Eklund J, Bradford G E
Genetics. 1977 Mar;85(3):529-42. doi: 10.1093/genetics/85.3.529.
A strain of mice (S1) was successfully selected for large litter size for 31 generations, increasing the mean by 4.2 young per litter. After generation 31, there was no further progress and it was concluded that a selection plateau had been reached. Realized heritability decreased during the course of the experiment from 0.16+/-0.06 for the first 15 generations to 0.00+/-0.03 for generations 30 through 45.--In order to explore the nature of the selection plateau, the following groups were derived from line S1 at generation 34 or 35: Upward selection with inbreeding (SF), random (relaxed) selection (SO), and downward selection (SR). Selections were carried out for 10 to 11 generations. The means of SO and SF were similar to those of S1, ruling out any major effect of natural selection or overdominance. SR decreased, the mean averaging 2.3 young per litter below that of S1 during the last three generations. The fact that SR responded to selection indicates that genetic variance was still present in the plateaued population. The SF sublines were crossed when the inbreeding was 95% and a new line, SX, was formed. SX was maintained for three generations and the difference of +0.7 young per litter above the contemporary generations of S1 was significant. The results from this experiment suggest that the selection plateau in line S1 was caused by reduction of additive genetic variance to a very low level. Some nonadditive genetic variance remained, however, and was attributed to recessive alleles at low frequency. In agreement with results reported by Falconer (1971), inbreeding with selection followed by crossing of the inbred sublines proved to be effective in overcoming a selection plateau in litter size.
成功选育出一个品系的小鼠（S1），其产仔数多的性状持续选育了31代，每窝平均产仔数增加了4.2只。在第31代之后，选育没有进一步进展，由此得出结论，已达到选择平台期。在实验过程中，实现遗传力从最初15代的0.16±0.06下降到第30至45代的0.00±0.03。——为了探究选择平台期的本质，在第34或35代从S1品系衍生出以下几组：近亲繁殖的向上选择（SF）、随机（放松）选择（SO）和向下选择（SR）。选择进行了10至11代。SO和SF的均值与S1的相似，排除了自然选择或超显性的任何主要影响。SR下降，在最后三代中，平均每窝产仔数比S1低2.3只。SR对选择有反应这一事实表明，在处于平台期的群体中仍存在遗传方差。当近亲繁殖达到95%时，对SF亚系进行杂交，形成了一个新的品系SX。SX维持了三代，每窝产仔数比当代S1品系多0.7只，差异显著。该实验结果表明，S1品系的选择平台期是由加性遗传方差降低到非常低的水平所致。然而，仍存在一些非加性遗传方差，这归因于低频的隐性等位基因。与Falconer（1971年）报道的结果一致，先进行选择的近亲繁殖，然后对近亲亚系进行杂交，被证明在克服产仔数选择平台期方面是有效的。
