Biology · Ch 8 — Improvement in Food Production
Hybridization, Selection and Release of New Varieties
Hybridization, Selection and Release of New Varieties
Once a plant breeding programme's objectives have been set, the actual work proceeds through a connected sequence of steps, each depending on the one before it.
Collection of genetic variability
The raw material of any breeding programme is genetic variability -- the naturally occurring differences between individual plants of a crop and its wild relatives. A germplasm collection -- the sum total of all the different alleles for every gene present in a crop species and its related wild forms, gathered and conserved (commonly as seed in a gene bank, or as living field collections) -- gives breeders the widest possible pool of traits, such as an unusual disease resistance found only in one wild relative, to draw on.
Evaluation and selection of parents
The assembled germplasm is evaluated to identify plants carrying the specific desirable traits the programme is targeting, and the best of these are selected to serve as parents for the next step. Pure lines (genetically uniform, self-pollinated stock) are commonly established from the selected parents first, so that the traits each parent contributes are well characterised and predictable.
Cross-hybridization between selected parents
The selected parents, each strong in a different desired trait, are then cross-pollinated (hybridized) to combine their traits in a single offspring. In a bisexual flower, this requires emasculation -- carefully removing the anthers from the flower bud chosen as the female parent before they mature and shed pollen, to prevent unwanted self-pollination -- followed by bagging the emasculated flower to keep out foreign pollen until the breeder deliberately dusts it with pollen collected from the chosen male parent.
Selection and testing of superior recombinants
The hybrid offspring segregate into a wide range of combinations of parental traits; the breeder selects those individual plants -- the superior recombinants -- that combine the desired traits most successfully, and multiplies and re-tests them over further generations until the line breeds true (remains genetically stable and uniform from one generation to the next).
Testing, release and notification of a new variety …
Worked out. A short explanatory note box on the two hand-technique steps that make controlled cross-hybridization possible in a bisexual flower. Emasculation is the careful removal, usually with fine forceps, of the anthers from a flower bud chosen as the female parent, carried out before the anthers mature and dehisce (split open to shed pollen), so the flower cannot pollinate itself. The emasculated flower is then covered with a small bag (bagging) to physically exclude any foreign, wind- or insect-carried pollen until the breeder is ready; at the right time, pollen collected from the chosen male parent is dusted onto the receptive stigma by hand, a controlled step called arti …