Esfandyari Hadi, Henryon Mark, Berg Peer, Thomasen Jørn Rind, Bijma Piter, Sørensen Anders Christian
Center for Quantitative Genetics and Genomics, Department of Molecular Biology and Genetics, Aarhus University, Tjele, Denmark.
Seges, Danish Pig Research Centre, Copenhagen, Denmark.
J Hered. 2017 May 1;108(3):318-327. doi: 10.1093/jhered/esw123.
Under the finite-locus model in the absence of mutation, the additive genetic variation is expected to decrease when directional selection is acting on a population, according to quantitative-genetic theory. However, some theoretical studies of selection suggest that the level of additive variance can be sustained or even increased when nonadditive genetic effects are present. We tested the hypothesis that finite-locus models with both additive and nonadditive genetic effects maintain more additive genetic variance (VA) and realize larger medium- to long-term genetic gains than models with only additive effects when the trait under selection is subject to truncation selection. Four genetic models that included additive, dominance, and additive-by-additive epistatic effects were simulated. The simulated genome for individuals consisted of 25 chromosomes, each with a length of 1 M. One hundred bi-allelic QTL, 4 on each chromosome, were considered. In each generation, 100 sires and 100 dams were mated, producing 5 progeny per mating. The population was selected for a single trait (h2 = 0.1) for 100 discrete generations with selection on phenotype or BLUP-EBV. VA decreased with directional truncation selection even in presence of nonadditive genetic effects. Nonadditive effects influenced long-term response to selection and among genetic models additive gene action had highest response to selection. In addition, in all genetic models, BLUP-EBV resulted in a greater fixation of favorable and unfavorable alleles and higher response than phenotypic selection. In conclusion, for the schemes we simulated, the presence of nonadditive genetic effects had little effect in changes of additive variance and VA decreased by directional selection.
根据数量遗传学理论，在无突变的有限位点模型下，当定向选择作用于种群时，加性遗传变异预计会减少。然而，一些选择理论研究表明，当存在非加性遗传效应时，加性方差水平可以维持甚至增加。我们检验了这样一个假设：当选择的性状受到截断选择时，同时具有加性和非加性遗传效应的有限位点模型比仅具有加性效应的模型能维持更多的加性遗传方差（VA），并实现更大的中长期遗传增益。模拟了包含加性、显性和加性×加性上位性效应的四种遗传模型。个体的模拟基因组由25条染色体组成，每条染色体长度为1M。考虑了100个双等位基因QTL，每条染色体上有4个。在每一代中，100个父本和100个母本进行交配，每次交配产生5个后代。对单一性状（h2 = 0.1）进行100个离散世代的选择，基于表型或最佳线性无偏预测估计育种值（BLUP-EBV）进行选择。即使存在非加性遗传效应，VA也会随着定向截断选择而降低。非加性效应影响长期选择反应，在遗传模型中，加性基因作用对选择的反应最高。此外，在所有遗传模型中，BLUP-EBV导致有利和不利等位基因的固定程度更高，且反应比表型选择更高。总之，对于我们模拟的方案，非加性遗传效应的存在对加性方差的变化影响不大，VA因定向选择而降低。
