Early Generation Testing, Mass Selection and Bulk Selection

1. Early Generation Testing, Mass Selection and Bulk Selection PLS 664 February 21, 2011

2. Early Generation Testing • Objective: identify those populations that are likely to contain superior lines • Strategy: eliminate populations with low potential from the inbreeding process • Goal: maintain and develop lines from populations with high genetic potential

4. Jenkins, 1935 • Usual method of estimating combining ability in maize was to inbreed lines, then mate them to a common tester • Jenkins saved seed from S0:1 lines through many selfing generations, then crossed them to common tester • Found that combining ability was already determined in S0:1 lines

5. Self-Pollinated Crops • Determine the generation for testing • If it is to be the F2, you will have to grow the F1 in an environment which favors seed production • A more common choice would be F2:3 lines

6. Self-Pollinated Crops • Harvest seed from individual F2 plants • Plant seeds in F2:3 progeny rows • Identify the superior rows • Harvest all seed in each selected row in bulk • Grow replicated tests of F2:4 lines • Grow replicated tests of F2:5 lines

7. Self-Pollinated Crops • Harvest selected F5 plants individually • Grow F5:6 lines in headrows • Test F5 - derived lines extensively

8. Breeder’s Decisions • Generation to test • Number of reps, locations and years - tradeoff between early and late generation testing • Separate program for inbreeding or not • Selected lines can be advanced by pedigree, bulk, or SSD • Number of plants chosen from each hetergeneous line may vary

9. Genetic Considerations Recall that there is all of the additive variance among F2:3 lines and one-half of the additive variance within F2:3 lines In later generations of F2 derived lines, there is still all of the additive variance among lines, and considerable variance within lines, as inbreeding progresses

10. Genetic Considerations • Therefore, one may need to take a large number of heads to adequately sample the variation within the F2 - derived line • Now one must decide how to allocate resources • Should you sample more lines or more selections within lines?

11. Pros • Inferior individuals and crosses are discarded early in the process • One hetergeneous line may yield more than one cultivar

12. Cons • When you commit a lot of resources to early generation testing, you cannot devote as much to thorough evaluation of more inbred material • If you spend a lot of time testing the early generations, cultivar release may be delayed

13. From Bernardo, 2003 CROP SCIENCE 43 : 1558-60

14. Bernardo • Simulation paper; no actual data • Demonstrated that even though the genetic correlation may be high, it was more strongly affected by non-genetic (ie environmental) effects than by the generation of selfing

15. Mass Selection Selective harvest of individuals from a heterogenous population. Oldest method of plant improvement. Humans have been Selecting desirable seeds for years, even since Neolithic times. Quickest way to make progress with undeveloped populations

16. Examples of selection criteria • height weight color • texture chemical content shippability (cookies) • storability timing of maturity taste!! • # tillers shape disease resistance • “vigor” clean leaves stay green • Selection success is related to the single plant • heritability estimate of each trait.

17. Mass Selection – How To Stage #1 = Selection of individuals within a heterogeneous population. Choose favorable/desirable plants and harvest the seed. Stage #2 = Sampling equal numbers of seed from selected individuals to plant the next generation.

18. Mass Selection in Oat : Romero & Frey, 1966 1958: Plant out heterogeneous population of F3 oat seed. Using a lawn clipper, cut oat plants to uniform height (determined by check cultivar). Harvest only top 10 cm in order to select against short plants. 10 cm Collected seed composed of complete panicles, partial panicles, or no panicle.

19. 1959 (F4), 1960 (F5), and 1961 (F6): Repeated the procedure. Evaluated 75 plants from the unselected group of each generation, 75 plants from the selected group of each generation, and 75 plants of a pure line check. Results Significant reduction plant height = 0.47” generation-1 Associated shift towards earliness = 0.23 days cycle-1 Shift towards higher yields = 0.41 grams plot-1. Non-significant

21. Mass selection in soybean for maturity and • calcareous soil tolerance : Fehr, unpublished • Yellowing of soybean indicates lack of ability to utilize available • iron. • Cultivar with desirable levels of tolerance and maturity were planted • as standards. • Plants with more yellowing than standards were removed before • flowering. • One pod/plant was harvest and bulked from selected individuals • ** These are two examples of selections based on individual • plants within the population. rg DI h2D h2I h2D = heritability of directly selected trait h2I = heritability of indirectly selected trait rg DI = genetic correlation between D&I traits

22. Mass selection of seed size in soybeans : Fehr & Weber, • 1968 • 1963: 4500 F6 plants were planted and 400 plants of early maturity • were selected. • Top 1/4 of main stem and all branches were removed to discard • poor seed; remainder of plant was threshed together. • Seed was passed over different sized sieves and 25% of the largest • and smallest seeds were retained.

