PBG 650 Advanced Plant Breeding Module 10: Recurrent Selection 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 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 Example: with 5 loci, all alleles have p=0.5 1/32 chance to get all of the good alleles • Recurrent selection: – systematically increases the frequency of favorable alleles Example: with 5 loci, all alleles have p=0.6 1/13 chance to get all of the good alleles – maintains the genetic variation within a population to permit continual progress from selection Recurrent selection in practice Why is it not used more often? (Bernardo) • Easy to apply in cross-pollinated crops; difficult in selfpollinating 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 Expected selection response Progeny Phenotype • • b • OP • • • • • •• • • • • • • • • Expected • • • • •• • • • •• •• • • • • • • • •• • • • • • • • • • • • selection •• ••• • •• ••• ••• • • • • • ••• ••• • • • • • • • • • • • • • • • • • •• • •• • ••• ••• • • • • •••• •• •• • •• response • • • •• • • •• • • • • • • •• ••••• • •••• ••• •• • •• • • •• ••••• • •••• ••• •• • •••• •• ••• • ••• • • • • R = h 2S • • • • • • • • • • • • • • ••• • • • • • • • •• • •• • •• • • •• •• • ••••• •• •••• ••• ••••••••• •••••••• •• •••••• •••• •• ••••• •• • • • • • • • • • • • • • •• • •• •••• • •••• • • •• • •••• • • • • • •• • • •••• •• •• ••• ••• ••••••••••••• •••••• ••••••• ••• •••• ••••••• ••••• •••• •• • •• • • • • • •• •• • • •••••• ••••• •• ••• ••• •••• • ••••• • •• •• •••• •••• •• •• • •••• • • • •• •• • • •• ••••• ••••••••• •••••••••• •••• ••••• ••• •• •••• ••• • • • ••• ••• •••• ••• • • • • • • • • • • •• • •• •• •• • •••• •• •••• ••• • ••• • • ••••• ••• • •• • • • • • • • • • • • • • Mean of • • • • ••• • ••••• • •• • •• • • • ••• • •• •• •• • • • selected • • • •• • • • •• • ••• • •••••• • • • • • • • • parents • •• •• • • Selection differential S Selected parents Truncation point Mean of all possible parents (selection candidates) Source: Lecture by Jean-Luc Jannink at Iowa State, 2004 h2 Mid-parent Phenotype Response to selection R=h2S X0 S Selection differential S XS X0 Response to selection Xs 60 70 80 90 100 110 Select best 10% of C0 120 130 140 150 120 130 140 150 Recombine to form C1 R X1 X0 X1 Realized heritability R 113 100 2 h 0.75 S 117 100 R 60 70 Falconer and Mackay, Chapt. 11 80 90 100 110 Predicting response to selection R=h2S • Need estimates of h2 and the selection differential • In theory, h2 is only applicable for a single generation, because heritability depends on gene frequencies. In practice, predictions seem to work for 5-10 generations. Selection differential • S can be predicted if we can assume: – normal distribution of phenotypes – truncation selection • Standardized selection differential (i) S = iσP i = S/σP = z/p p = proportion selected z = height of curve at truncation point i = standard deviations from the mean Values of standardized selection differential p i p i 0.90 0.20 0.09 1.80 0.80 0.35 0.08 1.86 0.70 0.50 0.07 1.92 0.60 0.64 0.06 1.99 0.50 0.80 0.05 2.06 0.40 0.97 0.04 2.15 0.30 1.16 0.03 2.27 0.25 1.27 0.02 2.42 0.20 1.40 0.01 2.67 0.15 1.55 0.005 2.89 0.10 1.76 0.001 3.37 Becker, 1984 – Appendix Tables 2 and 3 (infinite population size) Response to selection R=h2S S = iσP R=ih2σP • h 2 2 A 2 P R i 2 A P Applies to individual plants in a population – Selections made before flowering + controlled matings among selected individuals – Mass selection + selfing of selected plants Family selection (O) Parental plant in reference population (R) Recombination unit (X) Selection Unit (progeny mean) (W) Individual in improved population Cov(X,W) determines expected gain from selection Hallauer, Carena and Miranda (2010) Chapt. 