Mass Selection in Plants

Mass Selection

• A large number of phenotypically similar plants are selected, and their seeds are mixed to create a new variety.
• The selected plants generally do not undergo progeny testing.
• The resulting population contains a mixture of similar pure lines, leading to considerable genetic variation.
• Further mass or pure line selection can be implemented on this variety at a later stage.

Procedure of Mass Selection

• First year involves selecting a large number of phenotypically similar plants with desired traits like vigor, plant type, and disease resistance.
• Seeds from selected plants are combined to form the next generation.
• Second year involves planting the composited seed in a preliminary yield trial alongside standard varieties as a control.
• Including the original variety as a check helps determine if improvement has occurred through selection.
• The third through fifth years consist of a coordinated yield trial.
• In the sixth year, if the variety proves promising, it is recommended for release by the competent variety release committee.

Applications of Mass Selection

• It is used to purify existing pure-line varieties, as pure lines can become variable over time.
• Regular mass selection helps maintain purity.
• It is used to improve desi/local varieties of Self Pollinated Crops (SPC).
• Local varieties are often mixtures of several pure lines with varying characteristics.
• Eliminating less productive plant types can enhance the performance and uniformity of the original variety.
• Mass selection can improve local varieties without sacrificing their adaptability and stability.
• The new variety retains a composite of several superior pure lines from the original population.

Merits of Mass Selection

• It preserves the adaptation of the original variety.
• It retains considerable genetic variability, making future selection effective.
• It is conducted on homozygous and heterogeneous populations without crossing, thus reducing the need for extensive yield trials, saving time and money.
• It is less demanding, freeing up time for breeders to focus on other breeding programs.

Demerits of Mass Selection

• Varieties developed show variation and are not as uniform as pure-line varieties.
• Improvements are generally smaller compared to those achieved through pure line selection.
• Without a progeny test, determining if selected plants are homozygous is not possible.
• Cross-pollination can occur even in SPC, and some selected plants may be heterozygous.
• It can be difficult to ascertain whether phenotypic superiority is due to genetics or environment.
• Varieties developed are more challenging to identify compared to pure-line varieties in seed certification programs.
• It utilizes existing variability rather than generating new variability.

Hybridization

• Hybridization involves crossing two plants or lines with dissimilar genotypes.
• Successful hybridization requires preventing selfing (self-pollination) and unintended cross-pollination in female flowers through emasculation and bagging.
• Pollination must be done using selected male parents, which involves hand pollination and bagging of the male flower.
• The primary goal is to create genetic variation, which is influenced by the number of differing genes in the parents of the F1 generation.

Objectives of Hybridization

• Combination breeding involves transferring one or more oligogenic or polygenic traits into a single variety from different sources.
• It aims to increase yield by addressing weaknesses like tiller number, disease resistance, or grain/spike characteristics.
• Backcross and pedigree methods are useful for combination breeding.
• The parents’ genetic divergence is not as crucial as the high-intensity transfer of the desired trait.
• The intensity of the trait in the new variety may be similar to or lower than the source variety.
• Transgressive breeding aims to improve yield or contributing traits through transgressive segregation.
• Transgressive segregation is the appearance of individuals in the F2 or later generations that outperform both parents.
• It is caused by the accumulation of favorable genes through recombination of both parents.
• Pedigree methods, their modifications, and population approaches are suitable for high recovery of transgressive segregants.
• Unlike combination breeding, the genetic divergence between parents, each contributing favorable genes, is a prime consideration.
• As a result, the intensity of the trait is greater than both parents.
• Using hybrid varieties exploits that the F1 generation between two pure lines can be more vigorous than their parental lines.
• Hybrid varieties are commercially feasible.
• Genetic divergence and combining ability are key factors for successful hybrid varieties.

Types of Hybridization

• Based on the taxonomic relation between the cross parents you may have:
  • Intervarietal
  • Distant

Intervarietal Hybridization

• Intervarietal hybridization occurs between two strains, varieties, or races of the same species and is commonly called intraspecific hybridization.
• Simple or complex crosses can be performed, depending on the parents involved.
• Simple/single crosses involve crossing two parents to produce the F1 generation (e.g., A × B → F1 [A × B]).
• Complex/convergent crosses involve more than two parents to produce the F1 hybrid. Three-way crosses involve crossing the F1 generation with a third parent (e.g., F1 [A × B] × C → F1 [(A × B) × C]).
• Double crosses are when two F1 generations are crossed together (e.g., F1 [A × B] × F1 [C × D] → F1 [(A × B) × (C × D)]).

