Plant Breeding 3

Plant Breeding 3

Which breeding method requires the least record keeping?

Bulk method requires minimal documentation because seeds from entire F2-F5 populations are harvested together as bulk composite without labeling individual plants or rows. No pedigree tracking, no single plant numbering, and no progeny row maps are maintained during advancing generations; only final bulk lot identity is preserved with year and generation notation. Natural selection acts, and population evolves passively under environmental pressure. Labor for note-taking, tagging, and threshing individual plants is eliminated, and large populations exceeding 10,000 plants can be grown in high-

Ref: Suneson CA 1949 – Bulk method low record needs; Acquaah Chapter 11 comparison of methods

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Plant Breeding 3

Which method is best to handle segregating generations efficiently?

Efficiency in managing large segregating populations depends on ability to track ancestry, evaluate plant-to-row relationships, and apply selection with known pedigree. Pedigree breeding systematically documents parentage of each selected plant from F2 onward, maintaining detailed field diaries, numbering systems, and performance scores. This structured documentation enables breeder to handle 2500-3000 F2 plants as individually identifiable families, cull inferior families based on ancestral performance, and trace origin of superior recombinants for future crossing. Space-planted pedigree rows

Ref: Poehlman JM & Sleper DA. Breeding Field Crops, Chapter Pedigree handling. Allard RW. Principles of Plant Breeding

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Plant Breeding 3

A limitation of clonal selection is:

Vegetatively propagated clones maintain narrow genetic base because all individuals within variety are genetically identical, carrying same chromosomes and alleles fixed by mitosis. Uniformity, while commercially desirable, creates vulnerability to evolving pathogen races, insect biotypes, and environmental extremes, leading to boom-bust cycles such as potato late blight in Ireland. Lack of sexual recombination restricts generation of new variability for adaptation, slows response to climate change, and limited buffering through genotype mixtures reduces ecological resilience. Seed storage and

Ref: Esnault et al. Challenges of clonal crops; FAO report on clonal genetic vulnerability

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Plant Breeding 3

An advantage of clonal selection is:

Since clonal propagation involves mitotic division copying entire genome without meiosis, heterozygous advantage, epistatic interactions, and allelic combinations that produce superior performance are perpetuated unchanged across commercial plantations. Once elite genotype is identified from seedling population or mutation screening, it can be multiplied rapidly via cuttings, grafts, or in vitro micropropagation to produce millions of identical plants within year, bypassing slow seed multiplication and avoiding segregation that would break favorable gene complexes. This captures non-additive g

Ref: Bradshaw JE. Potato clonal selection advantages. Acquaah Chapter 18 – fixing heterozygous genotype

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Plant Breeding 3

Clonal degeneration is mainly due to:

Clonal degeneration is progressive decline in vigor, yield, and quality of a clone over successive vegetative generations despite genetic identity maintained by mitosis. Major cause is systemic accumulation of viruses, phytoplasmas, and viroids in phloem and meristems, transmitted mechanically through propagules because sexual filter absent that normally excludes many pathogens. Potato seed tubers accumulate Potato Virus Y and Leafroll Virus, sugarcane accumulates mosaic virus and ratoon stunting disease, banana accumulates Bunchy Top Virus, strawberries accumulate mottle viruses. Mixed infect

Ref: Loebenstein G & Katis NI. Plant Virus Diseases in clonal crops; Faccioli G. Potato degeneration review

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Plant Breeding 3

Genetic variation in clonal crops arises due to:

In clonally propagated crops meiosis is bypassed, so new genetic variation cannot arise through recombination or independent assortment each generation as in seed crops. Primary sources are somatic mutations in meristematic cells, known as bud sports, and chromosomal aberrations such as polyploidization, aneuploidy, and translocations occurring during mitosis. Transposable element activation, epigenetic changes like methylation shifts, and chimeral segregation also generate variation in long-lived clones. Mutations accumulate over vegetative cycles because they are not filtered by gametophytic

Ref: Dodds JH. Somaclonal variation in clonal crops; Larkin & Scowcroft 1981 Theor Appl Genet

