Plant Domestication: Selective Breeding (Lecture 5 and 6)

• Plant domestication is the process of modifying wild plants over time to make them more useful to humans.
• Several methods are used to domesticate plants, ranging from traditional selection to advanced techniques.

Selective Breeding (Mass Selection)

• Selective breeding relies on pre-existing natural variations within a plant population.
• Desired traits must be heritable, passing from parent to offspring.
• Farmers repeatedly select individuals with desired traits to be parents of the next generation.
• Selection is based on observable characteristics (phenotype) and how well the plant performs.
• Early selective breeding was often unintentional, like saving seeds from plants easier to harvest or with larger seeds
• Later, farmers consciously selected plants with specific desired traits.
• Selective breeding increases the frequency of desired traits through repeated selection.
• Early domestication relied on unconscious selection for manageability and harvestability.
• Over time, selection evolved to consciously target specific improvements, such as increased seed size.
• "Fishing in the dark" describes early farmers' lack of understanding of the underlying genetic mechanisms.

Hybrid Breeding

• Plant crossing, intentionally breeding two different plants together, was discovered in 1694.
• Hybrid offspring inherit traits from both parents, and the understanding of this inheritance occurred between 1761 and 1766.
• Hybrid vigor (heterosis) refers to hybrid offspring exhibiting superior traits compared to their parents; discovered around 1920.
• Hybrid breeding can occur between varieties of the same species or, sometimes, different, compatible species
• Development of inbred lines involves repeatedly self-pollinating plants to create homozygous lines for desired traits.
• This process aims to eliminate undesirable traits.
• Crossing homozygous lines produces hybrid seeds exhibiting hybrid vigour.
• Commercial hybrid corn exemplifies hybrid breeding and generates dramatically increased yields.
• Commercial hybrid corn requires farmers to purchase new seeds each year because subsequent generations may not retain the same high performance.
• Hybrid breeding uses the principle that crossing plants can create offspring with new trait combinations.
• Hybrid vigour has made hybrid breeding important for increasing crop yields.
• Modern hybrid breeding selects parent candidates, creates homozygous inbred lines through selfing and rigorous screening, and crosses these lines to produce hybrid offspring.
• This process is reproducible and predictable, but slow and requires starting a new program each season.

Mutation Breeding

• Mutation breeding induces mutations using chemical mutagens or physical radiation, like UV, X, or gamma rays, to increase the frequency of genetic changes in plants.
• The goal is to create plants with new, hopefully desirable, traits, ranging from minor to drastic changes.
• Mutation breeding requires screening large populations to identify individuals with desired traits.
• Only mutations occurring in the germline (cells producing reproductive cells) are heritable.
• Dominant mutations are easier to detect and work with, as they are expressed with one copy, whereas most mutations are recessive and require two copies.
• Mutations occur randomly, and most are harmful or lethal, with advantageous mutations being rare.
• Mutation breeding introduces genetic variation through induced mutations in hopes of generating desirable traits.
• Large populations must be screened due to the rarity of beneficial mutations.
• A key challenge is unpredictability and the recessive nature of most mutations.
• When a desirable and heritable mutation is found, it can lead to new plant cultivars, like the low-seeded Kinnow orange.

Backcross Breeding

• A donor parent possesses a specific desired trait but may lack other qualities of a cultivated variety.
• A target cultivar, also known as the elite cultivar, is already high-performing but lacks a particular desired trait.
• The goal is to transfer a specific gene or trait from the donor parent to the target cultivar.
• The hybrid offspring, carrying the desired trait, are repeatedly crossed back to the elite cultivar over several generations.
• In each generation, plants possessing the desired trait are selected for the next cross.
• Backcrossing recovers the elite genetic background so the offspring increasingly resemble the elite parent.
• Backcross breeding improves existing cultivars by adding a specific trait from a less desirable donor parent.
• This involves repeatedly backcrossing to the elite cultivar.
• The genetic makeup of the offspring becomes more similar to the elite parent.
• This is a more targeted approach than mutation breeding because the goal is to transfer a known beneficial trait.

Genetic Modification (Recombinant DNA Technology)

• Recombinant DNA technology involves taking DNA from one organism (donor) and inserting it into another (recipient).
• This technology facilitates introducing novel traits from distantly related species or even other types of organisms.
• Vector plasmids, like Agrobacterium tumefaciens, are often used to carry donor DNA into recipient plant cells.
• Agrobacterium tumefaciens, a soil bacterium, naturally transfers plasmid DNA into plant cells, causing tumors.
• Scientists have modified this system to insert genes of interest instead of tumor-inducing genes.
• The gene of interest is the specific DNA sequence coding for a desired trait, such as herbicide tolerance or Bt toxin production.
• Transformation is the process of introducing recombinant DNA into recipient plant cells, which often involves tissue culture.
• Common uses include herbicide tolerance (HT) in soy, corn, and cotton, and Bt toxin production for insect resistance.
• "Golden Rice" engineered to produce vitamin A, addresses nutritional deficiencies.
• Genetic modification is a precise domestication method using recombinant DNA technology.
• Vector plasmids transfer specific genes of interest.
• The process involves transformation of plant cells and regeneration of genetically modified plants.
• This technology enables rapid introduction of traits like herbicide and insect resistance.