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Module 10: Recurrent Selection PBG 650 Advanced Plant Breeding Slide 2 Recurrent selection Cyclical selection of populations form families evaluate in trials recombine selections Pedigree selection and improvement of elite lines are also cyclical processes, but the population structure is not so clearly defined Selfing and introgression of new germplasm are common features of both selection systems Recurrent selection and development of lines can be integrated into a comprehensive system Bernardo, Chapt. 10 Slide 3 Rationale for recurrent selection Selfing systems: Fixation of alleles is so rapid that the impact of selection is limited Probability of obtaining segregants with all of the favorable alleles controlling a quantitative trait is small Recurrent selection: systematically increases the frequency of favorable alleles maintains the genetic variation within a population to permit continual progress from selection Example: with 5 loci, all alleles have p=0.5 1/32 chance to get all of the good alleles Example: with 5 loci, all alleles have p=0.6 1/13 chance to get all of the good alleles Slide 4 Recurrent selection in practice Why is it not used more often? (Bernardo) Easy to apply in cross-pollinated crops; difficult in self- pollinating crops male sterility systems can be used Objectives are long-term several generations needed to complete a cycle Immediate output is an improved open-pollinated variety, not a line or hybrid Need to choose one or a few populations for selection not as much opportunity for speculation in use of germplasm Nonetheless, there are many examples of widescale use of varieties developed from recurrent selection schemes Slide 5 Expected selection response Source: Lecture by Jean-Luc Jannink at Iowa State, 2004 Slide 6 Response to selection R=h 2 S Selection differential Response to selection Realized heritability 60708090100110120130140150 60708090100110120130140150 Recombine to form C 1 Select best 10% of C 0 S R Falconer and Mackay, Chapt. 11 Slide 7 Predicting response to selection R=h 2 S Need estimates of h 2 and the selection differential In theory, h 2 is only applicable for a single generation, because heritability depends on gene frequencies. In practice, predictions seem to work for 5-10 generations. Slide 8 Selection differential S can be predicted if we can assume: normal distribution of phenotypes truncation selection Standardized selection differential ( i ) S = i P i = S/ P = z/p p = proportion selected z = height of curve at truncation point i = standard deviations from the mean Slide 9 Values of standardized selection differential pipi 0.900.200.091.80 0.800.350.081.86 0.700.500.071.92 0.600.640.061.99 0.500.800.052.06 0.400.970.042.15 0.301.160.032.27 0.251.270.022.42 0.201.400.012.67 0.151.550.0052.89 0.101.760.0013.37 Becker, 1984 Appendix Tables 2 and 3 (infinite population size) Slide 10 Response to selection R=h 2 S S = i P R=ih 2 P Applies to individual plants in a population Selections made before flowering + controlled matings among selected individuals Mass selection + selfing of selected plants Slide 11 Family selection (O) Parental plant in reference population (X) Selection Unit (progeny mean) (R) Recombination unit (W) Individual in improved population Hallauer, Carena and Miranda (2010) Chapt. 6 Cov(X,W) determines expected gain from selection Slide 12 Intrapopulation Improvement MethodProgenies testedRecombination unit Mass selection (both parents)Individual plants Mass selection (one parent)Individual plantsoutcrossed seed Half-sib (progeny selected)Half-sib families Half-sib (parent is selfed)Half-sib familiesS 1 family Modified ear-to-rowHalf-sib familiesoutcrossed seed Full-sibFull-sib families S 1 family S 2 family Slide 13 Intrapopulation Improvement MethodExpected GainGenerations/Cycle Mass selection (both parents)1 Mass selection (one parent)1 Half-sib (progeny selected)2 Half-sib (parent is selfed)3 S 1 /Testcross4 Modified ear-to-row1 Full-sib2 S 1 family*3 S 2 family*4 P is the square root of variance; pertains to selection units *additive variance for inbred progeny includes an additional component that is a function of the degree of dominance Slide 14 Phenotypic variance of families r = # replications e = # environments Half-sibs Full-sibs S 1 families S 2 families Error variance Variance due to genotype x environment interactions Slide 15 Interpopulation Improvement MethodProgenies testedRecombination unit Reciprocal recurrentHalf-sib familiesS 1 families Reciprocal full-sibFull-sib familiesS 1 families TestcrossTestcrossesS 1 families Slide 16 Reciprocal recurrent selection Half-sibs evaluated (Design I matings) Full-sibs evaluated B0B0 HS yield trials B 0 females A 0 females