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Breeding cross-pollinated crops. Cultivar Development in Cross-pollinated Species. Compared to self-pollinated species, cross-pollinated species differ in their gene pool structure, and in the extent of genetic recombination

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slide2

Cultivar Development in Cross-pollinated Species

  • Compared to self-pollinated species, cross-pollinated species differ in their gene pool structure, and in the extent of genetic recombination
  • Unselected populations typically consist of a heterogeneous mixture of heterozygotes; as a result of outcrossing, genes are re-shuffled in every generation
slide3

The breeder focuses more on populations, rather than individual plants, and on quantitative analysis, rather than qualitative traits

  • Progeny do not breed true, since the parent plant is pollinated by another plant with a different complement of alleles
slide4

Because progeny do not breed true in cross-pollinated crops, the usual progeny testing that would be employed in self-pollinated material is much less informative.

  • A more useful way to assess genetic potential is to examine combining ability:
    • General combining ability
    • Specific combining ability
slide5

Combining ability: the ability of an inbred line to give characteristic performance in hybrid combinations with other lines.

  • The progenies are tested for performance as populations and related back to parental.
  • More precision can be obtained by using a homozygous inbred line as the pollen donor (tester line).
slide6

General combining ability:

the average or overall performance of a line in hybrid combinations (open pollinated); represents additive genetic variance and additive x additive epistasis

Specific combining ability:

the performance of a line as compared to other lines when crossed with the same pollen donor (specific pollen source); represents non-additive genetic variance

slide7

Recurrent selection

  • Any breeding system designed to increase the frequency of desired alleles for particular quantitatively inherited characters by repeated cycles of selection
  • Identify superior genotypes for the trait under selection.
  • Inter-mate the superior genotypes and select improved progeny.
population structures
Population structures
  • Self-pollinators
    • mixture of homozygous lines
    • a single homozygous line
    • improve through cross, inbreed, select new superior homozygous line
  • Cross-pollinators
    • mixture of heterozygous plants (population)
    • maintain through cross-pollination (OP)
    • improve through selection of plants with desired genes, avoid too much inbreeding
progeny test vs combining ability test
Progeny test vs combining ability test
  • Self-pollinators
    • evaluate pure line offspring (Measure agric. value)
  • Cross-pollinators
    • evaluate selfed offspring (if possible) No C.A.)
    • evaluate test cross offspring (Measure combining ability)
      • homozygous line tester (specific combining ability)
      • heterogeneous population tester (general combining ability
recurrent selection principle
Recurrent selection principle

1. Select best plants

2. Intercross selected

to form next generation

Phenotypic recurrent sel.

Mass selection

Genotypic recurrent sel.

Evaluate offspring

mass selection
Mass selection

Very simple population improvement

Efficient only for high heritability traits

Population maintenance Remove off types First step in breeding programs

half sib selection
Half-sib selection

1. season

Source population

Select good looking plants

and intercross

2. season

Plants in each offspring

have female parent in

common. They are

half-sibs

They reveal combining

ability of selected plant

Progeny test of selected plants in isolation

3. season

A is less

efficient than B

A. Composite seed

from superior

progenies

B. Composite remnant

seed from plants with

superior progenies

slide13

Source population

Superior plants selected

Half-sib

selection

with

testcross

Tester can

be more or

less

uniform

Composite selfed

Composite open-pollinated

slide14

Source population

Cross pairs of selected plants

Full sib selection

based on

pair crosses

Measures specific combining ability between selected plants

Composite remnant cross seed

from combinations with superior progenies

slide15

Source population

self-pollinate selected plants

Selection from

S1 progeny

offspring test

Only if selfing

is possible

Composite remnant selfed seed

from selected plants with superior progenies

slide16

Formation of synthetic cultivars

Clones

Source population

Clone selected plants

The selected base

clones are kept

to form new Syn1

seeds regularly

Polycross selected

superior clones

Polycross offspring evaluation

The cultivar is propagated

until Syn2-Syn5 to obtain

enough seed

Clones with high

combining ability

With well combining clones

the synthetic can be more

uniform and vigorous than

traditional OP cultivars

Open pollinate

to form syn2

Intercross to

form Syn1

breeding clonally propagated species
Breeding clonally propagated species
  • Plants are highly heterozygous
  • Often semi-sterile
  • Often polyploids
  • Two major breeding methods
    • Spontaneous or induced mutations (sports)
    • Hybridization (often between subspecies / species)
hybridization in clonally propagated species
Hybridization in clonally propagated species

X

Clone A

Clone B

Potato

Begonia

Orchids

Select best looking seedlings

Evaluate clones 1-2 seasons

Multiply and market superior clones