6 Intrapopulation Improvement Method Progenies tested Recombination unit Mass selection (both parents) Individual plants Individual plants Mass selection (one parent) Individual plants outcrossed seed Half-sib (progeny selected) Half-sib families Half-sib families Half-sib (parent is selfed) Half-sib families S1 family Modified ear-to-row Half-sib families outcrossed seed Full-sib Full-sib families Full-sib families S1 family S1 family S1 family S2 family S2 family S2 family Intrapopulation Improvement Method Mass selection (both parents) Mass selection (one parent) Half-sib (progeny selected) Half-sib (parent is selfed) S1/Testcross Modified ear-to-row Full-sib S1 family* S2 family* Expected Gain Generations/Cycle i P 2 i(1/2) A P 2 i(1/4) A Phs 2 i(1/2) A Phs 2 A i(1/2) A 2 i(1/8) A 2 i(1/2) A i A2 P 2 i(3/2) A 2 S1 Phs Phs Pf s 1 1 2 3 4 1 2 3 P 4 S2 σP 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 Phenotypic variance of families Half-sibs P HS 1 re Full-sibs P FS 1 re S1 families P S1 1 re S2 families P 1 re S2 2 2 2 2 1 e 1 e 1 e 1 e 2 GE 2 GE 1 4 2 GE 2 A 1 2 2 GE 2 A 2 A 3 2 2 D 1 4 1 4 2 A 2 D 3 16 2 D Error variance Variance due to genotype x environment interactions 2 2 GE r = # replications e = # environments Interpopulation Improvement Method Progenies tested Recombination unit Reciprocal recurrent Half-sib families S1 families Reciprocal full-sib Full-sib families S1 families Testcross Testcrosses S1 families Reciprocal recurrent selection • Half-sibs evaluated (Design I matings) A0 S1 recombined A0 females HS yield trials HS yield trials B0 females B0 • A1 A1 x B1 (improved cross) S1 recombined B1 Full-sibs evaluated A0 S1 recombined A1 FS B0 yield trials S1 recombined Full-sib RRS • plants must have two “ears” • twice the number of plants can be evaluated • continue to inbreed and evaluate specific crosses A1 x B1 (improved cross) B1 Interpopulation Improvement Method Expected Gain Generations per cycle Reciprocal recurrent (i (1/2) A (P1) P(P1hs ) ) (i (1/2) A ( P 2 ) P(P 2hs ) ) 2 2 3 2 (i (1/2)( A (P1) A (P 2 ) ) P(P1xP2 f s) 3 Depends on choice of tester, but typically 3 Reciprocal full-sib 2 Testcross • Cross P1 plants to inbred line from P2 • Cross P2 plants to inbred line from P1 Phenotypic variance of families for RRS r = # replications e = # environments P(P1hs) P(P1xP2fs) 1 re 1 re 2 (P1) 1 e 2 GE(P1) ( 2 1 e 2 GE 1 2 2 A(P1) 1 2 2 A(P1) 2 A(P2) ) 1 4 2 D(P1P2) 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 Increasing selection response • Increase the selection differential (reduce proportion selected) • • • • Increase the coefficient of A2 Increase A 2 R Reduce nongenetic effects Reduce generations/cycle or increase generations/year i 2 A P 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 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 offseason nurseries • Seed quantities required for screening • Costs and resources available Maize families – seed quantity issues Family Crosses Seed quantity Comments Half-sibs 1. Collect pollen in bulk and cross to a female plant 2. Take pollen from one male and cross to several females 1. One ear 1. Controlled pollinations or by detasseling 2. ~4 ears 2. Full-sib families within half-sibs Full-sibs Cross two plants One or two ears With or without reciprocals S1, S2, etc. Self pollination One ear Testcrosses 1. Cross one male plant to a female tester and self 2. Cross an S1 line to a tester (population or inbred line) 1. ~4 ears 2. many ears Seed quantities decrease with inbreeding Can increase a line by selfing or by sib-mating Controlled pollinations or by detasseling (if S1 line is female) one ear at least four single-row plots Maintenance of Maize Streak Virus Resistance Modified full-sib family selection Year 1 Main season, savanna zone Evaluate full-sib families in target environments 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 F1 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 Off-season: data entry and analysis Reciprocal S1 Testcross Selection (modified) Year 1 First season Self Year 1 Evaluate ~500 S1 families (2 reps, 2 loc) Second season Select for disease resistance and other (high disease highly heritable traits pressure) 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 S1 families • Can stagger populations so that one is at the S1 stage and the other is at the testcross stage each year Meadowfoam - use of blue bottle flies as pollinators S1 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 S1 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 S1 seed of selected families in greenhouse • • Further selfing of selected S1 lines Evaluation of experimental varieties in yield trials 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