| The aim of breeding is to obtain continuous genetic advance whilst maintaining the genetic variation and limiting the inbreeding level of the population. Mating systems are critical in breeding practice. Several mating designs of variant scenarios were firstly studied with Monte-Carlo simulation using an infinitesimal additive model in this study. These investigated mating systems were designed according to the reproduction characteristics of aquatic organisms. The sizes of the testing population were set at 1200, 2400 and 4800. The studied heritability levels were 0.1 and 0.4. The tested mating systems include single pair mating design (♂:♀= 1:1), nested mating designs (♂:♀= 1:5, 1:10, 1:20) and factorial mating designs (♂:♀= 5:5, 10:10, 20:20). All populations were selected for 12 generations. The selection rate of each population was set such that the inbreeding coefficients were about equal after 12 generations of selection. The results show that factorial mating designs can always obtain the largest selection response. For example, in the scenarios heritability (h2) = 0.1 and testing population size (TPS) = 1200, 2400 and 4800, factorial mating designs and single mating design can increase the response up to 39%, 29%, 23% and 15%, 17%, 12% over nested mating designs, respectively. If h2 = 0.4, the figures are 37%, 30%, 20%, and 14%, 13%, 7% respectively.Based on the previous results, this study using the optimized mating ratio 5:5 made a further research on continuous selection in one line vs continuous selection in several (2, 4, 8) sublines (for the same breeding objective with the same total number of individuals tested) the so-called stratified selection. During the stratified selection crossbreeding was made among the sublines in every given generations. The same genetic model, tested population size and initial heritability levels as previous work were used in the second part of the study.The results of 12 generations'selection show that crossbreeding among sublines have a significant effect on gene complementation and gene recombination. Thus it makes an increase in genetic variation and a decrease in inbreeding coefficient of the population. And therefore it did get supernumerary genetic progress under continuous selection. The more the sublines are the more increment in genetic variation and decrease in inbreeding coefficient would be after several crossbreeding. However, the effect on selection response and inbreeding may diminish as the crossbreeding times increases, and it shows a bigger trend with less sublines. For each selected population combined by various tested population size and initial heritability levels, the population with 8 sublines can get the smallest inbreeding coefficient after 12 generations'selection if given a crossbreeding in each generation while the less figures for populations with 4 sublines and 2 sublines, and the biggest for single line.For example, for populations combined with TPS = 2400 and h2 = 0.1 and 0.4, populations of sublines by factors of 8, 4, 2 and single line can obtain inbreeding coefficients 5.30%, 6.84%, 8.41%, 9.36% and 7.58%, 10.35%, 13.36%, 15.24% respectively in generation of 12th after 11 times of crossbreeding. Compared to single line, 2 sublines can gain the same cumulative selection response with a significantly reduced inbreeding after 11 times of crossbreeding. For instance, for populations combined with TPS = 2400 and h2 = 0.1 and 0.4, the cumulative selection response are 2.13, 2.10 and 7.52, 7.56 for populations with 2 sublines and single line respectively. While 4 sublines gets a little less cumulative selection response than single line even if with 11 times of crossbreeding, and 8 sublines gains the least figure. Thus stochastic division of basic population into less than 4 sublines and mating ratio of 5:5 or little higher ones of factorial mating design are recommended in breeding practice of aquatic organism so as to significantly reduce inbreeding coefficient while achieving high genetic response.
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