#animal breeding

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 additive alleles while maintaining variability for future gains. Self-pollinated crops are homozygous, so recurrent recombination requires manual crossing, making scheme cumbersome and less efficient. Cross-pollinated crops benefit because heterosis and inbreeding depression are managed through outcrossing, and population improvement can precede extraction of inbred lines. Thus it remains core method for open-pollinated variety and base population enrichment before inbred extraction. Reciprocal recurrent selection extends concept to improve both populations simultaneously for cross performance, exploiting overdominance and epistasis to maximize heterosis in hybrid breeding programs where combining ability and allelic complementation are critical for commercial hybrid development.

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 controlled pollination, pollen from undesirable off-types contaminates selected individuals, diluting gains through random mating with poor types. Variability also complicates distinctness, uniformity, stability testing for variety release. In self-pollinated crops, mass selection quickly plateaus, and response to selection declines after few cycles because additive variance exhausts without deliberate recombination. Hence breeders shift to pedigree or progeny-based methods for uniformity and precise fixation. Statistical approaches like family-based selection, half-sib progeny testing, and use of augmented designs can partially overcome uniformity issues by incorporating pedigree information and spatial correction, but add complexity and cost compared to simple phenotypic mass selection in pure line crops.

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

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Plant Breeding 3

A major advantage of mass selection is:

Because mass selection avoids intensive inbreeding and retains many selected parents bulked together, resulting population preserves substantial genetic diversity and heterozygosity buffer. Broad base of alleles confers buffering against diverse environmental stresses, disease races, and climatic fluctuations, granting stable performance across years and locations. Unlike pure lines that may fail in off-type environments, mass-selected varieties contain mixture of genotypes complementary in stress tolerance, maintaining yield stability due to population buffering mechanism described by Allard. Historically, landraces developed by farmer mass selection in centers of diversity exhibit wide adaptation and resilience to marginal soils. Method also conserves rare minor genes and polygenic complexes underlying local adaptation. Low cost, no need for controlled pollination, and maintenance of variability make it attractive for low-input traditional systems and for initial improvement before advanced pedigree programs requiring purity. Heterogeneous populations also exhibit compensatory growth where different genotypes fill gaps, maintain canopy closure, reduce weed invasion, and provide insurance against complete crop failure under unpredictable monsoon variability common in traditional farming systems and marginal environments.

Ref: Harlan JR. Crops and Man – landrace adaptation; Ceccarelli S. Participatory breeding for wide adaptation

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Plant Breeding 3

Mass selection is more effective in:

Effectiveness hinges on population structure and genetic variance. Cross-pollinated species such as maize, pearl millet, and sunflower maintain high heterozygosity, large additive genetic variance, and expose extensive segregation each generation due to allogamy and random mating mediated by wind or insects. Mass selection in such highly variable base populations can shift allele frequencies rapidly because selected individuals outcross, recombining favorable alleles through pollen mixing. In self-pollinated wheat or rice, landraces already consist of homozygous pure lines; selecting best phenotypes merely picks best existing line without creating new recombination, and variability rapidly depletes after few cycles. Moreover, self-pollinated crops suffer from inbreeding depression less relevant, but mass selection cannot break linkage blocks efficiently. Recurrent phenotypic mass selection therefore became foundation for open-pollinated variety improvement, leveraging pollination biology to remix genes each cycle and sustain response. Open-pollinated crops have high outcrossing rate maintained by self-incompatibility, male sterility, or protandry, ensuring gene flow each cycle and providing continuous recombination to assemble favorable epistatic combinations that mass selection can exploit effectively for population improvement.

Ref: Frey KJ. Iowa State – Mass selection in open-pollinated crops. Hallauer & Miranda Quantitative Genetics in Maize

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Plant Breeding 3

Mass selection is based primarily on:

Mass selection operates directly on visible phenotypic values of individual plants in a population. Breeder walks field, visually scores traits such as plant height, earliness, seed color, or disease-free appearance, and harvests seeds from best looking individuals to constitute next generation bulk. Underlying genotype is not tested through progeny rows, molecular markers, or combining ability assays. Success depends on high heritability where phenotype mirrors additive genetic value, correlation between observable character and breeding value, and low environmental noise. It is simplest breeding method practiced since domestication, requiring no controlled crosses or pedigree books. However, it cannot distinguish homozygous PP from heterozygous Pp if dominance conceals recessive allele, and is confounded by macro-environmental variation across field blocks. Modification with stratification or selection of many individuals and progeny testing improves its genetic gain for low heritability characters. Heritability and selection intensity determine response to mass selection; high heritability traits like plant height respond quickly, whereas low heritability yield traits require stratified mass selection or honeycomb designs to reduce environmental variance and improve accuracy.

Ref: Gardner CO. Mass selection genetics; Acquaah Chapter 10 – Phenotypic selection basis

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Plant Breeding 3

Backcross breeding is commonly used for transfer of:

Backcrossing excels at transferring discrete, simply inherited traits governed by one or few genes where phenotype reliably identifies presence of donor allele. Disease resistance genes such as Sr2 for stem rust, Pi54 for blast, and Lr genes in wheat are typically dominant monogenic traits with major effect and high heritability, easily screened by inoculation in each backcross generation using pathogen isolates. Introgression of quantitative, polygenic traits like yield is inefficient because many loci would need simultaneous transfer, epistasis complicates selection, and linkage drag would impair elite background. Yield itself relies on hundreds of QTLs and strong G×E interaction, making backcross recovery less valuable. Therefore breeders employ backcross primarily for major gene resistances, male sterility, quality traits like waxy endosperm, and single-gene herbicide tolerance, often complementing with marker-assisted foreground selection using tightly linked markers to ensure gene presence. Typical protocol includes recurrent parent as susceptible elite cultivar needing improvement, donor as resistant landrace or wild relative, repeated inoculation and marker screening each backcross to maintain resistance allele while restoring agronomic superiority and quality characteristics.

Ref: Brar DS & Khush GS. Alien introgression in rice, Plant Molecular Biology 1997. Collard & Mackill 2008 MAS review

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