Distant Hybridization

• Distant hybridization involves crossing different species within the same or different genera
• Interspecific/intrageneric hybridization is when two species of the same genus are hybridized.
• Intergeneric hybridization occurs when two species belong to different genera.
• The goal is to transfer simply inherited traits, such as disease or stress resistance.
• It is likely to become more common to rectify specific issues in crops, driven by consumer demand for uniformity and reduced genetic erosion.

Handling Segregating Generations

• Hybridization is relatively straightforward, but managing segregating generations is challenging for crop improvement.
• The F2 and subsequent generations can consist of thousands of plants.
• Resources, labor, and land limitations affect the ability to raise large segregating generations from multiple crosses.
• Breeders need to select desirable plant types within these generations using scientific principles.
• Increasing the number of crosses is preferable to increasing the population size of each cross.

Pedigree Method

• The pedigree method involves selecting individual plants from the F2 generation onwards. Their progenies are grown, and pedigree, is maintained.
• It was first outlined by Love in 1927.

Pedigree Method Procedure

• Hybridization involves crossing selected parents to produce single or complex crosses.
• In the F1 generation, seeds are space-planted to maximize seed production.
• In the F2 generation, seeds are space-planted to facilitate selection, with a selection intensity of 1-10%.
• When crossing closely related varieties, fewer F2 individuals are selected compared to unrelated parents.
• For breeding quantitative traits, a larger number of F2 plants are selected.
• Selection in F2 is based on simply inherited traits.
• In the F3-F5 generations, individual plant progenies are space-planted and selected from within and between progeny rows.
• If multiple similar progenies come from the same row, only one is retained.
• In the F5 generation, the focus shifts to selection between progeny rows.
• In the F6 generation, selected progenies are planted in multi-row plots for visual comparison.
• Superior progenies are bulk-harvested to indicate they have high homozygosity, while segregating progenies are discarded individually.
• In F7, preliminary yield trials (PYT) with three replications are conducted with a standard check.
• Progenies are evaluated for plant height, lodging, disease resistance, flowering and maturity dates and yield with respect to the check.
• Outstanding superior lines are advanced to coordinated yield trials.
• In F8-F10, superior lines are tested in replicated yield trials at multiple locations.
• Evaluations include plant height, lodging, disease resistance, flowering and maturity dates yield and quality.
• A line superior to the commercial variety will be identified for release as new variety.
• In F11, when likely to be released, seed is multiplied for distribution to farmers.

Pedigree Record

• Documentation of the relationship between selected plants and their progenies.
• Advantages include the ability to trace each progeny back to its F2 origin, identify shared alleles, and predict the allelic composition of the final population.
• Two systems of maintenance are:
  • Pedigree based on location of progeny row in the field
  • Pedigree based on serial number of selected plants

Location based pedigree

• Each cross is given a number consisting of the year and serial number of the cross, such as 7911
• Plant progeny row in F3 and latter generations are assigned a row number based on its location in the plot
• E.g. F3 (7911-7) = Progeny in the 7th row in F3 plot
• Progeny in F4 and latter generations are identified by the row number of current generation and of the previous generation.
• E.g. 7911-7-4 is from the fourth row in the F4 row, selected from the F3 progeny in row seven.

Serial number based pedigree

• Each cross is given a number consisting of the year and serial number of the cross, such as 7911
• Selected individuals in F3 are given a serial number, and individuals in subsequent generations are given the serial number of previous generation.
  • E. g. F3 (7911-7) is progeny obtained from plat number 7 selected in F2
• Each progeny can be tracked back to its originating F2 individual without previous records.
  • E. g., F4 (7911-7-4) is progeny from plant 4, selected from F3 progeny derived from plant 7 selected in F2.
• The pedigree progenies are provided with a serial number and included in preliminary yield trials.

Applications of the Pedigree Method

• Most common method for selection from cross segregating generations in SPC
• Effective use in combination and transgressive breeding

Merits of Pedigree Method

• Maximizes the opportunity for breeders to use skills to select desirable plants from segregating generations.
• Transgressive segregants can be identified to improve yield and other quantitative traits.
• Breeders can obtain information about the inheritance of qualitative traits with pedigree record.
• Plant progenies are eliminated early if they exhibit visible defects/weaknesses.

