UENF 506 16: A new maize cultivar alternative for the state of Rio de Janeiro

Reciprocal recurrent selection is the selection of purebreds for crossbred performance and takes advantage of additive and non-additive variance by using pedigreed progeny performance records. Developed in maize, the adoption of this approach in livestock breeding has been limited to the pork and poultry industries; genomic selection may facilitate its extension into the beef industry by replacing pedigree. The literature regarding the relative importance of additive versus non-additive variance and reciprocal recurrent genomic selection models was reviewed. The potential for using reciprocal recurrent genomic selection in a terminal Wagyu × Angus cross scenario was examined. Non-additive variance is more important for fitness traits and accounts for a small proportion of variance related to production traits such as marbling. In general, reciprocal recurrent selection was not significantly better at improving performance of crossbreds than was traditional selection within parental breeds using only additive variance in the studies examined. Simulation studies showed benefits of including dominance or breed-specific allele effects in prediction models but advantages were small as more realistic simulations were examined. On the basis of the evidence, it is likely that in a terminal two-way cross-beef scenario utilising Wagyu sires and Angus dams, where selection emphasis is on marbling, selection of purebreds on the basis of additive variance will allow substantial progress to be realised.

For a reciprocal recurrent selection (RRS) program to succeed, it is essential to maintain genetic variability throughout the selection cycles and to obtain accurate estimates of genetic parameters, which in turn are directly related to the number of progenies and repetitions evaluated. This study evaluates the potential of maize progenies of the sixth cycle of RRS and proposes, using simulation methods, the ideal combination of the number of progenies and repetitions to employ in reciprocal full-sib recurrent selection. A total of 163 full-sib progenies were evaluated in a randomized block design with six repetitions. Based on the yield data, analysis of variance was carried out, and different scenarios were simulated using the Monte Carlo chain method. These scenarios varied in the number of repetitions (two, four, and six) and progenies (30 to 163). The contrast between progenies and controls was significant, revealing the potential of the progenies of the sixth cycle of RRS. The high magnitude of the selective accuracy (0.77) was reflected in high estimates of heritability, which allowed for efficient phenotypic selection, obtaining selection gains of 14.07%. From the estimates of phenotypic and genotypic variance, heritability, accuracy, and standard error, it was found that a repetition number above two results in drastic changes in the estimates of these parameters; however, with the use of 130 progenies, these estimates tend to stabilize, implying that a high number of progenies does not interfere decisively in the quality of most parameters, except for the limits of maximum and minimum variation.

BackgroundTo 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.ResultsWe 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.ConclusionsRRGS 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.

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.

Reciprocal recurrent selection (RRS) has proven to be a successful method for improving the performance of a cross population and to increase the heterosis between populations. However, RRS has not been widely adopted by the commercial breeders because RRS is not as efficient to recover inbred lines as other methods of inbred development. Use of two inbred lines as testers instead of the opposite population as reciprocal tester in a modified RRS (MRRS) scheme could overcome this limitation. A breeding research program was initiated in 1974 at Iowa State University to evaluate the modified RRS procedure and RRS in BS21 and BS22 maize populations. The modification used inbred line A632 as tester for BS21 and inbred line H99 as tester for BS22. After six cycles of selection were completed in BS21 and BS22 using MRRS and RRS, an experiment was conducted to evaluate the response to selection. The populations per se, testcrosses to inbred testers, and crosses between cycle populations of RRS and MRRS were evaluated in replicated yield trials. There were significant increases in grain yield in all six cross populations as a consequence of selection. The rate of direct response was greater for the RRS procedure than for the MRRS [4.4, 1.6, and 2.8 %cycle*' for BS21(R)xBS22(R), H99xBS22(HI), and A632xBS21(HI), respectively]. RRS was as effective as MRRS for improving the grain yield of the populations in crosses with the inbred lines, but MRRS was not as effective as RRS in the improvement of the cross population BS21xBS22, with a significantly lower rate of increase in yield of 1.6%cycle'V Realized heritability and response to selection for yield were 25 to 50% of their predicted values.

Key messageReciprocal recurrent selection sometimes increases genetic gain per unit cost in clonal diploids with heterosis due to dominance, but it typically does not benefit autopolyploids.Breeding can change the dominance as well as additive genetic value of populations, thus utilizing heterosis. A common hybrid breeding strategy is reciprocal recurrent selection (RRS), in which parents of hybrids are typically recycled within pools based on general combining ability. However, the relative performances of RRS and other breeding strategies have not been thoroughly compared. RRS can have relatively increased costs and longer cycle lengths, but these are sometimes outweighed by its ability to harness heterosis due to dominance. Here, we used stochastic simulation to compare genetic gain per unit cost of RRS, terminal crossing, recurrent selection on breeding value, and recurrent selection on cross performance considering different amounts of population heterosis due to dominance, relative cycle lengths, time horizons, estimation methods, selection intensities, and ploidy levels. In diploids with phenotypic selection at high intensity, whether RRS was the optimal breeding strategy depended on the initial population heterosis. However, in diploids with rapid-cycling genomic selection at high intensity, RRS was the optimal breeding strategy after 50 years over almost all amounts of initial population heterosis under the study assumptions. Diploid RRS required more population heterosis to outperform other strategies as its relative cycle length increased and as selection intensity and time horizon decreased. The optimal strategy depended on selection intensity, a proxy for inbreeding rate. Use of diploid fully inbred parents vs. outbred parents with RRS typically did not affect genetic gain. In autopolyploids, RRS typically did not outperform one-pool strategies regardless of the initial population heterosis.

