Positive Assortative Mating With Family Size as a Function of Predicted Parental Breeding Values

Abstract

While other investigations have described benefits of positive assortative mating (PAM) for forest tree breeding, the allocation of resources among mates in these studies was either equal or varied, using schemes corresponding only to parental rank (i.e., more resources invested in higher-ranking parents). In this simulation study, family sizes were proportional to predicted midparent BLUP values. The distribution of midparent BLUP values was standardized by a constant, which was varied to study the range of distributions of family size. Redistributing progenies from lower-to higher-ranking families to a point where an equal number of progenies were still selected out of each family to the next generation caused minimal change in group coancestry and inbreeding in the breeding population (BP), while the additive genetic response and variance in the BP were both greatly enhanced. This generated additional genetic gains for forest plantations by selecting more superior genotypes from the BP (compared to PAM with equal family sizes) for production of improved regeneration materials. These conclusions were verified for a range of heritability under a polygenic model and under a mixed-inheritance model with a QTL contributing to the trait variation. E ARLIER studies by computer simulation have demonstrated that positive assortative mating (PAM) applied in a long-term forest tree breeding program has the potential to generate extra genetic improvement in forest plantations (e.g., dissertations by Mahalovich 1990; Rosvall 1999). This is due to the enhancement of the additive genetic response and variance in the breeding population (BP), enabling the selection of more extreme genotypes from the BP. These genotypes can be established in orchards to produce seeds for production of reforestation nursery stock or alternatively can be vegetatively propagated and directly planted as clones. When both genetic gain and diversity are considered in a single selection criterion, PAM does not much alter the effective population size compared with that achieved under random mating, but increases the average inbreeding in the BP as a consequence of mating among more related individuals .Ideally, the contribution of individuals in the BP to the next generation should correlate with their breeding values (Lindgren 1986). Various weighting schemes have been proposed to assign mating frequencies to individuals in the BP. These are built on different assumptions about the distribution of breeding values and the function used to assign the corresponding mating frequencies Kang 1989;Wei and Lindgren 1995). Two approaches for controlling parental contributions during PAM were investigated by , using a stochastic model of a forest-tree breeding program. In the first approach, balanced mating (each individual involved in an equal number of combinations) was followed by unbalanced selection (more progenies selected from higher-ranking families and fewer from lower-ranking families). In the second, unbalanced mating (individuals of high...

Optimization of selection contribution and mate allocations in monoecious tree breeding populations

“…When PAM has been combined with static selection methods, little improvement in response to selection in the breeding population has resulted, compared with RM, i.e. [ 17 , 19 ]. As a mating method, PAM is not designed to improve population structure, but instead tries to separate the population into several sub-lines.…”

“…In forest tree breeding, most studies on the effect of mating schemes have compared positive assortative mating (PAM) and RM (e.g. [ 16 - 19 ]). The idea underlying PAM is to mate the best ranked trees with each other so that the between-family additive genetic variance of the population is increased [ 20 ].…”

“…As a result, selecting an elite part of the population could enhance the genetic merit further, i.e. selection for the deployment population [ 16 - 19 ]. None of the aforementioned studies used optimized dynamic selection methods; they used static selection methods where equal numbers of trees were selected from each generation irrespective of the pedigree of the total breeding population.…”

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“…Since that using of equal sex ratio in commercial herds is not economical, the use of less unequal ratio would be better to maintain genetic variability. Reducing family size variance with fewer selected animals within each family would result in lower rate of inbreeding for each unit of genetic progress [21].…”

Multivariate selection under adverse genetic correlations: impacts of population sizes and selection strategies on gains and coancestry in forest tree breeding

“…Parameter-based models have been excellent tools to help guide breeders in their examination of optimum strategies for mating, testing and selection (Mullin and Park 1995;Wei and Lindgren 1995;Kerr et al 1998;Rosvall and Anderson 1999;Lstiburek et al 2005) over early generations, as significant changes to genetic variances and covariances are generally not expected. However, for more than one to three generations, the use of locus-based 'gene' models may help improve our understanding of the changes to genetic diversity under different selection regimes over several generations, as well as highlight some of the genetic mechanisms behind biological constraints (e.g.…”