Theoretical Modifications of Reciprocal Recurrent Selection

Reciprocal recurrent selection (RRS), which assumes overdominant loci to be important, alters two genetically different populations to improve their crossbred mean. Individual plants from two populations (A and B) are selfed and also crossed with plants from the reciprocal female tester population (B and A, respectively). Selection is based on the mean of crossbred families, and the selected individuals are randomly mated within A and B to form new populations.—We propose two alternatives to RRS. The first (RRS-I) uses, as the tester of population A, a population (LB) that is derived from population B by family selection for low yield. The second (RRS-II) is similar to RRS-I, but also uses, as the tester of B, a population (LA) that is derived from population A by family selection for low yield.—The expected crossbred means of RRS, RRS-I, and RRS-II were compared, assuming equal σP, at several cycles of selection for incomplete and complete dominance, and for several cases of overdominance (depending on the gene frequencies in A and B, and on the equilibrium gene frequency).—The choice of selection method depends on the importance of the effects of overdominant loci compared to loci exhibiting incomplete or complete dominance. If overdominance is unimportant, RRS-II is the best selection method, followed by RRS-I and RRS. If overdominance is important, both RRS and RRS-I are superior to RRS-II; RRS is preferred to RRS-I if the effects of overdominant loci are sufficiently important. If the genetic model is a mixture of levels of dominance at different loci, a combination of selection systems is suggested.     

Training genomic selection models across several breeding cycles increases genetic gain in oil palm in silico study

Genomic selection (GS) is expected to increase the rate of genetic gain in oil palm. In a GS scheme, breeding cycles with progeny tests (phenotypic selection, PS) used to calibrate the GS predictive model and for selection alternate with GS cycles, making it possible to train the GS model with aggregated data from several cycles. To evaluate this possibility, we simulated four cycles of hybrid breeding for bunch production and compared two methods of calibrating the GS model, one using aggregated data from the two most recent cycles (Tr2Gen), the other using data from the last cycle (Tr1Gen). We also compared a GS scheme with two PS cycles and two GS cycles (2PT-2noPT), and a scheme with PS every other cycle and GS otherwise (PT-noPT). We showed that Tr2Gen had a 10.7% higher genetic gain per cycle than Tr1Gen, mostly due to increased selection accuracy, particularly in across-cycle selection, despite the decreased relationship between training individuals and selection candidates. After four cycles, Tr2Gen had a 5% higher cumulative genetic gain than Tr1Gen, with a lower coefficient of variation. PT-noPT benefited more from the advantages offered by Tr2Gen than 2PT-2noPT. Over four breeding cycles, combining PT-noPT and Tr2Gen largely outperformed conventional reciprocal recurrent selection (RRS), with an increase in annual genetic gain ranging from 37.6 to 57.5%, depending on the number of GS candidates. This study confirms the advantages of GS over RRS and indicated that oil palm breeders should train GS models using all data from past breeding cycles.     

Genetic gains from reciprocal recurrent and within-line selection for egg production in the fowl

Summary Selection for survivors percent egg production from first egg to 40 weeks of age was conducted for six generations. Within-line selection (WLS) on the basis of an index of individual records plus sire family and dam family means was compared with reciprocal recurrent selection (RRS) based on sire family selection among cross progeny. Genetically heterogeneous synthetic populations, the Cornell Control and the Purdue Pool strains were used.The responses to WLS (3.84 °) and RRS (2.57 °) were both significantly greater than zero, but were not significantly different from each other. Nevertheless, the responses were proportional to their predicted values (.363 vs. 340). The advantage of WLS was due to the mechanics of selection resulting in slightly greater selection intensity and an increased correlation between the criterion of selection and the trait being improved. On the contrary, the RRS method had a slightly larger realized heritability as would be expected in the presence of non-additive genetic variance. Four of five pure-lines selected under both methods had statistically significant declines in performance due to inbreeding depression effects. All of the results observed are comparable with known genetic theory.Some problems in comparing genetic gains from different selection methods or selection criteria are discussed.Journal Paper No. 4986 of the Purdue University Agricultural Experiment Station. Conducted in cooperation with the Regional Poultry Breeding Laboratory, Project N. C. 89. Supported in part by N. I. H. Grant No. ES00310.     

