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Ch 6. Selection 1. Selection for a single trait 2. Selection for multiple traits

Ch 6. Selection 1. Selection for a single trait 2. Selection for multiple traits 3. Gametophytic selection 4 . Marker-assisted selection . Positive selection --- select desirable types Negative selection --- eliminate undesirable types. 1. Selection for a single trait.

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Ch 6. Selection 1. Selection for a single trait 2. Selection for multiple traits

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  1. Ch 6. Selection 1. Selection for a single trait 2. Selection for multiple traits 3. Gametophytic selection 4. Marker-assisted selection Positive selection --- select desirable types Negative selection --- eliminate undesirable types

  2. 1. Selection for a single trait (1) Qualitative character의 선발 (Ref. 7. Bos외 1995, p.81~) ○ Time of a target trait expression Time of selection  before or after pollen distribution ?? ▷ In self-poll. crops  not important because the plants selected are simultaneously selected both as female and as male plants. ▷ In cross-poll. Crops  very important because it will affect the efficiency of selection. Ifthe trait is expressed after pollen distribution, there is no selection with regard to the plants as male parents: all plants have already contributed pollen from which the next generation is produced. -trait expression and consequent selection before flowering selfingor crossing between selected plants (both female and male) max. selection effect like self-poll. Crops : seedling traits, leaf or stem traits, growth habit, resistances expressed before flowering, etc - trait expression and consequent selection after flowering  all plants are already fertilized before selection  selection will be effective onlyon female(seed) plants : grain-yield traits, quality traits, panicle or ear traits, etc

  3. ① Incomplete dominance : The genotype of each plant can be derived from itsphenotype  desired plenotype can be selected faultlessly. Phenotype ≒ Genotype ▷ In self-poll. crops :selection effect ≒100% ▷ In cross-poll. crops - If the trait is expressed before flowering : selection effect ≒100% - If the trait is expressed after flowering : selection effect ≒50% To increase the selection effeciency : controlled mating between candidate plants (with numbering the plants) before selection  after trait evaluation  take the seeds only from crosses between selected plants (picking out via plant numbers used for crossing) ex) genotype phenotype AA ---------> 10 Aa ---------> 7 aa ---------> 0

  4. ②Complete dominance a. Selection favored the genotype "aa" - Ifthe trait is expressed before flowering in cross-poll. crops or in self-poll. crops  possible to eliminate the undesired allele A at once. - If the trait is expressed after flowering in cross-poll. crops  selfing or crossing a large number of plants with numbering the plants.  At harvest or later, breeder may select crosses in which both involved plants appear to have genotype aa b. Selection for genotype "AA“ - If selfing is possible  ⓐ line/pedigree selection will be effective for complete elimination of ‘a’ allele - If selfing is impossible  less efficient ⓑfull-sib family sel. ⓒ half-sib family sel. ⓓ mass selection * genotype assessment ---> by progeny test

  5. ⓐ line/pedigree selection ○ in Self-poll. cropsor in cross-poll. crops (trait expression before flowering) S0 : Elimination ‘aa’ by phenotype, Selfing AA, Aa plants ---> selection S1 : planting by ear to row Aa lines ---> eliminated before pollen release AA lines ---> selected S2 : only AA lines (a allele is eliminated) ○ In cross-poll. crops (When trait expressed after flowering) S0 : - selfing plants as many as possible - after trait evaluation, harvest seeds from only AA (or Aa) plants S1 : - planting by ear to row in isolated plots  open–pollination within line - distinguish AA orAa lines after trait evaluation  harvest in bulk from AA lines S2 : - planting in bulk of AA plants

  6. ⓑ full-sib family sel. : - SI crops, sugar beet, grasses, oil palm, etc ○ If the trait is expressed before flowering Type of Crosses in G0 Genotypes in G1 Segregation visible AA Aaaa family 1: Aa x Aa ¼ ½ ¼ yes family 2: AA x Aa ½ ½ - no family 3: AA x AA 1 - - no - G0: Remove ‘aa’ plants before flowering  make crosses between plants of phenotype ‘A’ - G1 : - isolated planting of seeds harvested from phenotype‘A’ into each family (I plant 1 family)  discard the families in which ‘aa’ is segregating (family 1) - Select and harvest seeds in family 2 and 3 - G2 : - isolated planting of eachfamily (family-derived bulk)  discard the families in which ‘aa’ is segregating before flowering ( family 2type)  select only family 3 (AA x AA) ○ If the trait is expressed after flowering - G0 : make pairwise crosses between selection-candidate plants in G0.  after trait expression, discard the crosses in which ‘aa’ plants were used as parents - G1 : isolated planting of eachfamily (orcontrolled crossing within each FS-family)  discard the families in which ‘aa’ is segregating - G2 : - isolated planting of selected families (family-derived bulk) discard the families in which ‘aa’ is segregating before flowering ( family 2type) select only family 3 (AA x AA)

  7. ⓒ Half-sib family selection --- Only one of either parents can be selected - rye, maize, grasses, (animal breeding) ○ If the trait is expressed before flowering * in base population, AA: Aa: aa = 1/4: 1/2: 1/4 Maternal Genotype in G1 Segregation visible genotype(G0) AA Aaaa (Crosses in G0) Aa x 1/3 AA 1/6 1/6 - x 2/3 Aa 1/6 2/6 1/6 --------------------- 2/6 3/6 1/6 yes AA x 1/3 AA 1/3 x 2/3 Aa 1/3 1/3 - --------------------- 2/3 1/3 no - G0 :Remove ‘aa’ plants before flowering and open pollination  harvest from seed parents (AA, Aa) - G1 : - isolated planting of each HS-family  discard families in which ‘aa’ phenotype is segregating (G0 : Aa)  Select seed plants(AA, Aa) - G2 : repeat G1 ○ If the trait is expressed afer flowering - Same as above - In this case, since phenotype ‘aa’ plants participate in mating, elimination of ‘a’ allele in population becomes much slower. - remnant seed procedure may be applicable: because it may not require successiveHS-mating and isolation of populations.

