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. |
