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

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

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

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).

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

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 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 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 1. Select best plants 2. Intercross selected to form next generation Phenotypic recurrent sel. Mass selection Genotypic recurrent sel. Evaluate offspring

Mass selection Very simple population improvement Efficient only for high heritability traits Population maintenance Remove off types First step in breeding programs

Select good looking plants 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

Superior plants selected Source population Superior plants selected Half-sib selection with testcross Tester can be more or less uniform Composite open-pollinated Composite selfed

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

self-pollinate selected plants 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

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 Intercross to form Syn1 Open pollinate to form syn2

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 Clone A X Clone B Potato Begonia Orchids Select best looking seedlings Evaluate clones 1-2 seasons Multiply and market superior clones