PB Ch 21. Multiline Varities

Concept and Origin

Why Multilines Work — The Epidemiological Basis

• In a multiline field, different pathogen races may overcome individual component lines. However:
• A new race that overcomes Line A is still blocked by Lines B, C, D, etc.
• Only a small fraction of the total plant population becomes susceptible in any one season
• Disease cannot spread rapidly through the entire field — only through susceptible component plants
• This is unlike a uniform susceptible variety where ALL plants become susceptible simultaneously when a new race appears
• Reserve component lines (carrying other resistance genes) are maintained and can replace susceptible components as new races appear
• Borlaug suggested 15-20 component lines for durability; if only a reduced disease level is the objective, a smaller number (5-10) is adequate.

Development of Multiline Varieties

Step 1: Development of Component Lines (Near-Isogenic Lines)

• Resistance genes are incorporated into the elite variety (recurrent parent) through conventional backcross programmes. 
• A SEPARATE backcross programme is run for each resistance gene, using the same recurrent parent. 
• After 5-6 backcrosses, each resulting near-isogenic line (NIL) is genetically almost identical to the recurrent parent, differing only in the specific resistance gene. 
• Alternatively, limited backcrossing (2-3 BC) followed by pedigree selection may be used, though lines may then differ in agronomic features.

Step 2: Evaluation and Grouping of Components

Component lines evaluated for:

• Agronomic uniformity — must be visually indistinguishable in the field
• Disease reaction to all known pathogen races — each line resistant to some races and susceptible to others
• Yield performance — mixture must not yield less than recurrent parent
• Compatibility — lines must not reduce each other's yield when grown as a mixture
• If a component line later becomes susceptible to a newly emerged race, it is removed from the mixture and replaced by a new resistant line kept in reserve (maintained in seed bank).

Characteristics of a Good Multiline 

• Genetic diversity for vertical resistance genes for the concerned disease
• Vertical resistance genes should be strong enough — each component line must show clear resistance to specific pathogen races
• Should have normal resistance to other diseases as well
• Components of multiline should be uniform for agronomic and other features — same height, maturity, grain colour, seed size
• Should have yield advantage over a single pure line — the mixture should not yield less than the RP

Merits and Demerits of Multiline Varieties

Achievements — Indian Multiline Varieties in Wheat 

• In India, four multiline varieties have been released in wheat. Kalyan Sona and Sonalika (the most popular varieties during the Green Revolution) were used as the recurrent parents to produce these varieties:
• KSML 3: 8 component lines with rust resistance genes from Robin, Ghanate, K1, Rend, Gabato, Blue Bird, Tobari, and others. Kalyan Sona as recurrent parent.
• MLKS 11: 8 component lines; resistance derived from E6254, E6056, E5868, Frecor, HS19, E4894, etc. Kalyan Sona as recurrent parent.
• KML 7406: 9 component lines deriving rust resistance from different sources. Kalyan Sona as recurrent parent.
• Sonalika Multiline-1: 6 component lines. Sonalika as recurrent parent; released for cultivation in Punjab state.