Plant breeding Lecture 13

Plant breeding The science of improving the heredity of plants for the benefit of humankind

The transition form wild plant to a cultivated plant started early in the history of mankind

Wild form Cultivated form

The process of beginning of a wild species under human management is referred to domestication. This is likely to give rise to better type than the wild one. Thus domestication may be regarded as a method, in fact the most basic method of plant breeding.

Where Crops were Domesticated Five areas where crops were domesticated independently are: • Southwest Asia (Middle East): – wheat, pea, olive China: Rice, Millet Mesoamerica: corn, beans, squash Andes: potato (Ireland) America: sunflower

Wild plant versus cultivated plant Difference in: - yield - content germination synchronity - maturation - resistance T. monococcum ; T. aestivum

Breeding objectives Yield Resistance to lodging (bending or breaking over of plants before harvest) and shattering (fall out of seeds before harvest) Winter hardiness (to survive winter stress) Heat and drough resistance Soil stress Resistance to plant pathogens Resistance to insect pests Product quality Content germination Synchronity maturation

Highlights of plant breeding • 1865 Mendelian genetic principles • 1923 first hybrid corn • 1960 ‚Green Revolution, breeding, plant management (fertilization, pest control..) • 1983 first transgenetic plants.

Plant Breeding: the process of selectively mating plants A basic type of plant breeding is Selection. Selection is when plants with desirable traits are chosen from a population and then reproduced. Natural selection Artificial selection

Different types of breeding Selection breeding Artificial selection- 1. Mass selection 2. Pureline selection 3. Clonal selection Hybridization Introgression breeding Important for transferring genetic material from one species to the other. (Some might be adaptively important. Some might be neutral genes.)

Introgression breeding

Backcrossing Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent. It is used in horticulture, animal breeding and in production of gene knockout organisms.

Backcross breeding enables breeders to transfer a desired trait such as a transgene from one variety (donor parent, DP) into the favored genetic background of another (recurrent parent, RP). If the trait of interest is produced by a dominant gene, this process involves four rounds of backcrossing within seven seasons. If the gene is recessive, this process requires more generations of selfing and thus nine or more seasons are needed. The rate at which the DP genes are removed and the RP genes are recovered in the genetic makeup of the plant can be calculated using the number of backcross generations utilized. This rate is dramatically increased with the recent advances in marker technology which allow breeders to control the gene of interest and control the genetic background.

Mass selection Key features: Oldest method for self-pollinated species Variable population Self- and cross-pollinated species Solely selection (no variability) Recurrent Based on phenotype Applications: Maintain purity of a cultivar Develop cultivar from base population Initial inexpensive screening

Mass selection Procedure: Plant heterogeneous population Generalized steps in breeding by mass selection for (a) cultivar development, and (b) purification of an existing cultivar Procedure: Plant heterogeneous population negative mass selection positive mass selection Disadvantages: should have > heritability Uniform environment Cannot distinguish homozygous from heterozygous Advantages:?

Pure line selection Key features: Lines that are genetically different may be isolated from within a population of mixed genetic types. Variation is due to environmental factors and ??? Applications: Cultivars Parents for hybrid production (uniform etc.) Generalized steps in breeding by pure-line selection

Mendel’s laws of inheritance (1) The Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. (2) The Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.  Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

Pedigree selection Key features: Records of the ancestry of the cultivar Hybridization is used to generate variability Followed by selection of a segregating population Applications: Mainly crop plants, Improvement of quantitative traits

Emasculating Zinnias: Collecting Pollen: Images kindly provided by Goldsmith Seeds

Fertilization Images kindly provided by Goldsmith Seeds

Breeding vegetatively propagated crops Maintenance of desired genotypes

Through: bulbs and corms (tuplips, gladioli) Budding or grafting (?) Cuttings (pelargoniums, carnations) Division (hardy herbaceous perennials)

“Grafted before planting” “Field grafting”

Basic scheme for breeding clone cultivars.

Overview: Cultivars Pure inbred lines (species that are highly self-pollinated) homogeneous, homozygous series of self- pollinations Open-pollinated populations (species that are naturally cross-pollinated) heterogeneous, heterozygous recurrent selection Hybrids (crossing inbred lines) homogeneous, heterozygous Clones (asexually propagation)

Modern breeding methods Examples: - Mutation breeding - Breeding of polyploids - Generation of haploids

Mutation Breeding The gamma field is a circular field of 100m radius with 88.8TBq Co-60 source at the center.

Gamma Greenhouse for chronic irradiation of sub-tropical plants. Cs137 source (4.81TBq, 130 Ci) is used in the octagonal greenhouse with 7 m radius at the Institute of Radiation Breeding, NIAR, Ibaragi, Japan.

Breeding of Polyploids * Produced by:- a) doubling of chromosomes in 1 species (autopolyploidy) b) doubling after hybridization of 2 species (allopolyploidy) * Common in plants (80% of species) - many commercially important examples. * Rarer in animals (some fishes, flatworms, shrimp, amphibians). Triploid oysters (sterile)

Polyploids Autopolyploids • between genetically very similar plants • parents the same species • chromosomes homologous • at meiosis quadrivalents Allopolyploids • between genetically dissimilar plants • parents different species • chromosomes not homologous at meiosis bivalents

Allopolyploidy in Brassica

• gain and loss of coding or uncoding DNA Possible changes taking place between the component genomes following allopolyploidyzation • gain and loss of coding or uncoding DNA • structural rearrangements at the chromosome level changes in gene expression - gene silencing or activation

Haploid plants A. Reduce time for variety development, e.g. 10 to 6 years or less. B. Homozygous recombinant line can be developed in one generation instead of after numerous backcross generations. C. Selection for recessive traits in recombinant lines is more efficient since these are not masked by the effects of dominant alleles. Agricultural applications for haploids - Rapid generation of homozygous genotypes after chromosome doubling. Haploid - gametic number of chromosomes

Processes Leading to Production of Haploid Plants Androgenesis – haploid plant derived from male gamete, most common method in vitro. Parthenogenesis - from unfertilized egg Chromosome elimination - chromosome elimination in somatic cells, most common method used with plant breeding.

Production of Haploids through Anther and Microspore In Vitro Culture Anther and microspore (pollen) culture - haploid plants are derived from microspores (pollen) cultured individually or in anthers Background – micro-sporogenesis and microgametogenesis leading to pollen development.

Features of Anther/Microspore Culture

Anther/Microspore Culture Factors • Genotype – As with all tissue culture techniques • Growth of mother plant – Usually requires optimum growing conditions • Correct stage of pollen development – Need to be able to switch pollen development from gametogenesis to embryogenesis • Pretreatment of anthers – Cold or heat have both been effective