Lindgren D, Mullin T
Complementing inbreeding coefficient information with status number: Implications for structuring breeding populations
New Zealand Journal of Forestry Science: 1997 27:255-271
Using stochastic simulations, the effects of size of unrelated groups, mating designs, and selection strategies were investigated to address the issue of assuring long-term genetic gains. The parameters analysed were status number (Ns), inbreeding coefficient (F), and genetic gain for two heritabilities (0.05 and 0.2). Under a fixed-resource scenario, unrelated and non-inbred founders were clustered into variable group sizes (from 12 to 128), with 0.5 to 5.5 crosses per parent. Also considered were phenotypic selection and combined index selection, with and without restrictions on the number of individuals selected per family. Breeding schemes with small, disconnected groups were slightly more efficient in preserving status number through a large number of generations than breeding schemes with large groups, but medium- to large-size groups showed larger expected gains. Inbreeding in small groups may become so severe as to cause fertility problems and considerably reduce the efficiency of selection for additive gene effects. Hence, using very small groups would probably not provide a sustainable long-term breeding strategy. Nevertheless, small groups may form a critical component of breeding strategies that employ marker-assisted selection, since the maintenance of marker-QTL associations would be facilitated in these small populations. Substantial extra gain resulted from restricted combined index selection if more crosses per parent were made. Gain almost doubled during the first four generations with the increase from 0.5 to 1.5 crosses per parent. Constraints on the number of individuals selected per family severely curtailed gain, especially early on at low heritability and with one or fewer crosses per parent. Small groups could be the means of delivering rapid gains through a concentration of breeding resources combined with a reasonably short breeding cycle, but small groups by themselves will probably not constitute a sustainable long-term breeding strategy.