CSS 650 Advanced Plant Breeding

2. Selection for a high mean Success is a function of the population mean ? the deviation of the best segregants from ? ability to identify the best segregants Advanced Cycle Breeding = �inbred recycling� cross best by best (often related) pedigree and backcross selection emphasis on high mean at the expense of ?G2 need methods for predicting ?

3. Probability of fixing favorable alleles during inbreeding Three approaches to increase chances of fixing favorable alleles selection before inbreeding selection during inbreeding one or more backcrosses to the better parent before inbreeding

4. Mean with selfing Inbreeding decreases the mean if there is dominance At fixation (with no selection):

5. Mean of recombinant inbreds from a single-cross Mean of recombinant inbreds derived from F2 of a single-cross

6. Selfed families from a single-cross

7. Selfed families from a single-cross

8. Variance among and within selfed families

9. Genetic variance with selfing

10. Testcrosses The choice of tester will determine if an allele is favorable or not

11. Effect of alleles in testcrosses

12. Testcross mean of recombinant inbreds Testcross mean of recombinant inbreds derived from F2 of a single-cross

13. Testcross means Testcross mean of the heterozygote is half-way between the two homozygotes Cross �good� by �good� But, the correlation between the performance of inbred lines per se and their performance in testcrosses is very poor for yield and some other agronomic traits

14. Heterosis or Hybrid Vigor Quantitative genetics: superiority over mean of parents Applied definition superiority over both parents economic comparisons need to be made to nonhybrid cultivars Various types population cross single-, three-way, and double-crosses topcrosses modified single-cross definition of a modified single-cross advantages and disadvantages of double-crossesdefinition of a modified single-cross advantages and disadvantages of double-crosses

15. Heterosis Amount of heterosis due to a single locus = d 50% is lost with random-mating

16. Theories for heterosis Dominance theory: many loci with d ? a Should be possible to obtain inbred ? single-cross Expect skewed distribution in F2 (may not be the case if many loci control the trait) Overdominance theory: d > a Pseudo-overdominance - decays over time

17. Heterosis � some observations Epistasis can also contribute to heterosis does not require d>0 Experimental evidence suggests that heterosis is largely due to partial or complete dominance Yields of inbred lines per se are poor predictors of hybrid performance due to dominance due to heritability <1

18. Heterotic groups Parents of single-crosses generally come from different heterotic groups Two complementary heterotic groups are often referred to as a �heterotic pattern�

19. Identifying heterotic patterns Diallel crosses among populations Crosses to testers representing known heterotic groups Use molecular markers to establish genetic relationships, and make diallel crosses among dissimilar groups initial studies were disappointing markers must be linked to important QTL

20. Exploiting heterosis Recycle inbreds within heterotic groups Evaluate testcrosses between heterotic groups elite inbreds often used as testers BLUP can predict performance of new single-crosses using data from single-crosses that have already been tested fairly good correlations between observed and predicted values

21. Predicting hybrid performance