Hybridization techniques and its consequences

Transcript

1. Hybridization techniques and its consequences Hybridization techniques and its consequences Meaning Meaning Objectives Objectives Types Types Program Program Procedures Procedures Consequences Consequences Deependra Dhakal Deependra Dhakal 2019/04/21 (updated: 2020-11-06) 2019/04/21 (updated: 2020-11-06) 1 / 15 1 / 15
2. Hybridization Hybridization 2 / 15 2 / 15
3. Meaning Hybrid : Any sort of sexual or somatic combination of genetically more or less differentiated parental cells, individuals, or taxa; specifically, an individual plant from a cross between parents of differing genotypes; any heterozygote represents dissimilar alleles at a given locus; or a hybrid graft gene. Hybrid breeding (syn heterosis breeding): Involves the transfer of genes from one source or genetic background to another, or combining genes from different sources that complement each other, with the hope that the new cultivar will combine the best of both parents. The term hybridization is prone to ambiguity. The concept of a cross between individuals from two distinct groups is superficially simple, but uncertainty arises in defining distinct groups. Harrison, 1990 defined hybridization as a cross between individuals from separate populations that differ in one or more heritable traits. In flowering species, sexual hybridization is the commonly occuring mode of hybridization. Natural hybridization Artifical hybridization 3 / 15
4. Applications of crossing in plant breeding Gene transfer Recombination Break undesirable For heterosis For maintenance of breeding For maintenance of diversity in a gene pool For evaluation of parental lines For genetic analysis 4 / 15
5. Arti cial hybridization Arti cial hybridization 5 / 15 5 / 15
6. Preconditions Parents should belong to same or closely related species Parents should together supply the critical genes needed to accomplish the breeding objective. One parent is usually designated female. Whereas some breeding methods may not require this designation, breeders usually select one parent to be female and the other a male (pollen source). The female parent usually needs some special preparation (Emasculation in complete flowers) Pollen is often physically or manually transferred Pollination control techniques Mechanical control Chemical control Genetic control 6 / 15
7. Flower and owering issues in hybridization Flower health and induction Synchronization of flowering Selecting female parent and suitable flowers Considerations of pollination Successful pollination depends on pollen maturity, quality (freshness), and timing of pollination, among other factor. Considerations: Collection and storage Application of pollen Tagging after pollination 7 / 15
8. Genetic issues in hybridization Genetic issues in hybridization 8 / 15 8 / 15
9. Immediate e ect Expression of recessive lethal gene Hybrid necrosis Heterosis Transgressive segregation Genome-plastome incompatibility 9 / 15
10. Subsequent e ects The variability in an F2 population as affected by the number of genes that are different between the two parents Number of heterozygous loci (n) Number of heterozygotes in the F2 (2^n) Number of different genotypes in the F2 (3^n) Minimum population size for chance to include each genotype (4^n) 1 2 3 4 2 4 9 16 3 8 27 64 4 16 81 256 5 32 243 1024 10 1024 59049 1048576 15 32768 14348907 1073741824 10 / 15
11. Subsequent e ects (...continued) Gene recombination in F2 Factors that determine the number of gene recombinations that would be observed in an F2 population: The number of gene loci for which the parents in a cross differ. The number of alleles at each locus. The linkage of the gene loci. ... 11 / 15
12. Types of populations generated through hybridization Divergent crossing Single cross Three way cross Double cross Diallel cross 12 / 15
13. The basic types of crosses used by plant breeders. Some are divergent (a) while others are convergent (b) 13 / 15
14. Convergent cross Backcross Wide cross 14 / 15
15. Bridge cross An example of a bridge cross. To hybridize Italian ryegrass and tall fescue, the breeder may firstly make an intermediary cross with meadowgrass, followed chromosome doubling. 15 / 15