Demerits of Pedigree Method

• Maintaining pedigree records is tedious and a consuming job.
• Labour-intensive selection limits the crosses a breeder can handle.
• Success of this method is on the breeder’s skill.
• No opportunity is provided for natural selection.
• Selection for yield in F2/F3 is ineffective, and valuable genotypes can be lost if care is not taken.

Bulk Method

• The F2 and subsequent generations are harvested in bulk until homozygosity is attained or a favorable environment for selection is encountered.
• Individual plants are selected and evaluated.

Bulk Method Procedure

• Cross selected parents to produce single or complex crosses.
• The F1 generation is space-planted and harvest in bulk.
• For F2 – F6 generations, planting is in commercial rates/spacings and harvest is in bulk.
• During bulking, natural selection alters genotypic frequencies without artificial selection being applied.
• For the F7 generation, space plant and harvest seeds from phenotypically superior plants separately.
• For the F8 generation, grow plant progenies in single/multiple rows and bulk harvest superior progenies.
• For the F9 generation, a PYT is conducted with standard commercial checks to rate plant height, lodging, disease resistance (such as flowering and maturity date), yield and quality to determine if 2-5 stand-out lines will be moved into coordinated yield trials.
• For generations F10-F12, lines are tested in replicated locations, and performance data is taken for plant height, lodging, disease resistance, and yield of plant. One stand-out is chosen for new variety.
• For the F13 strain likely released and multiplied

Applications of the Bulk Method

• Method has three prominent applications:
  • Isolation of homozygous lines
  • Waiting for opportunity for isolation
  • Enabling natural selection to alter genetic composition
• Homozygous lines are isolated using minimum amount of expense and effort, by starting plant selection with end of buking period. The PYT will occur in the second year following the end of the bulking period.

Waiting For Opportunity for Selection

• This helps by bulking segregating generation, if there is no suitable place for cultivation until things improve
• Harlan called this method mass-pedigree of method

Natural Selection to Alter Genetic Composition of Population

• Opportunity for natural selection acts on genetic composition via long-bulking generation
• Natural selection favours high yielding and eliminates those that do not
• From a population with a long period of bulk, expect better isolation lines
• Evolutionary method of breeding is what Suneson terms bulk methods, as natural selection is given the oppurtunity
• It changes genetic composition of population

Merits of Bulk Method

• It assists in natural selection to improve genotypes
• Greater chance of isolating transgressive segregants as the population is better
• Individual plant selection completed after reaching homozygosity allows for effective selection for quantitative traits
• Suited for small grain crops for their high number
• Suitable for study to see how genes and genotype survuve
• Simple, cheap and convenient to allow breeder for other projects

Demerits of Bulk Method

• Only after F10 to do natural selection better which can require 20 seasons
• Limited for using heterozygousness and for short term isolation lines to have little effort on population

Single Seed Descent (SSD)

• SSD are bulk method modifications
• Single seed from plant F2 or subsequenet populations bulked to increase nezt population, selection is made out of progeny and superior progeny rows are harvest to conduct PYT
• Rapid advancement of cross generations help
• Helps gain RILs from selected cross and do so quickly
• It advances generation at maximal speed
• very little space, labour and effort is needed.
• The end populations display random genotypes for homozygous

Disadvantages of SSD

• Permits no kind of selection be used while segregating
• Each new generation will be less than prior, which is due to factor like dying off, bad germination

Backcross Method

• A cross between parents is knows as backcross, in short the hybrid and progreny will get continually crossed between each other.

Requirements for backcross

• Needed for programme of:
  • Recurrent for using popualr breed without characters
  • Donor parent

Backcross method (Dominant Gene transfer)

• Hybrid A will cross with B

Limitations of Backcross Method

• Some variety not be good with recurrent

Hardy-Weinberg Law

• G.H. Hardy and W. Weinberg both discovered a method that allows to estimate genetopye numbers out of genes if population is random
• It states it will remina constant, and there is no change in selection - mutation migration and drife, Hardy Weinberg Law will maintaing constency over change

Derivation

• The basic is how gamete form, and create 3 classes Genotypes - AA, Aa, aa
• Where gene will be identified, those are proporption alleles A or a

First Generation of Random Mating

• Gene number will show

Limitations to Hardy Weinburg Law

• Breeding groups can mainin to be limited
• The evolution is primary application

Element That Effect Hardy-Weinburg

• This includes selection, mutation, mating.
• It may be a gene change in genetic drift
• Or an allele from breed
• Inbreeding will occur in group, mating may be random or non-favourable