Comparison of full and half-sib reciprocal recurrent selection

Summary Full and half-sib reciprocal recurrent selection (RRS) were compared algebraically and with computer simulation. The relative performance of the two schemes depended on the selection intensity and the environmental variance. Full-sib RRS was favoured at less intense selection and when the environmental variance was large relative to the total genetic variation. As selection intensity increased its advantage declined. Full-sib RRS enables a breeder to combine the efficient development of new hybrids with population improvement and should be a valuable technique in plant breeding.Published with the approval of the Director as paper No. 2466 Journal series, Nebraska Agr. Exp. Sta., Lincoln, Nebr. and as Atomic Energy Commission Technical Information Document No. COO-1512-17.     

An experimental comparison of selection alternatives to plateaued response

Three alternative selection methods for extending selection limits or breaking response plateaus were compared over ten generations in a replicated model experiment using two unrelated populations of Drosophila melanogaster that no longer responded to purebred selection for high egg number, a heterotic polygenic trait. The three methods were: (1) reciprocal recurrent selection (RRS) with selection within each of the plateaued populations based solely on crossbred performance, (2) a modification of reciprocal recurrent selection (MRRS) with selection within each population based on both purebred and crossbred performance, and (3) purebred selection within a new synthetic population formed by crossing the two plateaued populations.--Conflicting estimates were obtained for heritability of purebred egg number in each of the plateaued populations. The realized heritability values and estimates from diallel analyses indicated an absence of additive genetic variation for both populations; however, estimates from conventional intraclass correlation methods were positive. The diallel analyses revealed significant amounts of nonadditive gene effects for purebred egg number in each population, while the significant gene effects for crossbred egg numbers were additive. Estimates of the genetic correlation between purebred and crossbred egg number were negative (-0.85 +/- 0.68 and -0.32 +/- 0.25) for the two base populations.--All three alternatives to continued purebred selection gave significant responses, with the average gain per generation from MRRS being significantly superior to the other two methods. Observed purebred and crossbred responses under RRS were in agreement with quantitative genetic theory. Such was not the case for MRRS, which suggested the possibility of major gene segregation.--Evidence supporting a negative genetic correlation between purebred and crossbred performance and the possibility of overdominance is presented and discussed.     

Long-term genomic selection for heterosis without dominance in multiplicative traits: case study of bunch production in oil palm

Background

To study the potential of genomic selection for heterosis resulting from multiplicative interactions between additive and antagonistic components, we focused on oil palm, where bunch production is the product of bunch weight and bunch number. We simulated two realistic breeding populations and compared current reciprocal recurrent selection (RRS) with reciprocal recurrent genomic selection (RRGS) over four generations. All breeding strategies aimed at selecting the best individuals in parental populations to increase bunch production in hybrids. For RRGS, we obtained the parental genomic estimated breeding values using GBLUP with hybrid phenotypes as data records and population specific allele models. We studied the effects of four RRGS parameters on selection response and genetic parameters: (1) the molecular data used to calibrate the GS model: in RRGS_PAR, we used parental genotypes and in RRGS_HYB we also used hybrid genotypes; (2) frequency of progeny tests (model calibration); (3) number of candidates and (4) number of genotyped hybrids in RRGS_HYB.

Results

We concluded that RRGS could increase the annual selection response compared to RRS by decreasing the generation interval and by increasing the selection intensity. With 1700 genotyped hybrids, calibration every four generations and 300 candidates per generation and population, selection response of RRGS_HYB was 71.8 % higher than RRS. RRGS_PAR with calibration every two generations and 300 candidates was a relevant alternative, as a good compromise between the annual response, risk around the expected response, increased inbreeding and cost. RRGS required inbreeding management because of a higher annual increase in inbreeding than RRS.

