Prediction of response to selection within families

Hybrid maize breeding with doubled haploids. IV. Number versus size of crosses and importance of parental selection in two-stage selection for testcross performance

Parental selection influences the gain from selection and the optimum allocation of test resources in breeding programs. We compared two hybrid maize (Zea mays L.) breeding schemes with evaluation of testcross progenies: (a) doubled haploid (DH) lines in both stages (DHTC) and (b) S(1) families in the first stage and DH lines within S(1) families in the second stage (S(1)TC-DHTC). Our objectives were to (1) determine the optimum allocation regarding the number of crosses, S(1) families, DH lines, and test locations, (2) investigate the impact of parental selection on the optimum allocation and selection gain (DeltaG), and (3) compare the maximum DeltaG achievable with each breeding scheme. Selection gain was calculated by numerical integration. Different assumptions were made regarding the budget, variance components, correlation between the mean phenotypic performance of the parents and the mean genotypic value of the testcross performance of their progenies (rho( P )), and the composition of the finally selected test candidates. In comparison with randomly chosen crosses, maximum DeltaG was largely increased with parental selection in both breeding schemes. With an increasing correlation rho( P ), this superiority increased strongly, while the optimum number of crosses decreased in favor of an increased number of test candidates within crosses. Thus, concentration on few crosses among the best parental lines might be a promising approach for short-term success in advanced cycle breeding. Breeding scheme S(1)TC-DHTC led to a larger DeltaG but had a longer cycle length than DHTC. However, with further improvements in the DH technique and the realization of more than two generations per year, early testing of S(1) families prior to production of DH lines would become very attractive in hybrid maize breeding.

Relaxation of selection with equalization of parental contributions in conservation programs: an experimental test with Drosophila melanogaster

Equalization of parental contributions is one of the most simple and widely recognized methods to maintain genetic diversity in conservation programs, as it halves the rate of increase in inbreeding and genetic drift. It has, however, the negative side effect of implying a reduced intensity of natural selection so that deleterious genes are less efficiently removed from the population with possible negative consequences on the reproductive capacity of the individuals. Theoretical results suggest that the lower fitness resulting from equalization of family sizes relative to that for free contribution schemes is expected to be substantial only for relatively large population sizes and after many generations. We present a long-term experiment with Drosophila melanogaster, comparing the fitness performance of lines maintained with equalization of contributions (EC) and others maintained with no management (NM), allowing for free matings and contributions from parents. Two (five) replicates of size N = 100 (20) individuals of each type of line were maintained for 38 generations. As expected, EC lines retained higher gene diversity and allelic richness for four microsatellite markers and a higher heritability for sternopleural bristle number. Measures of life-history traits, such as egg-to-adult viability, mating success, and global fitness declined with generations, but no significant differences were observed between EC and NM lines. Our results, therefore, provide no evidence to suggest that equalization of family sizes entails a disadvantage on the reproductive capacity of conserved populations in comparison with no management procedures, even after long periods of captivity.

Balanced vs. slightly unbalanced selection

Balanced selection, defined as that all parents have equal contributions to the next generation, is expected to minimize the loss of gene diversity and thereby to allow a steady increase of selection response even at very advanced generations. In this study, balanced selection was compared with two degrees of slightly unbalanced selection (equal contributions from most of the families) and with unrestricted selection (unequal contributions) in terms of the retrieved response and genetic diversity consumed (group coancestry) over 30 generations of selection. A simulation was used varying the heritability and considering two population sizes and two family sizes. Results showed that a slight unbalance can bring a favourable relation between retrieved response and genetic diversity when compared to balanced selection at low and moderate heritabilities, and when small families and population size were assumed. The implicit constraint of a constant number of contributing families imposed in balanced and slightly unbalanced selection reduced the likelihood of gene loss compared to unrestricted selection. I conclude that a slight unbalance often could be more favourable than complete balance regarding selection efficiency, and that gain retrieved per unit of group coancestry declined with further unbalance.