PB Ch 20. Back Cross Method

Definition 

Historical context: 

• Early 20th century breeders found that varieties with disease resistance genes from hybridization were inferior to the original well-adapted parent in yield and quality. 
• Backcross method was developed to solve this — transfer the resistance gene while recovering the rest of the adapted parent's genotype.

Terminology 

• Recurrent parent (RP) = Recipient parent: Well-adapted, high-yielding variety; repeatedly used in backcross programme; receives the transferred gene.
• Donor parent = Non-recurrent parent (NRP): Source of the target gene(s); used only once to produce the initial F1 hybrid.

Requirements of a Successful Backcross Programme 

• A good recurrent parent variety needing improvement in one qualitative character or a quantitative character with high heritability
• Suitable donor parent with the character to be transferred in a highly intense form
•  High expressivity of the character through several backcrosses in the RP genetic background
• Character to be transferred must have high heritability — preferably one or few genes
• Simple, reliable, economical testing technique for detecting presence of character in each BC generation
• Recovery of the recurrent parent genotype in a reasonable number of backcross generations

Transfer of a Dominant Gene 

Scenario: 

• Variety A (RP — high yielding but susceptible to rust) x Variety B (NRP — rust resistant, dominant gene R). 
• Objective: transfer R into Variety A.

Recovery of Recurrent Parent Genome  

Transfer of a Recessive Gene

• When resistance gene is RECESSIVE (rr), BC plants in each generation are all phenotypically susceptible (Rr — heterozygous = looks susceptible). Cannot select by phenotype in BC generations.
• Solution: 

• After every 2 BC generations, SELF the BC plants to produce F2 progenies. 
• Inoculate F2 to identify rr (resistant) plants. 
• Select rr plants and resume backcrossing. 
• This alternation approximately DOUBLES the time required compared to dominant gene transfer.

Scheme:

• F1 (Rr, susceptible) -> BC1 to RP -> BC1 (all susceptible, Rr or rr)
• BC1 -> SELF -> BC1F2 -> inoculate -> select rr (resistant) -> BC2 to RP
• BC2 -> SELF -> BC2F2 -> inoculate -> select rr -> BC3 to RP
• After BC5F2 or BC5F3: select rr + RP plant type -> new variety

MABC solution: Molecular markers flanking the recessive gene (foreground selection) can identify Rr plants in each BC generation without selfing — eliminating the extra seasons needed.

Transfer of Two or More Characters 

Strategy 1: Simultaneous Transfer

• Both genes transferred in same BC programme — requires larger population. 
• If genes are LINKED, only one selection needed. 
• Wheat examples of linked genes: Lr24+Sr24; Lr19+Sr25; Lr26+Sr31 (linked leaf rust + stem rust resistance pairs — transfer one, get both).

Strategy 2: Stepwise Transfer

• RP first improved for character 1 (complete BC programme). 
• Improved RP then used for character 2. Sequential — takes ~twice as long.

Strategy 3: Simultaneous but Separate Transfers 

• Each character transferred into same RP in separate parallel BC programmes. 
• Resulting improved versions crossed together. 
• Homozygous lines with all genes selected by pedigree from the segregating populations. 
• 'This approach appears to be the most suitable of the three strategies.'

Merits and Demerits of Backcross Method

Marker-Assisted Backcrossing (MABC)

• MABC combines molecular markers with conventional backcrossing for precise, rapid, efficient introgression. 
• Foreground selection markers: SSR/SNP/KASP markers flanking target gene (ideally within 5 cM on both sides). Scored in every BC generation to confirm gene presence — even in Rr heterozygotes for recessive genes. Eliminates need for selfing generations in recessive gene transfer.
• Background selection markers: 80-200 genome-wide markers across all chromosomes. Measure % RP genome in each BC plant. Select plants with highest RP genome recovery. Reduces BC generations from 6 to 2-3 for equivalent RP recovery. Minimizes linkage drag.

MABC advantages over conventional backcrossing:

• Reduces generations from ~6 to 2-3 for >95% RP genome
• Eliminates alternating BC-selfing cycle for recessive genes
• Minimizes linkage drag through background selection
• Multiple target genes handled simultaneously in one genotyping run

Indian MABC Success Stories 

• Improved Samba Mahsuri (ISM): ICAR-IIRR Hyderabad — three BB resistance genes (Xa21, xa13, xa5) pyramided via MABC into Samba Mahsuri. Retains grain quality + broad-spectrum bacterial blight resistance.
• Improved Pusa Basmati 1 (IPB-1): IARI — xa13 + Xa21 transferred into Pusa Basmati 1 via MABC. Premium basmati quality maintained.
• Swarna-Sub1: IRRI + CRRI — Sub1A gene (submergence tolerance from FR13A) transferred into Swarna via MABC. Now grown on millions of hectares in flood-prone eastern India.

Backcross Method — Achievements

• Two cotton varieties — 170-Co-2 and 134-Co-2m — developed by backcross for disease resistance 
• Kalyan Sona wheat (susceptible to leaf rust) — resistance transferred from diverse sources: Robin, K1, Blue Bird, Tobari, Frecor, HS-19 — through BC programmes producing multiline series (KSML 3, MLKS 11, KML 7406)
• Tift 23A pearl millet (susceptible to downy mildew) — backcrossed with MS-521A, MS-541A, MS-570A — resistant hybrids produced 
• Norin-10 dwarfing genes (Rht1, Rht2) transferred from Japanese wheats into CIMMYT and Indian varieties by backcrossing — foundation of Green Revolution in wheat

Pedigree vs Backcross — Comparison Table