Haif-sih and full-sib reciprocal recurrent selection (RRS) have been successful methods of reciprocal improvement of two maize (Zca mays L.) populations to maximize the performance of the population cross. The objective of our study was to use computer simulation to compare responses to selection of populations themselves and their crosses for half-sib and full-sib RRS and one modification. The modification included an additional generation of inbreeding before producing testcrosses (S 2 plants as recombination units). Parental populations included 110 individuals throughout 20 cycles of selection. Evaluation of each individual in both RRS methods was based on the mean performance of 10 testcross progeny. Selection intensity was 10% in all instances. Assumptions for the simulation studies included diploid individuals with two alleles per locus, where expression of the quantitative trail was determined by 40 independent loci; genotype x environmental interaction was not included. There were 22 initial conditions corresponding to two sets of initial parental population allele frequencies and 11 genetic models. The two sets of initial parental populations were equal and unequal allele frequencies. The 11 genetic models included additive genetic variance, three levels of dominance, and seven epistasis combinations. Half-sib and full-sill RRS and their modification were simulated for each of the 22 initial conditions with three replications of each condition for 20 cycles of RRS selection. Genetic response of full-sib RRS was similar to half-sib RRS for 21 initial conditions with S 1 s as the recombination units. The linear response of half-sib RRS was 1.7 times (P ≤ 0.01) greater than full-sib RRS for the genetic model that included complete dominance and dominance x dominance epistasis with S 1 recombination units. There were no significant differences between half-sib and full-sib RRS with S 2 s as recombination units but use of S 2 s increased selection response for both half-sib and full-sib RRS. Compared with half-sib RRS, full-sib RRS requires 50% fewer test-crosses, but full-silo RRS has the same selection response as half-sib RRS. Full-silo RRS with S 2 s as the recombination units would lie the more efficient method of interpopulation improvement in maize breeding.

We evaluated three maize (Zea mays L.) crosses among BSCBI(R)C5, BSSS(R)C5, and BSSS(HT)C6 populations in six environments. BSCBI(R)C5 and BSSS(R)C5 improved populations from reciprocal recurrent selection, and BSSS(HT)C6 was obtained from testcross selection with a double‐cross tester. Also, we used five selected S3 lines from each of BSCBI(R)C5 and BSSS(R)C5 and five selected S3 or S4 lines from BSSS(HT)C5 in three sets of line ✕ line crosses among the sources. Average yields of the population crosses were essentially equal, which was unexpected for BSSS(R)C5 ✕ BSSS(HT)C6 Most of the variation among crosses in each set was accounted for by average line performance (general combining ability) in all traits.Average yield of line crosses in a set was significantly higher than the population cross in each set and in the BSCBI(R)C5 ✕ BSSS(R)C5 and BSCBI(R)C5 ✕ BSSS (HT)C6 sets, the best cross yielded 35% more than the population cross. Two BSCBI(R)C5 ✕ BSSS(R)C5 crosses yielded significantly higher than B37 ✕ Oh43, and one of these crosses was equal to B37 ✕ Oh43 for lodging resistance. None of the crosses in the other two sets was equal to B37 ✕ Oh45 for all traits, mainly because of lodging. A modified reciprocal recurrent selection procedure using an inbred tester is proposed as a more efficient method of simultaneously improving breeding populations and developing elite single crosses.

Recurrent full‐sib family and reciprocal recurrent selection have resulted in significant increases in grain weight of crosses between the varieties of maize (Zea mays L.) Jarvis Golden Prolific and Indian Chief. The objectives of this study were to (i) compare increases due to 14 cycles of recurrent full‐sib family selection, and reciprocal recurrent selection for yield in maize and (ii) determine whether increased grain weights were accompanied by parallel increases in total dry matter and total N accumulation. Unless total dry matter acquisition increases in parallel with grain weight during selection, sustained root and shoot functions would be restricted by excessive diversion of carbohydrates to grain. After 14 selection cycles on sandy soils of the North Carolina coastal plain, grain weight, dry matter accumulation, and N accumulation in the population hybrid increased 23, 12, and 4%, respectively, with full‐sib family selection and 27, 21, and I0%, respectively, with reciprocal recurrent selection. Greater proportions of the dry matter and N accumulated were partitioned to grain after full‐sib family selection than after reciprocal recurrent selection. Therefore, photosynthate production after full‐sib family selection was not sufficient to fully satisfy both the demand by grain and the requirements for root and shoot processes. Reciprocal recurrent selection resulted in a greater increase in photosynthate production, so that the hybrid population was able to meet the demand for carbohydrate by higher grain yield and still sustain N acquisition by roots. The conclusions were also supported by heterosis for grain weight, total dry matter, and total N accumulation, which was greater after reciprocal recurrent selection than after full‐sib family selection. Selection to increase grain weight will result in efficient N acquisition only when the greater grain weight is supported by an adequate increase in photosynthate 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.

