EGT and Mass Selection PLS 664 April 10, 2007

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

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 S 0:1 lines through many selfing generations, then crossed them to common tester Found that combining ability was already determined in S 0:1 lines

Self-Pollinated Crops Determine the generation for testing If it is to be the F 2, you will have to grow the F 1 in an environment which favors seed production A more common choice would be F 2:3 lines

Self-Pollinated Crops Harvest seed from individual F 2 plants Plant seeds in F 2:3 progeny rows Identify the superior rows Harvest all seed in each selected row in bulk Grow replicated tests of F 2:4 lines Grow replicated tests of F 2:5 lines

Self-Pollinated Crops Harvest selected F 5 plants individually Grow F 5:6 lines in headrows Test F 5 - derived lines extensively

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

Recall that there is all of the additive variance among F 2:3 lines and one-half of the additive variance within F 2:3 lines In later generations of F 2 derived lines, there is still all of the additive variance among lines, and considerable variance within lines, as inbreeding progresses 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?

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

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

Mass Selection : The Prologue The formation of a composite propagation stock by the 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 and therefore get tenure!! Today’s mass selection is performed mostly in concert with an established breeding program in a specific population.

Mass selection can be based upon any method a breeder can design to “accept or reject” plants in the population. It is only limited by the creativity of the researcher. Examples of selection criteria heightweightcolor texturechemical contentshippability storabilitytiming of maturitytaste!! # tillersshapedisease resistance “kick test” Of course, selection success is related to the single plant heritability estimate of each trait.

Modes of Implementation Method #1 = Selection of individuals within a heterogeneous population. Choosing favorable/desirable plants and harvesting the seed. Method #2 = Sampling seed from selected individuals to plant the next generation.

10 cm 1958: Plant out heterogeneous population of F 3 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. Collected seed will be composed of complete panicles, partial panicles, or no panicle. Mass Selection in Oat : Romero & Frey, 1966

1959 (F 4 ), 1960 (F 5 ), and 1961 (F 6 ):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

h 2 D h 2 I r g DI h 2 D = heritability of directly selected trait h 2 I = heritability of indirectly selected trait r g DI = genetic correlation between D&I traits Mass selection in soybean for maturity and calcareous soil tolerance : Fehr, unpublished Yellowing of genotypes 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 two examples have been of selections based on individual plants within the population.

Mass selection of seed size in soybeans : Fehr & Weber, : 4500 F 6 plants were planted and 400 plants of early maturity were selected. Top 1/4 of main stem and all branches were removed; remainder of plant was threshed together. Seed was passed over different sized sieves and 25% of the largest and smallest seeds were retained. 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 : 2300 F 7 seeds from each group were planted: 400 selected for harvest in same fashion. Repeat sizing and specific gravity tests.

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 : 2300 F 7 seeds from each group were planted: 400 selected for harvest in same fashion. Repeat sizing and specific gravity tests.

1965 : Process repeated with 1000 F 8 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.

Mass Selection : The Epilogue Selection can be applied to 1) individual plants, 2) seeds. Selection involved is 1) artificial selection, 2) naturalselection, 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).

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 h 2 of trait on a single plant basis. Improving h 2 will improve gain.

Double Edged Sword Appears Again Advantages Rapid, inexpensive procedure for increasing frequency of desired genotypes. Allard likes it because of the safety and rapidity with which mass selection can affect improvement in landraces. 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 h 2 traits.

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

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

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

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; 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