9 Breeding Hybrid Cultivars

Methods of Cultivar Development

• Open pollinated Cultivar

• Hybrid Cultivar

• Genetic Engineering

Hybrid Cultivar• Hybrid cultivars are the first generation offspring of a cross between

different inbred line parents

• In self pollinated crops, segregating populations are selfed for 5/6 generations to attain homozygosity, after which a pure line is selected and cultivated as cultivar. Prior to 1900 same was practiced for cross pollination. This procedure was a mild form of inbreeding that led to a reduction in heterozygosity of breeding lines, and gradual loss in vigor and productivity.

• GH Shull in 1909 proposed a method for producing hybrid cultivars of corn. Hybrid cultivar development has now been extended to a large number of crops to date

Hybrid Cultivar is produced in 3 steps• Development of inbred lines (normally by several generations of

inbreeding in a natural or segregating population of a cross pollinated species).

• Production of single-cross F1 hybrid cultivar with many heterozygous loci by crossing pairs of unrelated inbred lines

• Seed Increase of the single-cross hybrid cultivar for distribution to the growers

Inbreeding and Inbreeding depression• Inbreeding consists of any system of mating that leads to an increase in

homozygosity in cross pollinated crops

• Inbreeding in cross pollinated crops may be practiced through– Self pollination (in case of naturally cross pollinating crops like corn)

– Sib mating (in crops where seed setting is problem due to self incompatibility)

1. Half-sib mating: mating between plants that have one parent in common

2. Full-sib mating: mating between plants within the progeny of a single plant

• Decline in vigor and size in plants due to inbreeding is referred to as inbreeding depression

Reduction in vigor and size in corn with successive generations of inbreeding

An inbred line is a homozygous breeding line developed and maintained by self pollination

S0

S1

S3 S4 S5S6

S2

Heterosis or Hybrid vigor• Increased in size, vigor or productivity of a hybrid plant over the mid-

parent average or better parent. Theories explaining Heterosis• Dominant-gene Theory:

– Based on the assumption that hybrid vigor may from bringing together an assortment of favorable dominant gene.

Inbred A X Inbred B(AABBccddEE) (aabbCCDDEE)

F1 Hybrid (AaBbCcDdEe)

• Over-Dominance theory: based on the assumption that heterozygote contributes more than a homozygote.

Each allele produces favorable effect therefore a1a2 will produce better effect as compared to a1a1

A third explanation of heterosis

• An alternative theory was proposed by Milborrow (1998).

• He suggested that growth of a plant may be limited by the genes that regulate certain metabolic pathways down to lower levels than the maximum possible.

• Heterozygote may partially escape this regulation because they have two slightly different alleles for these genes, allowing greater flow on these pathways.

• This is not over-dominance; but, like the over-dominance hypothesis, it predicts that heterozygote have an inherent advantage in vigor that cannot be duplicated by any amount of selection in open-pollinated homozygous lines.

Techniques in hybrid seed productionHand Emasculation and pollination

Hybrid in monoecious species• Pistillate trait is controlled by recessive gene (ff). Ff pistillate result in

50% pistillate & 50% bisexual plants. Pistillate plants are used as seed source and bisexual plants are removed.

Hybrids in Dioecious species• F1 hybrid seed may be produced by inter-planting male and female

clones

Clonal propagation of hybrids• F1 hybrids can be produced vegtatively e.g. sugarcane & potato

Apomictically propagated hybrids• If F1 hybrid plant is produced in apomictic species, this plant is

converted to obligate apomixis (production only through apomixis)

Genetic male sterility• GMS trait is controlled by recessive gene (msms). Msms will result

in 50% msms & 50% Msms plants. msms plants are used as seed source and Msms plants are removed which require more labour and time, thus making it less practical.

Chemically Induced Male sterility• Chemically induced male sterility will eliminate the need to go for

genetic/ cytoplasmic male sterility and restoration system. Male sterility of any line can be obtained by spraying.

Self Incompatibility• Alternate strips of self-incompatible, yet cross-compatible clones are

transplanted. Cross pollination will take place resulting in hybrid seed whereas selfed seed will not be produced due to self- incompatibility.

Cytoplasmic male sterility for

• The A-line, B-line and R-line model for hybrid seed production• A-Line is female or seed producing line. It is recessive non-restorer

gene carrier. Cytoplasmic male sterility is induced through back crossing

• B-line is exactly alike A-line but has normal cytoplasm instead of male sterile cytoplasm.

• R-line is the pollen parent in the cross to produce hybrid seed. The function of R-line is– Pollinate A-Line

– Restore fertility in hybrid seed

– Combine with A-line to produce vigorous hybrid seed