Breeding Self-polinated crops

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Breeding Self-polinated crops


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1 Breeding Self Pollinated Crops GEETIKA


Cultivars 2 Cultivar Is a group of genetically similar plants, which may be identified (by some means) from other groups of genetically similar plants Essential Characteristics: Identity: cultivar must be distinguishable from other cultivars Reproducibility: the distinguishing characteristic(s) need to be reproduced in the progeny faithfully

Types of Cultivars:

Types of Cultivars 3 Open-Pollinated cultivars O.P. seeds are a result of either natural or human selection for specific traits which are then reselected in every crop. The seed is kept true to type through selection and isolation; the flowers of open-pollinated or O.P. seed varieties are pollinated by bees or wind.

Types of Cultivars:

Types of Cultivars 4 Synthetic cultivars A population developed by inter-crossing a set of good combiner inbred lines with subsequent maintenance through open-pollination. The components of synthetics are inbreds or clones so the cultivar can be periodically reconstituted.

Types of Cultivars:

Types of Cultivars 5 Multi-line cultivars A mixture of isolines each of which is different for a single gene controlling different forms of the same character (e.g., for different races of pathogens) F1 cultivars The first generation of offspring from a cross of genetically different plants Pure-line cultivars The progeny of a single homozygous individual produced through self-pollination

Cultivars and Self-pollinated Crops:

Cultivars and Self-pollinated Crops 6 In self-pollinated species: Homozygous loci will remain homozygous following self-pollination Heterozygous loci will segregate producing half homozygous progeny and half heterozygous progeny Plants selected from mixed populations after 5-8 self generations will normally have reached a practical level of homozygosity


7 In general, a mixed population of self-pollinated plants is composed of plants with different homozygous genotypes (i.e., a heterogeneous population of homozygotes If single plants are selected from this population and seed increased, each plant will produce a ‘pure’ population, but each population will be different, based on the parental selection Cultivars and Self-pollinated Crops


8 Selection involves the ID and propagation of individual genotypes from a land race population, or following designed hybridizations Genetic variation must be identified and distinguished from environment-based variation Selection procedures practiced in mixed populations of self-pollinated crops can be divided into two selection procedures Breeding Self-pollinated Crops


9 Breeding Methods of Self Pollinated Crops Pure line Mass Bulk Pedigree Single Seed Descent ( modified pedigree) Backcross


10 Pure Line


11 Pure Line: (Recount Johannsen. 1903) usually no hybridization Initial parents (IPs) selected from a heterogenous population (i.e. genetically variable) procedure continues until homogeneity is achieved last phase is field testing

Pure-line Selection:

Pure-line Selection 12 A pure line consists of progeny descended solely by self-pollination from a single homozygous plant Pure line selection is therefore a procedure for isolating pure line(s) from a mixed population

Pure-line Selection:

Pure-line Selection 13 Pure line cultivars are more uniform than cultivars developed through mass selection (by definition, a pure line cultivar will be composed of plants with a single genotype) Progeny testing is an essential component of pure line selection Improvement using pure line breeding is limited to the isolation of the ‘best’ genotypes present in the mixed population

Pure-line Selection:

Pure-line Selection 14 More effective than MS in development of self-pollinated cultivars However, leads to rapid depletion of genetic variation Genetic variability can be managed through directed cross hybridizations Essential to progeny test selections

Pure-line Selection-Steps:

Pure-line Selection-Steps 15 Select desirable plants Number depends on variation of original population, space and resources for following year progeny tests Selecting too few plants may risk losing superior genetic variation A genotype missed early is lost forever Seed from each selection is harvested individually

Pure-line Selection-Steps:

Pure-line Selection-Steps 16 Single plant progeny rows grown out Evaluate for desirable traits and uniformity Should use severe selection criteria (rogue out all poor, unpromising and variable progenies) Selected progenies are harvested individually In subsequent years, run replicated yield trials with selection of highest yielding plants After 4-6 rounds, highest yielding plant is put forward as a new cultivar


Advantages 17 ID of best pure line reflects maximum genetic advance from a variable population; no ‘poor’ plants maintained Higher degree of uniformity Selection based on progeny performance is effective for characters with relatively low h 2