S1S1 recombined B1B1 A0A0 S1S1 A1A1 A 1 x B 1 (improved cross) B0B0 FS yield trials S1S1 recombined B1B1 A0A0 S1S1 A1A1 A 1 x B 1 (improved cross) Full-sib RRS plants must have two ears twice the number of plants can be evaluated continue to inbreed and evaluate specific crosses Slide 17 Interpopulation Improvement MethodExpected GainGenerations per cycle Reciprocal recurrent 3 Reciprocal full-sib 3 Testcross Depends on choice of tester, but typically Cross P1 plants to inbred line from P2 Cross P2 plants to inbred line from P1 3 Slide 18 Phenotypic variance of families for RRS r = # replications e = # environments Slide 19 Comprehensive breeding program Development of breeding populations from diverse sources such that the performance of the population cross is maximized while maintaining high levels of genetic variance within each population Application of an effective recurrent selection procedure Development of inbreds from each population with good combining ability and recycling of superior inbreds back into the base populations Eberhart et al., 1967 Slide 20 Increasing selection response Increase the selection differential (reduce proportion selected) Increase the coefficient of A 2 Increase A 2 Reduce nongenetic effects Reduce generations/cycle or increase generations/year Slide 21 Choice of selection method I. Breeding Objectives Open-pollinated varieties, synthetics or hybrids Status of commercial seed sector Strategy for distribution of seed Elite variety or genetic resource Target production environments Low or high inputs? Narrow or broad adaptation? Number of traits, relative importance of traits Slide 22 Choice of selection method II. Genetic, Environmental, External Factors Heritability of the trait(s) Extent of GXE Type of gene action Effects of inbreeding on the trait Expected gain per cycle Number of seasons per cycle Growing seasons per year and availability of off- season nurseries Seed quantities required for screening Costs and resources available Slide 23 Maize families seed quantity issues one ear at least four single-row plots FamilyCrossesSeed quantityComments Half-sibs1.Collect pollen in bulk and cross to a female plant 2.Take pollen from one male and cross to several females 1.One ear 2.~4 ears 1.Controlled pollinations or by detasseling 2.Full-sib families within half-sibs Full-sibsCross two plantsOne or two ears With or without reciprocals S 1, S 2, etc.Self pollinationOne ear Seed quantities decrease with inbreeding Can increase a line by selfing or by sib-mating Testcrosses1.Cross one male plant to a female tester and self 2.Cross an S 1 line to a tester (population or inbred line) 1. ~4 ears 2. many ears Controlled pollinations or by detasseling (if S 1 line is female) Slide 24 Modified full-sib family selection Maintenance of Maize Streak Virus Resistance Year 1 Main season, savanna zone Evaluate full-sib families in target environments Off-season: data entry and analysis Year 2 First season, forest zone Recombine selected full-sib families by making plant to plant crosses between families Year 2 Second season (high disease pressure) 1)Plant F 1 families ear-to-row under MSV infestation 2)Remove susceptible plants and offtypes before flowering 3)Make reciprocal crosses between best plants in good rows to generate new full-sib families Slide 25 Reciprocal S 1 Testcross Selection (modified) Year 1 First season Self Year 1 Second season (high disease pressure) Evaluate ~500 S 1 families (2 reps, 2 loc) Select for disease resistance and other highly heritable traits Testcross to the reciprocal population Year 2 Main season Evaluate ~ 200 testcrosses (3 reps, 4 loc) in the target environments Select for yield and other agronomic traits Year 2 Off-season Recombine selected S 1 families Can stagger populations so that one is at the S 1 stage and the other is at the testcross stage each year Slide 26 Meadowfoam - use of blue bottle flies as pollinators Slide 27 S 1 testcross selection in meadowfoam Year 1 (spring) Self ~300 plants in the greenhouse with blue bottle flies Year 1 (fall) + Year 2 (spring) Plant rows of ~5 seeds per family in isolation with bees, 2 blocks Reject S 1 families with poor agronomic characteristics (disease, insect damage, small seeds, etc) Harvest ~6000 seeds per family in bulk Year 2 (fall) + Year 3 (spring) Evaluate ~150 testcross families in yield trials, select ~30 Year 3 (fall) Recombine S 1 seed of selected families in greenhouse Further selfing of selected S1 lines Evaluation of experimental varieties in yield trials Slide 28 Balancing resources for recurrent selection *Daylength can be controlled in the greenhouse to complete a generation in four months Makes efficient use of greenhouse space New experimental varieties can be evaluated every year