Conclusions

RRGS appeared as a valuable method to achieve a long-term increase in the performance for a trait showing heterosis due to the multiplicative interaction between additive and negatively correlated components, such as oil palm bunch production.     

Full-sib and reciprocal recurrent selection in relation to pearl millet improvement

Summary During the years 1973 to 1976 two populations of Pearl millet with wide genetic base, namely, Delhi composite (DC) and Vijay composite (VC) were used to compare the response to selection by the full-sib family method from biparental material and reciprocal recurrent selection (RRS). The results indicated that it was possible to advance grain yield with one cycle of RRS by about 23 percent in the case of population DC and 21 percent in population VC, while for the full-sib selection method, the improvement in grain yield was not so rapid. The studies on the nature of gene action indicated that both additive and dominance gene actions were important for grain yield, ear length and ear girth. The coefficient of variation as a result of RRS was reduced in population DC, while it was comparable to base population in the other population. The correlation studies indicated that the magnitude of favourable correlation of different characters with grain yield were higher in case of RRS compared to the full-sib system. The presence of negative correlation of plant height with grain yield in both the improved populations indicated the possibility of breakage of unfavourable gene combinations through RRS and full-sibs developed from biparental mating.     

Combined S2 and crossbred family selection in full-sib reciprocal recurrent selection

Summary A method (CRRS) that combines S₂ and crossbred family selection in full-sib reciprocal recurrent selection (FSRRS) is proposed. The method requires four generations per cycle in single-eared maize populations. Selection is based on performance of S₂ and full-sib families by applying selection index theory. Equations to estimate the coefficients included in the index are given. These estimates are functions of the genetic and phenotypic variances and covariances among and between the two kinds of families. Comparisons of FSRRS and CRRS under equivalent amount of effort show that CRRS has some advantage over FSRRS for low heritability of the trait being selected (e.g., maize yield) and when only one or two locations with two replications are involved in the selection experiment.Joint contribution: Institute Nacional de Investigaciones Agrarias, La Coruna, Spain; and Agricultural Research, Science and Education Administration, U.S. Department of Agriculture, and Journal Paper No. J-10118 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA 50011. Project 2194     

Comparison of selection methods for improvement of the population hybrid

Summary The objective of this study was to compare several selection procedures with respect to expected genetic gain in the population hybrid across a range of initial allelic frequencies, degrees of dominance, and environmental variances. The methods compared were intrapopulation recurrent selection using full-sib or S₁ families, full-sib and two half-sib reciprocal recurrent selection procedures, and convergent improvement applied to populations. Comparisons were made by calculating expected allelic frequency changes for each method. The optimal selection method for a given set of allelic frequencies and degree of dominance depended little on the environmental variance. Partly because of its short cycle, full-sib intrapopulation selection was the most effective method for the majority of allelic frequency combinations when the degree of dominance was small and an off-season nursery could be used to make recombinations. With larger values for the degree of dominance, S₁ and reciprocal full-sib methods became optimal, the former method especially when favorable alleles had a high frequency and the latter when populations were highly divergent. When off-season nursery use was restricted to making self-pollinations or was absent, S₁ selection was optimal for the majority of allelic frequency combinations. Convergent improvement was superior only for extremely divergent allelic frequencies and then only when the degree of dominance was less than 0.10. Half-sib reciprocal methods were never optimal, although the gain for the standard half-sib reciprocal procedure differed little from that of full-sib reciprocal selection when the degree of dominance was 0.75.     