Disadvantages 18 Requires relatively more time, space, and resources for progeny testing than MS to develop new cultivar High degree of genetic uniformity; more genetically vulnerable and less adaptable to fluctuating environments ID and multiplication of one outstanding pure-line depletes available genetic variation; leads to fast genetic erosion


19 How long will a cultivar remain pure? As long as the commercial life of the cultivar, unless: Seed becomes contaminated with seed from other sources (e.g. from harvesting and seed cleaning equipment) Natural out-crossing occurs (amount varies by species but seldom exceeds 1-2% in self-pollinated crops) Mutations occur To maintain purity, off-types arising from mutation or out-crossing must be rogued out


20 Mass Selection


21 May or may not include hybridization Make IP selections based on single, ideal or desirable phenotype and BULK seed May repeat or go directly to performance testing Mass Selection has 2 important functions: Rapid improvement in land-race or mixed cultivars Maintenance of existing cultivars (sometimes purification) * Many pb’ers of self pollinated crops believe that combining closely related pure lines imparts “genetic flexibility” or buffering capacity and so are careful to eliminate only obvious off types Mass Selection


22 Success depends on extent of variation and h 2 of the traits of interest Land races make an ideal starting source High genetic variability accumulated over generations of mutation and natural hybridization

Mass Selection:

Mass Selection 23 Initial selection Can be either a positive or a negative selection Negative screening: A screening technique designed to identify and eliminate the least desirable plants. positive screening: which involves identifying and preserving the most desirable plants.

Mass Selection - 1st Year:

Mass Selection - 1 st Year 24 Select plants with respect to height, maturity, grain size, and any other traits that have ‘production’ or ‘acceptability’ issues Bulk seed (may ‘block’ these bulks if wide variation is present for certain traits; e.g. height) May be able to use machines to select Harvest only tall plants, or save only large seed passed through a sieve

Mass Selection - 2nd Year:

Mass Selection - 2 nd Year 25 MS really only takes 1 yr because selected seed represents a mixture of only the superior pure lines that existed in the original population However, additional rounds of selection and bulking will allow for evaluation under different environments, disease and pest pressures. Also, multiple years will allow you to compare performance with established cultivars over years and environments.


26 Objectives of Mass Selection: To increase the frequency of superior genotypes from a genetically variable population Purify a mixed population with differing phenotypes Develop a new cultivar by improving the average performance of the population


Disadvantages 27 Selection based on phenotypic performance; not effective with low h 2 traits Without progeny testing, heterozygotes can be inadvertently selected Population cannot realize maximum potential displayed by the ‘best’ pure line, due to bulking Final population is not as uniform as those developed through pure-line selection

Mass selection vs pure line selection:

Mass selection vs pure line selection 28 Line mixture Bulk of phenotypically similar plants Cultivar register and marketing Single plant offsprings L1 L2 L3……. LN Register and market the best pure lines Mass selection Pure line selection Heterogenous cultivars Homogenous cultivars Line mixture Bulk of phenotypically similar plants Cultivar register and marketing Single plant offsprings L1 L2 L3……. LN Register and market the best pure lines Mass selection Pure line selection Heterogenous cultivars Homogenous cultivars Line mixture Bulk of phenotypically similar plants Cultivar register and marketing Single plant offsprings L1 L2 L3……. LN Register and market the best pure lines Mass selection Pure line selection Heterogenous cultivars Homogenous cultivars


29 Bulk Method


30 Bulk Inbreed in bulk to have homozygous lines Select superior lines after F6 Crosses with no high heritability traits segregating


31 Natural selection changes gene freq. via natural survival Breeder may assist nature and discard obviously poor types Relieves breeder of most record keeping Most of us treat bulks with extremely low inputs and low expectations. Points to consider in Bulk Method


32 The bulk method is a procedure for inbreeding a segregating population until a desired level of homozygosity is reached. Seed used to grow each selfed generation is a sample of the seed harvested in bulk from the previous generation. In the bulk method, seeds harvested in the F 1 through F 4 generations are bulked without selection; selection is delayed until advanced generations (F 5 -F 8 ). By this time, most segregation has stopped.