  8. ⓓ Mass selection ○ If the trait is expressed before flowering - Discard ‘aa’ plants before flowering AA Aaaa ‘a’ allele freq. ¼ 2/4 ¼ (discard) - G0 selection 1/3 2/3 1/3 selected seeds AA x AA 1/9 (planting in G1) “ x Aa 1/9 1/9 Aa x AA 1/9 1/9 “ x Aa 1/9 2/9 1/9 Total4/9 4/9 1/9 (discard) - G1selection1/2 1/2 1/4 selected seeds AA x AA 1/4 (planting in G2) “ x Aa 1/8 1/8 Aa x AA 1/8 1/8 “ x Aa 1/16 2/16 1/16 9/16 6/16 1/16 (discard) - G2 selection3/5 2/5 1/5 . . . . . . . q0 : freq. of ‘a’ allele in base population qt: freq. of ‘a’ allele in Gtselection generation The freq. of ‘a’ allele reduces in a very low rate. ○ If the trait is expressed after flowering - Same as above - phenotype ‘aa’ plants participate in mating, elimination of ‘a’ allele in population becomes much muchslower.

  9. ⓔ Progeny testing - very effective in estimating the genotypes in previous generation - more effective accompanied by selfing * test crossing (2) Selection of quantitative characters--- heritabilities Genetic advance under selection(Ref. 2:281-292)

  10. Selection intensity (q) and selection differential (k) Factors affecting (=Gs=Rs) = Strong selection (적은선발)  increase High variation in pop (변이가 큰 집단)  increase High heritability 

  11. Predicted response to selection vs. selection intensity for different heritabilities q= =

  12. Mean of offspring population Mean of selected plants in the base population

  13. Ex) Selection experiment with Korean lespedeza (둥근매듭풀) 284 familiesfrom 3 different hybrids. In F3 and F4, 2 replications, 2 years Phenotypic variance Additive genetic variance  ○ 5% selection  Gs ?? q=5%  =2.06 Gs= = 64 g/plot = 164.23 g (selection differential)

  14. 2. Selection for multipletraits (1) Tandem selection (순차적선발) • To select one trait at a time  serial improvement • • several generations required • • genetic correlation between traits ignored • Ex) 1st generation  short stalk • 2nd “  resistance to bacterial disease • 3rd “  high lysine content • - - - (2) Truncation selection (independent curling selection; elimination selection) (절단선발) • To select multiple traits in one generation • • set the selection criteria for each trait • • strong selection • • genetic correlation between traits ignored • • Breeder’s experience • Ex) 50% of pop for trait A  40% for trait B  50% for trait C •  0.5 x 0.4 x 0.5 = 10% of the pop are selected

  15. (3) Index selection : Simultaneous selection of multiple traits considering the relationship among traits and economic importance of certain trait. Function for index score Variables measured Weighting coefficient of each variable A. Selection of a single complex trait (ex. Yield) : y = : Ph. variance of trait 1 : Ph. covariance of trait 1 & 2 = . . . . . . . . . . . . . . . . . . . . . . . , Genetic covariance of trait y & others =

  16. B. Simultaneous selection of multiple traits : yield and others = : economic importance of each trait imposed by the breeder = . . . . . . . . . . . . . . . . . . . . . . . = 3 traits :

  17. C. Response to selection of Index value (score)

  18. 3. Gametophytic and sporophytic selection (Hayward 등:332-352) HormazaJI, and Herrero M, 1992. Pollen selection, TAG 83:663-672 ○ Pollen competition - Two types ① A direct competition among haploid gametophytes a. physical competition: the rate of pollen tube growth b. chemical competition based on pollen inhibition (allelopathy) ② Interaction between haploid and diploid genes : selective reception by pistils (pollen-pistil interactions) - intense pollen competition --> decrease in genetic variability of the offspring (lower frequency of extreme genotypes) - a positive correlation between pollen competition ability and several sporophytic traits such as seed and seedling wt, number of seeds per fruit, root growth, etc ---> The higher pollen tube growth rate, the more vigorous the offspring. - pollen tube growth rate : controlled by polygenes 4. Marker-/Genomics-assisted selection

  19. ○ Requirement for gametophytic selection - Overlap in genetic expression between the sporophyte and gametophyte ' isozymes ' mRNAs - Correlation between sporophytic and gametophytic responses to external factors ' Temperature ' Salinity and osmotic pressure ' Metals ' Herbicides and other toxic compounds ○ Pollen selection and practical plant breeding - Pre-germination of pollens -->vigorous pollen tube growth --> density gradient (The larger the pollen tube, the lower the density.) --> selection using centrifugation, flow cytometry --> pollination in vivo or in vitro (after style-cutting) - Grow whole plant under selective pressure. * Progeny from crosses made at low temperatures had better cold adaptation than progeny from crosses at normal temp. (Zamir and Gadish, 1987 Pollen selection for low temperature adaptation in tomato. TAG 74(5): 545-548) - Treatment of selective agents on the stigma : herbicides, salts, etc

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