Genetic variation for foot-rot and Fusarium head-blight resistances among full-sib families of a self-incompatible winter rye (Secale cereale L.) population

The amount of genetic variation for resistance to foot rot caused by Pseudocercosporella herpotrichoides, Fusarium spp., and Microdochium nivale and for resistance to head blight caused by Fusarium culmorum are important parameters when estimating selection gain from recurrent selection in winter rye. One-hundred and eighty-six full-sib families of the selfincompatible population variety Halo, representing the Petkus gene pool, were tested for foot-rot resistance at five German location-year combinations (environments) and for head-blight resistance in three environments with artificial inoculation in all but one environment. Foot-rot rating was based on 25 stems per plot scored individually on a 1–9 scale. Head-blight resistance was plotwise scored on a 1–9 scale and, additionally, grain-weight per spike was measured relative to the non-inoculated control plots. Significant estimates of genotypic variance and medium-sized heritabilities (h ²=0.51–0.69) were observed in the combined analyses for all resistance traits. In four out of five environments, the amount of genetic variance was substantially smaller for foot-rot than for head-blight rating. Considerable environmental effects and significant genotype-environment interactions were found for both foot-rot and head-blight resistance. Coefficients of error-corrected correlation among environments were considerably closer than phenotypic correlations. No significant association was found between the resistances to both diseases (r=-0.20 to 0.17). In conclusion, intra-population improvement by recurrent selection should lead to substantial higher foot-rot and head-blight resistances due to significant quantitative genetic variation within Halo. Selection should be carried out in several environments. Lack of correlation between foot-rot and head-blight resistance requires separate infection tests for improving both resistances.     

Non-additive gene effects in populations under different methods of selection

Summary The genetic parameters of two quantitative traits, 13-day larval weight and pupal weight, in Tribolium populations developed by reciprocal recurrent selection (RRS) and by within-line purebred selection (WLS) were compared each with the other and also with the parameters of the unselected base populations using the genetic model of Carbonell, Nyquist and Bell. The variability for two and three-way crosses of inbred lines derived from companion populations (two strains, breeds, or varieties used for a terminal cross or hybrid) was analyzed into genetic effects: autosomal additivity (^* g), autosomal heterosis (^* s), sex-linked additivity (L), sex-linked heterosis (LL), general maternal (m), specific maternal or reciprocal (r), additive by additive epistasis (aa), and deviations from the model due, among other causes, to higher order epistasis (dev). One series of crosses involved companion populations with diverse origins. For contrast, a second series of crosses involved companion populations originating from a common heterogenous base population. For the heterotic trait larval weight, ^* g and ^* s effects were equally important and accounted for over 50% of the total variation. The aa epistasis contributed another 20% and was followed in importance by higher order epistasis and general maternal effects. For the more highly heritable trait, pupal weight, ^* g effects were most important with ^* s, aa, and m effects having smaller but significant influences. Sex-linked and reciprocal effects were statistically significant for many crosses, but they were relatively unimportant overall. In general, the unselected base populations showed higher ^* g variation than either RRS or WLS populations with the reverse true for ^* s effects. In agreement with theoretical expectations, RRS was more effective than WLS in exploiting ^* s effects. The aa epistatic effects for larval weight were of major importance in the unselected populations, but RRS and WLS did not differ significantly for exploiting superior aa gene combinations. Companion populations with diverse origins revealed significantly larger variation due to ^* g and ^* s effects in crosses than did populations initiated from a common heterogeneous base.Journal Paper No. 11559 from Purdue University Agricultural Experimental Station     

