglossenen New Hampshire Linie
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Abstract
In a closed population, mating of related individuals (inbreeding) occurs because of only a few number of ancestors. Related individuals have an increased proportion of their genome in common. The probability for the offspring increases to inherit the same allele of one common ancestor from each of the parents and to be homozygous for the locus. The closer the relationship of the parents the faster the proportion of homozygous loci rises in the progeny. Based on these relations, the inbreeding coefficient F (Wright 1921) reflects the increase of homozygosity in a population or the expected proportion of allele pairs identical by descent for an individual, respectively. Beside this estimation method, individual homozygosity can be measured directly with different genetic markers in chickens, for example with RFLP, with protein polymorphism or with microsatellite markers. The objective of the presented study was to analyse genetic variability in a closed New Hampshire line by comparison of estimated (by inbreeding coefficients) and realized (by microsatellite analysis) inbreeding. The New Hampshire line was maintained at the research station of the institute of Animal Sciences, Humboldt-University of Berlin near Berlin/Germany since 1955. Divided into 15 sublines, the hens (10 to 15 per subline) were mated in a rotation system once a year in order to avoid close inbreeding. The individual inbreeding coefficients F (Wright) were estimated including complete pedigree data of about 8100 chickens. For the molecular analysis, DNA was isolated from blood samples of generation 1994 (79 chickens) and plasma samples of generation 1982 (58 chickens). The individuals were typed by 17 markers located in non-coding regions and 6 markers within coding regions. Based on the frequencies of genotypes, allele frequencies and expected genotype frequencies were calculated and their deviations from Hardy Weinberg equilibrium (HWE) were checked. The individual realized inbreeding was determined as the proportion of homozygous loci of all investigated loci. In the investigated New Hampshire line 22 markers were polymorphic showing only few alleles per locus (2 to 4). Comparing the generations of 1982 and 1994, 80% of the loci changed allele frequencies significantly, partly accompanied by losses of alleles in 1994. In 5 loci the observed genotype frequencies deviated significantly from HWE expectations in both generations, but in contrast to the theory mainly with tendency to heterozygosity. Mean realized inbreeding increased from 56,4% (in 1982) to 61,6% (in 1994). At the same time the mean estimated inbreeding (Wright) increased from 18,8% to 24,3% and reached 26,6% after 43 generations. The low rate of inbreeding was caused by the rotation mating scheme which was used to avoid close inbreeding. The increase of realized inbreeding (molecular analysis) was higher than predicted by the increase of estimated inbreeding (Wright). In general, an overestimation of homozygosity by the inbreeding coefficient was expected because of heterozygote advantages. These advantages could be explained by heterozygous genes which are responsible for fertility and vitality. The microsatellites for the presented analysis were chosen from coding and non-coding regions. In contrast to the majority of 17 markers located in non-coding regions with increasing homozygosity, the 6 microsatellites located within coding regions showed on average an increase of heterozygosity in the New Hampshire line. Slow inbreeding and slight selection to maintain the line compensated inbreeding depressions and could have counteracted the expected decrease in heterozygosity.
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