Plant Breeding 4

Plant Breeding 4

Main advantage of DH in breeding is:

Primary bottleneck in conventional line breeding is time required to attain homozygosity through repeated selfing, typically six generations to reach acceptable uniformity and distinctness for release. Doubled haploid technology converts heterozygous material into completely homozygous lines immediately, compressing 5-6 year inbreeding cycle into less than 18 months including haploid induction and doubling steps. Haploid inducer lines or anther culture produce haploid embryos, chromosome doubling yields instant true-breeding lines that can be multiplied for yield trials in next season with hig

Ref: Geiger HH & Gordillo GA. 2009 Plant Breeding Reviews – DH rapid homozygosity; Weyen 2008 J

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Plant Breeding 4

Ph1 gene in wheat ensures:

Pairing homoeologous1, Ph1 locus on long arm of chromosome 5B in hexaploid wheat Triticum aestivum encodes ZIP4-B2 protein that suppresses homoeologous chromosome pairing between related A, B, and D subgenomes, enforcing strictly homologous bivalent pairing during meiosis. Without Ph1, homoeologs from different subgenomes would synapse forming multivalents and translocations, causing sterility and aneuploidy, as seen in ph1 mutants where extensive homoeologous pairing occurs and multivalents observed cytologically. Ph1 promotes homolog recognition via telomere bouquet organization and restrict

Ref: Riley R & Chapman V 1958 Nature – Ph1 homologous pairing discovery; Griffiths et al. 2006 Nature cloning of Ph1

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Plant Breeding 4

Autotetraploids are generally:

Autotetraploids contain four homologous chromosome sets of same species genome, AAAA, derived from doubling of diploid AA genome via colchicine treatment that inhibits spindle formation. Presence of four allele copies increases cell size, nuclear volume, and organ dimensions termed gigas effect due to increased gene dosage and protein production per cell. Leaves become thicker, stomata larger, flowers bigger, fruits heavier, and biomass enhanced, contributing to increased vigor in forage crops like alfalfa and rye grass and ornamental flowers. Photosynthetic rate may rise due to larger chlorop

Ref: Randolph LF. 1932 Autopolyploid vigor; Stebbins GL. Gigas effect. Acquaah Autotetraploid breeding

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Plant Breeding 4

MAS is most effective when markers are:

Efficiency of marker-assisted selection hinges on recombination frequency between marker and causal gene influencing linkage disequilibrium persistence. If marker is loosely linked at 20 cM, crossover occurs in 20% gametes each generation, decoupling marker allele from desired trait allele, leading to false positive selections and loss of target and reduced genetic gain. Tightly linked markers situated within 1-2 cM or preferably gene-based intragenic markers such as functional SNPs in coding region show recombination below 1%, ensuring near-complete co-segregation with trait across breeding c

Ref: Michelmore RW. 1995 Molecular dissection – tight linkage. Collard & Mackill 2008 tightly linked effectiveness

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Plant Breeding 4

Gene pyramiding refers to:

Gene pyramiding, also called gene stacking, assembles multiple favorable alleles, often for disease or insect resistance, into single elite genotype to achieve broader spectrum, durable, and synergistic protection. Unlike single gene deployment that is vulnerable to pathogen race evolution breaking resistance within few seasons due to simple mutation, combining two to four unlinked major resistance genes forces pathogen to acquire multiple virulence mutations simultaneously, which is epidemiologically rare and fitness costly. Example includes stacking bacterial blight genes Xa4, xa5, xa13, Xa2

Ref: Sanchez AC et al. 2000 – Gene pyramiding multiple genes concept; Sundaram et al. Rice bacterial blight pyramiding

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Plant Breeding 4

True-breeding mapping populations include:

True-breeding or immortal mapping populations maintain homozygous, non-segregating genotype across generations so that same genotype can be phenotyped repeatedly in different environments and years. Examples include doubled haploid lines where chromosome doubling produces instant homozygosity, recombinant inbred lines after F6 selfing, and near-isogenic lines that differ only at introgressed segment. F2, BC1F1, and testcross populations are transient and heterozygous, segregating each generation, unsuitable for replicated phenotyping because genotype changes upon selfing and allele frequencies