Reciprocal recurrent selection (RRS) has not been widely adopted by maize (Zea mays L.) breeders because pedigree selection methods were effective in developing improved inbred lines. RRS as been used successfully, but modified reciprocal recurrent selection (MRRS) uses elite inbred lines as testers, which may be more useful in applied breeding programs. A study was conducted to compare responses to selection via RRS and MRRS in the BS21 and BS22 maize populations. After six cycles of RRS and MRRS, an experiment was conducted to determine the direct and the indirect responses to selection. The populations themselves, testcrosses to inbred testers, and crosses between BS21 and BS22 were evaluated at four locations for 2 yr. Grain yield increased significantly in all population crosses. Direct response to selection was greater for the RRS method than for the MRRS method: 4.4% cycle−1 for BS21(R) × BS22(R); 2.8% cycle−1 for BS21(HI) × A632; and 1.6% cycle for BS22(HI) × H99. RRS was as effective as MRRS for improving grain yield of BS21(R) and BS22(R) in crosses with A632 and H99, but MRRS was not as effective as RRS in the improvement of the BS21(HI) × BS22(HI) cross populations: 1.6%−1 cycle for BS21(HI) × BS22(HI) vs. 4.4% cycle−1 for BS21(R) × BS22(R).There was no evidence that the genetic variation among testcrosses for grain yield was greater with use of inbred lines as testers compared with use of populations as testers.

The objective of the research was to evaluate recurrent selection strategies in maize (Zea mays L.) by determining the expected and realized responses to selection, as well as the alterations in heterosis, of hybrids from the cross between the original populations (C0) and the third cycle (C3). The genetic variances and covariances were obtained for intrapopulational (P11 and P22) and interpopulational (P12 and P21) half-sib progenies from populations 1 and 2 in cycles C0 and C3. One-hundred progenies of each type were evaluated [husked ear weight (g plant−1)] at two separate locations by conducting 10 × 10 triple lattice design experiments contiguous for each cycle. Four different recurrent selection strategies were investigated: reciprocal recurrent selection (RRS), interpopulation testcross half-sib selection (THS 1 and THS 2) and intrapopulational half-sib recurrent selection (HSS). THS 1, which employed population 1 (with the lowest frequency of favorable alleles) as tester, was most efficient in the simultaneous improvement of the hybrid and the two populations per se. RRS showed the highest efficiency in improving the interpopulational hybrid but not in the parental populations. In order to estimate heterosis and realized responses to RRS, representative samples of seed from intra and inter-populational half-sib progenies from C0 and C3 were employed in randomized block design experiments with 10 repetitions carried out at two separate locations. RRS increased heterosis in the interpopulational hybrid from 12.3 % (C0) to 24.9 % (C3), and the productivity gain [husked ear weight (t ha−1)] was 13.5 % over the three cycles.

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.

This paper reports the effects of three cycles of reciprocal recurrent selection (RRS) on the means, genetic variances, and on the genetic correlations for several traits in the IG-1 and IG-2 maize (Zea mays L.) populations. Interpopulation full-sib progenies from cycle zero (C0) and from cycle 3 (C3) of RRS were evaluated in two locations. RRS was highly effective to improve the traits according the objectives of the program: grain yield and prolificacy increased significantly, while plant height, ear height, and ear placement decreased significantly. Genetic variances for all traits decreased significantly from C0 to C3, but the genetic correlations did not change consistently across the cycles of selection. The expected responses to the fourth cycle of RRS and the probability of selecting double-crosses from C3 that outperform those from C0 showed that the decreases in the genetic variances were not great enough to limit the continued improvement of the populations as well as the use of the improved populations as sources of inbred lines to develop commercial hybrids. However, if the magnitudes of the genetic variances continue to decrease, new sources of improved germplasm should be incorporated into both populations to allow the continued improvement of the interpopulation by RRS.

SUMMARYPrevious studies have shown that reciprocal recurrent selection (RRS) would be ineffective in the early cycles of selection if over-dominant loci were at equilibrium gene frequencies in both selected populations. It is shown in the present study that response to RRS may be reduced by this unstable equilibrium even when gene frequencies are as much as 30 % removed from the theoretical equilibrium frequency. Reducing one population to a bottleneck of two individuals for one generation or more before initiating RRS (RRSB) was very effective in overcoming the unstable equilibrium. RRS with recurrent inbreeding, then outcrossing in both populations each cycle of selection (RRSC) was not effective in overcoming the unstable equilibrium, but yielded greater response per cycle after selection response began. The effectiveness of RRSC was inversely proportional to the heritability of the trait. Use of these modifications to increase the effectiveness of RRS in poultry breeding is discussed.