Wide- versus specific-adaptation strategy for lucerne breeding in northern Italy

This study is aimed at comparing wide- versus specific-adaptation strategies for lucerne in northern Italy on the basis of actual dry matter yield gains over 12 harvests from phenotypic selection, assessing the value of specific genetic bases and selecting environments for the contrasting subregion A (no drought stress/sandy-loam soil) and subregion C (summer drought stress/silty-clay soil). A second aim is to investigate the adaptive responses of five sets of 18 half-sib progenies. The following selected populations were evaluated along with five cultivars: GW–SW, GA–SA, GA–SC, GC–SC and GC–SA (where GW, GA and GC are the genetic bases for wide adaptation, subregions A and C; SW, SA and SC are the selection environments for wide adaptation, subregions A and C). The selection and test environments were four artificial environments created by the factorial combination of two drought stress levels by two soil types. Two environments represented the subregions A and C whereas the combination of the other two environments represented the intermediate subregion B. Genotype × environment interaction (P ≤ 0.001) due to both environmental factors and implying cross-over interaction between the contrasting subregions occurred for the populations and the five selections. Specific genetic bases (GA and GC) implied gains in their target subregions of 5.2% for subregion A and 2.9% for subregion C compared with the widely adapted one (GW). The gain of SA (‘no stress/sandy-loam soil’) over SC (‘stress/silty-clay soil’) decreased from subregion A (10.6%) through subregion C (1.7%) but exhibited an advantage per se across environments of 5.4%. The best specific selections (GA–SA for subregions A and B; GC–SA for subregion C) implied higher yields of 9.8% in subregion A and 6.5% in subregion C, and over twofold greater selection efficiency across the region, relative to GW–SW. Half-sib progeny × artificial environment interaction (P ≤ 0.05) occurred in three sets of progenies whose parents belonged to cultivars with different or similar adaptation.     

Relative efficiency of the genotypic value and combining ability effects on reciprocal recurrent selection

Reciprocal recurrent selection (RRS) has been successfully applied to maize breeding for more than 60 years. Our objective was to assess the relative efficiency of the genotypic value and the effects of general and specific combining abilities (GCA and SCA) on selection. The GCA effect reflects the number of favorable genes in the parent. The SCA effect primarily reflects the differences in the gene frequencies between the parents. We simulated three traits, three classes of populations, and 10 cycles of half- and full-sib RRS. The RRS is a highly efficient process for intra- and interpopulation breeding, regardless of the trait or the level of divergence among the populations. The RRS increases the heterosis of the interpopulation cross when there is dominance, and it decreases the inbreeding depression in the populations and the genetic variability in the populations and in the hybrid. When there is not dominance and the populations are not divergent, the RRS also determines population differentiation. The half-sib RRS, which is equivalent to selection based on the GCA effect, is more efficient than the full-sib RRS based on the genotypic value, regardless of the trait or the level of improvement of the populations. The full-sib RRS based on the SCA effect is not efficient for intra- and interpopulation breeding.     

Comparison of two methods of reciprocal recurrent selection in maize (Zea mays L.)

Reciprocal recurrent selection (RRS) was proposed for simultaneously improving two populations and their cross. A modification of the classical full-sib RRS (FS-RRS) was proposed in which the performance of full-sibs and S₂ families is combined in a selection index (FS-S₂-RRS). The Mediterranean corn borer (MCB) is the main corn borer species in the Mediterranean and adjacent areas and produces important yield losses. We started two RRS programs (FS-RRS and FS-S₂-RRS) from the same maize population in which the selection criterion was grain yield under artificial infestation with MCB eggs. Original populations, two cycles of selection derived from them by both RRS methods, and population crosses were evaluated under MCB attack and under insecticide treatment in three different environments. The objective was to compare the efficiency of the FS-RRS and the FS-S₂-RRS methods for improving grain yield. We found that the FS-S₂-RRS method was successful for improving the yield of the population cross under optimum conditions (the regression coefficient over cycles was b = 0.87** Mg ha⁻¹ cycle⁻¹) without losing yield under high pressure of MCB attack (b = 0.07). On the contrary, FS-RRS failed to improve the yield of the population cross under optimum conditions (b = 0.65) and tended to decrease the yield under high levels of MCB attack (b = −0.26). We conclude that for developing high yielding and stable varieties, FS-S₂-RRS is more efficient than the classical FS-RRS method.     