23. Specific gravity tests in glycerol-water solutions were performed for each size group. 25% of the seeds with a high density and 25% of the seed with a low density were selected. 1964 : 2300 F7 seeds from each group were planted: 400 selected for harvest in same fashion. Repeat sizing and specific gravity tests.

24. 1965 : Process repeated with 1000 F8 seeds from each group. Results Linear change in seed size Progress for high protein- low oil was best in large seed/high specific gravity set. Progress for high oil - low protein was best in small seed/low specific gravity set. This is an example of selections and sampling being performed simultaneously on the harvested seed.

25. Mass Selection Selection can be applied to 1) individual plants, 2) seeds. Selection involved is 1) artificial selection, 2) natural selection, 3) both. Selection for 1) one trait, 2) multiple traits Sampling methods : 1) random sample of selected seeds bulked 2) equal quantities of seeds harvested & bulked from selected individuals 3) selection performed on seeds which are used to plant out the next generation. (Selection & sampling done simultaneously).

26. Cultivar Purification : mass selection is used routinely • in the maintenance of purity for self-pollinated cultivars • or inbreds of cross-pollinating species. It involves roguing of • off-types (removal of individuals that do not conform to the • normal types). • Genetic Considerations: • Leads to higher percentages of desired genotypes. • Effectiveness is a function of h2of trait on a single plant basis. • Improving h2 will improve gain.

27. Advantages Rapid, inexpensive procedure for increasing frequency of desired genes. Can repeat over years until no more progress seems apparent. Disadvantages Can only be used in environment where character is expressed. Prevents use of off-season nurseries. Limited value for low h2 traits (like yield).

28. Additional thoughtsMass selection is distinguished from other forms of selection by two criteria:1. Mass selection is the selection of individuals rather than families.2. It is based on the performance of the individual in that generation, not on a progeny test. Mass Selection

29. History:1. Natural selection: removes less fit individuals.2. Leaming, 1825, began selecting the best ears in a field of OP corn and improved the Leaming variety.3. Pearl, 1907, conducted mass selection for egg production in chickens.Overall, the results were mixed. This led to the conclusion that mass selection was not very successful with low heritability traits, i.e. those in which the environmental effect was large.

30. In a plant breeding context, we think of two forms of mass selection:1. Selection of pre-flowering traits, followed by intermating the selected individuals that same season.2. Selection on an individual plant basis of top performing females (in an cross pollinated crop).3. Selection on an individual plant basis, no intermating (eg in self pollinated crops)

31. The best-known example of mass selection is the work of C. O. Gardner (Crop Sci. 1: 241-245). He began with the OP variety Hays Golden. To begin with, Gardner determined yield from each plant at 10% moisture. He then bulked 20 seeds from each selected plant to provide seed for the next cycle of selection.

32. In 1956 he imposed a grid system on his selection scheme: what was the motivation for doing this? Micro environmental variation was impeding his selection response. He planted 40 plants per cell and selected the top 4 yield plants in each cell, to reduce problems associated with soil variability, etc. Equal numbers of seeds from each cell were bulked for the next cycle of selection.

33. Selection Response

34. The C0 was Hays Golden. In order to evaluate selection response, new seed from each cycle of selection would have to be generated and the experiment would be grown in replicated plots at several locations. To determine actual selection response, the usual approach is to regress cycle means on cycle numbers, and the linear regression coefficient is used as the estimate of selection response.

35. Selection Response over 15 Cycles of Selection % of ck. 150 C0 C5 C15

36. Modified Bulk System • Also known as mass selection • Grow the segregating bulk • Harvest seed from superior individuals • Combine or “bulk” the harvested seed • This bulk constitutes the population for the next year, or next cycle of selection

37. Modified Bulk System • Very simple to implement • Has an excellent track record (dating back to Neolithic times!) • Combines selection and inbreeding • May be difficult to perceive superior individuals in a heterogeneous population

38. Modified Bulk System • Does not work well with low heritabililty traits • Does allow the breeder to shape the population while inbreeding

39. UK Modified Bulk System • F2: select bright clean heads from early, short disease free plants; thresh in bulk • F3: same procedure • F4: select bright clean heads from early, short disease free plants; keep each head separate and thresh into headrow trays • F5: plant F4:5 headrows; harvest superior headrows separately • F6: test F4:6 lines in single rep trials at 2-3 locations