33 Advantages Less record keeping than pedigree Inexpensive Easy to handle more crosses Natural selection is primarily for competitive ability More useful than pedigree method with lower h 2 traits Large numbers of genotypes can be maintained Works well with unadapted germplasm Can be carried on for many years with little effort by the breeder


34 Environmental changes from season to season so adaptive advantages shift Most grow bulk seed lots in area of adaptation Less efficient than pedigree method on highly heritable traits (because can purge non-selections in early generations) Not useful in selecting plant types at a competitive disadvantage (dwarf types) Final genotypes may be able to withstand environmental stress, but may not be highest yielding If used with a cross pollinated species, inbreeding depression may be a problem Disadvantages


35 Pedigree Method


36 Most popular Essentially a plant to row system to develop near pure lines Followed by performance testing of resulting strains This method and its variants require a lot of record keeping Pedigree Method


37 Pedigree Selection during inbreeding Early generations: High heritability traits Late generations: low heritability traits


38 Genetic Considerations: Additive genetic variability decreases within lines and increases among lines, assuming no selection recall the movement toward homozygosity following the hybridization of unlike and homozygous parents Dominant genetic variability complicates pedigree selection homozygous and heterozygous individuals look alike and therefore you may continually select the heterozygote THUS, selection can be discontinued with phenotypic uniformity within a line is obtained


39 Advantages Eliminates unpromising material at early stages; Multi-year records allow good overview of inheritance, and more effective selection through trials in different environments; Multiple families (from different F 2 individuals) are maintained yielding different gene combinations with common phenotype Allows for comparison to other breeding strategies


40 Disadvantages Most labor, time and resource intensive method; usually compromise between # crosses and population sizes; Very dependent on skill of breeder in recognizing promising material; Not very effective with low h 2 traits; Slow; can usually put through only one generation per year, and the right environmental conditions must be at hand for accurate selection. Upper ceiling set by allelic contents of F 2 ; can not purge selections of undesirable alleles once ‘fixed’.


41 Single Seed Descent


42 Single Seed Descent Inbreed with one seed from each plant in each generation Select superior line after F6 Crosses with no high heritability traits segregating


43 Advantages Rapid generation advance; 2-4 generations/yr Requires less space,time and resources in early stages, therefore accommodates higher # crosses; Superior to bulk/mass selection if the desired genotype is at a competitive disadvantage; natural selection usually has little impact on population. Delayed selection eliminated confusing effects of heterozygosity; more effective than pedigree breeding when dealing with low h 2 traits; Highly amenable to modifications and can be combined with any method of selection.


44 Disadvantages May carry inferior material forward Fewer field evaluations, so you lose the advantage of natural selection Need appropriate facilities to allow controlled environment manipulation of plants for rapid seed production cycles (day length, moisture and nutrient control)


45 Backcross


46 Same form whether self or cross pollinated species Only difference is pollination control With backcross we approach homozygosity at the same rate as with selfing Goal is to move 1 to a few traits from a donor parent (deficient in other traits) to a recurrent parent (deficient in the trait of interest) Backcross




48 Limited use of BC to create a population for selection that fosters wider genetic variance and modest introgression is a separate issue than a repeated BC to derive a new cultivar Jensen suggested that a 3-way (a backcross to another recurrent or superior parent following he single cross of a desirable and an undesirable parent) was superior to single cross followed by pedigree or other selection methodology Backcross


49 BC must be used with other, more exploratory procedures; otherwise G s =0 Must have a suitable recurrent parent # of BCs to make? usually 4 Use several RP plants! WHY? To incorporate > 1 trait, use parallel programs and then converge Evaluation phase can be less stringent because you should already know the utility of the recurrent parent! Backcross


50 Backcross Breeding Recovery of the recurrent parent genotype follows this pattern: % recurrent % donor F 1 50 50 BC 1 75 25 BC 2 87.5 12.5 BC 3 93.7 6.3 BC 4 96.9 3.1 BC m 1-(1/2) m+1 (1/2) m+1



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