Change in ribosomal DNA spacer-length composition in maize recurrent selection populations. 2. Analysis of BS10, BS11, RBS10, and RSSSC

Four maize (Zea mays L.) populations selected for grain yield (BS10, Iowa Two-ear Synthetic; BS11, formerly Pioneer Two-ear Composite; RBS10, Illinois strain of BS10; and RSSSC, Illinois strain of Iowa Stiff Stalk Synthetic) were assayed for molecular variation in the ribosomal DNA (rDNA) intergenic spacer (IGS) at initial and advanced cycles of selection. RSSSC and RBS10 underwent reciprocal recurrent selection with an inbred tester in a high-yield environment, whereas BS10 and BS11 were subjected to full-sib reciprocal recurrent selection. Maize rDNA, which encodes the ribosomal RNA genes, is highly repetitive and shows IGS length variation within and among individuals. Five different ribosomal spacer-length variants (rslvs) and a polymorphic SstI restriction site in the IGS were detected in the four populations. The five rslvs and the polymorphic restriction fragment were observed in 20 different combinations or hybridization fragment patterns (HP). RSSSC, RBS10, and BS11 showed significant changes in the overall rslv and HP frequencies between cycle 0 and the advanced cycle of selection, whereas BS10 did not. In general, two specific HPs were more frequent in the majority of the advanced cycles of the four populations. The frequency changes between initial and advanced cycles were more dramatic for HPs than rslvs. These results are consistent with earlier findings and further support the hypothesis that certain rDNA HPs and/or linked loci may be responding to selection for grain yield and may be associated with a selective advantage in US Corn Belt environments.     

Genetic similarity and relationships of DNA fingerprints with performance and with heterosis in Japanese quail lines from two origins and under reciprocal recurrent or within-line selection for early egg production

DNA fingerprints of Japanese quail male and female pure line breeders were obtained with probes 33.6, 33.15, and R18.1 and they yielded a total of 59 scoreable bands. Bandsharing (0 < BS < 1) was calculated within and between six quail lines of two origins, and under reciprocal recurrent (AA and BB), within-line (DD and EE) or no (PP and FF) selection. Twenty one pair types were compared. BS was 0.30 higher within line than between lines. BS with the control line was smaller for reciprocal recurrent selection lines than for lines under individual selection. Bandsharing between the two reciprocal recurrent selection lines was 0.19 lower than between lines under individual selection. These results indicate that the two selection methods had different effects on the genetic constitution of the lines, in agreement with previous observations made from the analysis of biochemical polymorphisms with the same set of birds. Egg production and weight traits of pure and crossbred progeny from fingerprinted quail were obtained and compared, and a linear relationship with the measure of bandsharing was estimated. No significant regression coefficient of performance on BS was found over all progeny genetic types. Heterosis from individual matings could also be estimated under the two selection methods since the same birds were parents of both pure and crossbred performance-tested quail. The association of heterosis with the difference between BS of parents of the purebreds and BS of parents of their half-sib crossbreds was favourable and significant for early production traits in lines DD and EE, but no relationship was found in lines AA and BB. These results indicate that the high level of heterosis obtained through reciprocal recurrent selection, and the heterosis observed under within-line selection may have, partly at least, a different genetic determinism. Therefore, the relationship of heterosis with BS may also depend on the past history of selection in the lines.     

A new aspect of genetic diversity of Indonesian oil palm (Elaeis guineensis Jacq.) revealed by isoenzyme and AFLP markers and its consequences for breeding

Oil palm (Elaeis guineensis Jacq.) plays an important economic role in some countries of Southeast Asia like Indonesia, which is the world’s second producer of palm and palm kernel oil. The quality improvement of planting material needs a better understanding of the genetic relationships between genotypes from different populations used in the breeding programmes. In this study, 48 parents, representative of four populations used in Indonesian Oil Palm Research Institute (IOPRI) breeding programmes, were analysed with five selected AFLP primer pairs and four isoenzymatic systems. One hundred and fifty eight scorable band levels were generated of which 96 (61%) were polymorphic. AFLP allowed us to identify off–type descendants which were excluded from analysis. The use of unbiased Rogers distance clearly separated the four studied populations. The Neighbor-Joining method re-groups two African populations which are known as originating from different regions. Nevertheless, the variability revealed is in accordance with oil palm breeders’ knowledge. The results obtained with AFLP showed that the crosses among the African sub-population, which is excluded in oil palm reciprocal recurrent selection (RRS) breeding programmes, may be more interesting than the crosses between the African and the Deli populations. Received: 9 August 1999 / Accepted: 28 February 2000     