Ref: Paterson AH et al. Mendelian mapping populations; Snape & Parker DH as true-breeding mapping resource 2007

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Plant Breeding 4

RILs are developed by:

Recombinant Inbred Lines are immortal homozygous mapping population developed by repeatedly selfing progeny from single cross through single seed descent without selection for six to eight generations. Starting from F2, one seed per plant is advanced randomly; heterozygosity halves each generation, approaching 99% homozygosity by F6, genotype fixed as mixture of parental alleles in homozygous form through recombination. Each RIL represents unique mosaic of parental chromosomes after multiple recombination events, capturing increased mapping resolution compared to F2 due to accumulated crossove

Ref: Burr & Burr 1991 RIL development; Singh & Singh Plant Breeding – SSD repeated selfing method

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Plant Breeding 4

Marker-assisted selection avoids:

Phenotypic selection suffers from confounding effects of environmental variation, microclimate heterogeneity, soil fertility gradients, and developmental stage, which obscure genetic differences especially for low heritability traits governed by many QTLs. DNA markers represent fixed sequence differences that are independent of external environment, plant age, and tissue, expressed constitutively in genome regardless of moisture or pathogen pressure. Therefore selection based on marker genotype is unaffected by seasonal fluctuations, field heterogeneity, or inoculum pressure that may cause esc

Ref: Tanksley et al. 1989; Xu Y. Molecular Plant Breeding – MAS avoids environmental influence

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Plant Breeding 4

QTLs are associated with:

Quantitative Trait Loci are genomic regions contributing to continuously varying traits that show distribution overlapping between phenotypes rather than discrete classes. Traits like grain yield, plant height, days to flowering, oil content, and drought tolerance are governed by many genes of small effect plus environmental modification, producing normal bell curve distribution in segregating population. Each QTL explains proportion of phenotypic variance, detected via statistical linkage between marker genotype and trait mean in mapping populations such as F2, RILs, or DH using interval mapp

Ref: Lynch M & Walsh B Quantitative Genetics; Tanksley SD 1993 – Mapping polygenes continuous traits – QTL Genetics

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Plant Breeding 4

Co-dominant markers are preferred in MAS because they:

Codominant markers reveal both alleles at locus simultaneously, displaying two distinct bands in heterozygote versus one band in each homozygote on gel or capillary electropherogram. This dosage information is invaluable in marker-assisted breeding because breeder can identify heterozygous carriers Aa that contain one copy of recessive or dominant donor allele while retaining recurrent background, and distinguish from homozygous donor AA that may exhibit drag penalty or lack recurrent adaptation. In backcrossing, BC progeny segregate 1:1 Aa:aa, and only heterozygous carrier advances, allowing

Ref: Collard BCY & Mackill DJ 2008 Marker-assisted breeding review: co-dominant heterozygote detection. Euphytica

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Plant Breeding 4

Background selection in MABC helps in:

Background selection accelerates recovery of recurrent parent genome beyond theoretical expectation using genome-wide molecular markers distributed across all chromosomes. While foreground selection tracks donor gene, background selection scans chromosomes unlinked to target using polymorphic SSR or SNP markers distinguishing recurrent and donor alleles by size or sequence. In each backcross generation, individuals carrying target gene are genotyped across background, and those with highest proportion of recurrent parent alleles are chosen as parents for next backcross. Conventional backcrossi

Ref: Frisch M et al. 1999 Crop Sci – background selection for RPG recovery; Ribaut JM MAS strategies

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Plant Breeding 4

Foreground selection in MABC focuses on:

Marker-assisted backcrossing comprises two selection layers that accelerate recovery and ensure gene retention. Foreground selection focuses directly on target locus introgressed from donor, using gene-specific or tightly linked markers less than 2 cM to ensure presence of desired allele in each backcross generation. In early generations BC1F1, BC2F1, individual seedlings genotyped for foreground markers, carriers retained, non-carriers discarded, avoiding waiting for phenotypic expression that may require disease inoculation or recessive homozygosity assessment. This prevents accidental loss

Ref: Hospital F & Charcosset A 1997 Genetics – MABC foreground selection. Collard & Mackill marker review

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