Transcript Hybrid Cultivar

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
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–
1.
2.
•
Self pollination (in case of naturally cross pollinating crops like corn)
Sib mating (in crops where seed setting is problem due to self
incompatibility)
Half-sib mating: mating between plants that have one parent in common
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
An inbred line is a homozygous breeding line
developed and maintained by self pollination
S0
Reduction in vigor and size in corn with
successive generations of inbreeding
S1
S2
S3
S4
S5
S6
Heterosis or Hybrid vigor
• Increased in size, vigor or productivity of a hybrid plant over the midparent 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
(AABBccddEE)
X
Inbred B
(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 production
Hand 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 selfincompatibility.
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