Genetic variation and selection response in model breeding populations of Brassica rapa following a diversity bottleneck

Domestication and breeding share a common feature of population bottlenecks followed by significant genetic gain. To date, no crop models for investigating the evolution of genetic variance, selection response, and population diversity following bottlenecks have been developed. We developed a model artificial selection system in the laboratory using rapid-cycling Brassica rapa. Responses to 10 cycles of recurrent selection for cotyledon size were compared across a broad population founded with 200 individuals, three bottleneck populations initiated with two individuals each, and unselected controls. Additive genetic variance and heritability were significantly larger in the bottleneck populations prior to selection and this corresponded to a heightened response of bottleneck populations during the first three cycles. However, the overall response was ultimately greater and more sustained in the broad population. AFLP marker analyses revealed the pattern and extent of population subdivision were unaffected by a bottleneck even though the diversity retained in a selection population was significantly limited. Rapid gain in genetically more uniform bottlenecked populations, particularly in the short term, may offer an explanation for why domesticators and breeders have realized significant selection progress over relatively short time periods.     

Accelerating the domestication of trees using genomic selection: accuracy of prediction models across ages and environments

? Genomic selection is increasingly considered vital to accelerate genetic improvement. However, it is unknown how accurate genomic selection prediction models remain when used across environments and ages. This knowledge is critical for breeders to apply this strategy in genetic improvement. ? Here, we evaluated the utility of genomic selection in a Pinus taeda population of c. 800 individuals clonally replicated and grown on four sites, and genotyped for 4825 single-nucleotide polymorphism (SNP) markers. Prediction models were estimated for diameter and height at multiple ages using genomic random regression best linear unbiased predictor (BLUP). ? Accuracies of prediction models ranged from 0.65 to 0.75 for diameter, and 0.63 to 0.74 for height. The selection efficiency per unit time was estimated as 53-112% higher using genomic selection compared with phenotypic selection, assuming a reduction of 50% in the breeding cycle. Accuracies remained high across environments as long as they were used within the same breeding zone. However, models generated at early ages did not perform well to predict phenotypes at age 6 yr. ? These results demonstrate the feasibility and remarkable gain that can be achieved by incorporating genomic selection in breeding programs, as long as models are used at the relevant selection age and within the breeding zone in which they were estimated.     

Reciprocal Recurrent Selection Compared to within-Strain Selection for Increasing Rate of Egg Lay of Tribolium under Optimal and Stress Conditions

A replicated comparison of reciprocal recurrent selection (rrs) based on crossbred performance and within strain-selection (wss) based on purebred performance was made in three diverse environments over ten generations for the improvement of a heterotic trait, 4-day virgin egg lay of Tribolium castaneum. A selection intensity of 20% based on performance in either an optimum (33°), a mild stress (38°), or a severe stress (28°) environment was applied uniformly. Periodically, the performance of each population was measured in all three environments to provide both direct and correlated responses.—Heritability of egg lay in the base population ranged from 0.36 ± 0.03 in optimum to 0.26 ± 0.03 in severe stress. Estimates of dominance effects assumed significant proportions in severe stress only. Genetic correlations for egg lay in diverse environments were large and positive (.6 to.8).—Only in severe stress did the rrs response significantly exceed that for wss. Quadratic adjustments fitted to response curves revealed that small initial genetic gains under rrs were followed by significantly increasing rates of gain in late generations of selection. The reverse was true for wss. This and evidence from realized heritabilities and genetic correlations suggested that rrs had utilized both additive and dominance effects, but wss response was limited to additive effects.—These results agree with selection theory in demonstrating that purebred selection is more efficient than crossbred selection in utilizing additive gene effects. The latter method has merit when non-additive effects assume significant proportions, and this is the more probable case for severe stress conditions.