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Plant Breeding 3

Clonal crops propagate mainly by:

Clonal crops rely predominantly on asexual propagation bypassing sexual cycle: stem cuttings in sugarcane and grape, tubers and eyes in potato, rhizomes in ginger and banana, stolons in strawberry, bulbs in garlic and onion, grafts and budding in mango and apple, and tissue culture micropropagation in banana and ornamentals. Sexual reproduction is rare, irregular, or produces offspring that deviate from mother type due to high heterozygosity and segregation, losing commercial quality traits governed by complex epistasis. Vegetative organs contain somatic meristems capable of organized growth,

Ref: Hartmann HT & Kester DE. Plant Propagation Principles, Chapter cloning. George EF et al. Micropropagation

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Plant Breeding 3

Clonal selection is applicable to:

Clonal selection targets crops propagated vegetatively where genotype is conserved intact through mitosis without meiosis or segregation. Examples include potato propagated via tubers, sugarcane via stem cuttings, banana via suckers, sweet potato via vine cuttings, and fruit trees via grafting and budding. These species are highly heterozygous, often polyploid, and suffer severe inbreeding depression if selfed, making seed propagation undesirable and genetically unstable. Superior clone arises via natural mutation, introduction, or hybridization followed by segregation in seedling population w

Ref: Acquaah Chapter 18 Vegetatively propagated crops. Hartman & Kester Plant Propagation – clonal selection principles

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Plant Breeding 3

Progeny testing is essential in recurrent selection because:

Phenotype of a single cross-pollinated plant poorly reflects its transmitting ability because heterozygosity, dominance, and micro-environment mask genotype. Superior appearance may result from favorable interlocus interactions or non-heritable environmental boost, not additive alleles that can be passed to progeny. Progeny testing evaluates half-sib or S1 families derived from selected plant in replicated trials, providing estimate of breeding value, general combining ability, and heritability. In recurrent selection, selected S1 plants are selfed and testcrossed to a common tester; progeny m

Ref: Lush JL. Animal Breeding Plans; Acquaah Chapter 13 – importance of progeny testing in recurrent selection

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Plant Breeding 3

The objective of recurrent selection is to:

Recurrent selection functions as a population improvement strategy aimed at slowly accumulating favorable alleles at multiple loci controlling polygenic traits. Each cycle selects individuals or families with superior phenotypic value or good combining ability, then recombines them to break repulsion linkages and generate new genotypic arrays. Over cycles, frequency of positive additive alleles rises, additive genetic variance converts to breeding value, and mean performance of population for traits like grain yield, kernel weight, and stress tolerance increases. Unlike pedigree breeding which

Ref: Comstock RE et al. 1949 Genetics – recurrent selection theory. Hallauer Quantitative Genetics

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Plant Breeding 3

Recurrent selection is mainly practiced in:

Recurrent selection was conceptualized by Hayes and Garber for improving cross-pollinated populations where individuals are heterozygous and open-pollinated progeny expose combining ability and additive effects. In maize, brassicas, and alfalfa, gamete pool is shared through random mating, allowing allele frequencies to be altered gradually without immediate fixation. Recurrent cycles involve selection of superior individuals based on phenotype or testcross performance, intercrossing selected individuals in isolation to form new population for next cycle, increasing frequency of favorable addi

Ref: Hallauer AR & Darrah LL. Recurrent selection in maize; Sprague GF. Iowa St. J. Sci.

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Plant Breeding 3

A disadvantage of mass selection is:

Retaining many parental plants without progeny testing yields genetically heterogeneous population where individual plants differ in maturity, height, and quality, reducing market acceptability and certification standards. Seed lot lacks uniformity required for mechanized harvesting, DUS testing, and uniform product quality demanded by modern industry. Phenotypic selection confounds environment with genotype, so inferior heterozygous plants with favorable micro-environment may be selected, limiting genetic advance, especially for low heritability traits like yield where h² below 0.2. Without c

Ref: Allard RW. Chapter 8 – Limitations of mass selection. Poehlman 1995: uniformity issues

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