Breeding cross-Breeding cross-pollinated cropspollinated crops

Cultivar Development in Cross-Cultivar Development in Cross-pollinated Speciespollinated Species

Compared to self-pollinated species, Compared to self-pollinated species, cross-pollinated species differ in their cross-pollinated species differ in their gene pool structure, and in the extent gene pool structure, and in the extent of genetic recombinationof genetic recombination

Unselected populations typically Unselected populations typically consist of a heterogeneous mixture of consist of a heterogeneous mixture of heterozygotes; as a result of heterozygotes; as a result of outcrossing, genes are re-shuffled in outcrossing, genes are re-shuffled in every generationevery generation

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

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

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

A more useful way to assess A more useful way to assess genetic potential is to examine genetic potential is to examine combining ability:combining ability:

• General combining abilityGeneral combining ability• Specific combining abilitySpecific combining ability

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

The progenies are tested for The progenies are tested for performance as populations and performance as populations and related back to parental.related back to parental.

More precision can be obtained by More precision can be obtained by using a homozygous inbred line as using a homozygous inbred line as the pollen donor (tester line).the pollen donor (tester line).

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

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

Recurrent selectionRecurrent selection

Any breeding system designed to Any breeding system designed to increase the frequency of desired increase the frequency of desired alleles for particular quantitatively alleles for particular quantitatively inherited characters by repeated cycles inherited characters by repeated cycles of selectionof selection

Identify superior genotypes for the trait Identify superior genotypes for the trait under selection.under selection.

Inter-mate the superior genotypes and Inter-mate the superior genotypes and select improved progeny.select improved progeny.

Population structuresPopulation structures

Self-pollinatorsSelf-pollinators– mixture of homozygous linesmixture of homozygous lines– a single homozygous linea single homozygous line– improve through cross, inbreed, select improve through cross, inbreed, select

new superior homozygous linenew superior homozygous line Cross-pollinatorsCross-pollinators

– mixture of heterozygous plants mixture of heterozygous plants (population)(population)

– maintain through cross-pollination (OP)maintain through cross-pollination (OP)– improve through selection of plants with improve through selection of plants with

desired genes, avoid too much inbreedingdesired genes, avoid too much inbreeding

Progeny test vs combining Progeny test vs combining ability testability test

Self-pollinatorsSelf-pollinators– evaluate pure line offspring (Measure agric. evaluate pure line offspring (Measure agric.

value)value)

Cross-pollinatorsCross-pollinators– evaluate selfed offspring (if possible) No C.A.)evaluate selfed offspring (if possible) No C.A.)– evaluate test cross offspring (Measure evaluate test cross offspring (Measure

combining ability)combining ability)• homozygous line tester (specific combining homozygous line tester (specific combining

ability)ability)• heterogeneous population tester (general heterogeneous population tester (general

combining abilitycombining ability

Recurrent selection principleRecurrent selection principle

1. Select best plants1. Select best plants2. Intercross selected2. Intercross selectedto form next generationto form next generation

Phenotypic recurrent sel.Phenotypic recurrent sel. Mass selectionMass selectionGenotypic recurrent sel.Genotypic recurrent sel. Evaluate offspringEvaluate offspring

Mass selectionMass selectionVery simple population Very simple population improvementimprovement

Efficient only for high heritability traitsEfficient only for high heritability traits

Population maintenance Remove off types First Population maintenance Remove off types First step in breeding programsstep in breeding programs

Half-sib selectionHalf-sib selection

Source populationSelect good looking plants

and intercross

Progeny test of selected plants in

isolation

A. Composite seedfrom superior

progenies

B. Composite remnantseed from plants with

superior progenies

1. season

2. season

3. seasonA is lessefficient than B

Plants in each offspringhave female parent in

common. They arehalf-sibs

They reveal combiningability of selected plant

Source populationSuperior plants selected

Composite open-pollinatedComposite selfed

Half-sibselection

with testcross

Tester canbe more or

less uniform

Composite remnant cross seedComposite remnant cross seedfrom combinations with superior progeniesfrom combinations with superior progenies

Full sib selectionFull sib selectionbased onbased on

pair crossespair crosses

Measures Measures specific specific combining combining ability ability between between selected plantsselected plants

Source populationSource populationCross pairs of selected plantsCross pairs of selected plants

Source populationSource populationself-pollinate selected plantsself-pollinate selected plants

Composite remnant selfed seedComposite remnant selfed seedfrom selected plants with superior progeniesfrom selected plants with superior progenies

Selection fromSelection fromS1 progenyS1 progeny

offspring testoffspring test

Only if selfing Only if selfing is possibleis possible

Source Source population population

Clone selected Clone selected plantsplants

ClonesClones

Polycross selectedPolycross selectedsuperior clonessuperior clones

Polycross offspring evaluationPolycross offspring evaluation

Clones with high Clones with high combining abilitycombining ability

Intercross to Intercross to form Syn1form Syn1

Open pollinateOpen pollinateto form syn2to form syn2

Formation of synthetic Formation of synthetic cultivarscultivars

The selected The selected basebase

clones are keptclones are keptto form new to form new

Syn1Syn1seeds regularlyseeds regularlyThe cultivar is propagatedThe cultivar is propagated

until Syn2-Syn5 to obtainuntil Syn2-Syn5 to obtainenough seedenough seed

With well combining clonesWith well combining clonesthe synthetic can be more the synthetic can be more uniform and vigorous thanuniform and vigorous than

traditional OP cultivarstraditional OP cultivars

Breeding clonally propagated Breeding clonally propagated speciesspecies

Plants are highly heterozygousPlants are highly heterozygous Often semi-sterileOften semi-sterile Often polyploidsOften polyploids

Two major breeding methodsTwo major breeding methods– Spontaneous or induced mutations Spontaneous or induced mutations

(sports)(sports)– Hybridization (often between subspecies Hybridization (often between subspecies

/ species)/ species)

Hybridization in clonally Hybridization in clonally propagated speciespropagated species

Clone AClone A Clone BClone BXX

Select best looking seedlingsSelect best looking seedlings

Evaluate clones 1-2 seasonsEvaluate clones 1-2 seasons

Multiply and market superior clonesMultiply and market superior clones

PotatoPotatoBegoniaBegoniaOrchidsOrchids