HORT 101 Plant Improvement and Breeding

Last time, we began a discussion of basic concepts of genetics in relation to selecting improved horticultural crops. Today, I want to continue and discuss different approaches that have been used to develop improved cultivars.

1. Today's Objectives

a) Learn the difference between simply inherited and quantitative traits.

b) Learn that in agricultural production, we often use plants representing a narrow genetic base, and may need to increase genetic diversity for subsequent crop improvement.

c) Learn about different strategies for improving crop plants.

2. The inheritance of some traits is very complex.

a) Simply inherited traits

b) quantitative traits

- Complex traits

- Several individual genes contribute to a characteristic.

- examples of quantitative traits

yield, drought tolerance, fruit quality, etc.

c) It is difficult to study and evaluate quantitative traits.

=>For example if 6 individual genes contributed to a quantitative trait, there would be 2⁶ or 64 possible combinations.

3. Since the genetics of plants is complex, plant breeders strive to make small incremental improvements.

a) Identify plants with complimentary traits

b) Controlled crosses

c) Evaluate progeny in F₁, F₂ or later generations.

d) Selection and continued efforts toward developing new cultivar.

=>Procedure of choice varies depending upon reproductive characteristics of plant, and other factors.

For example: With self-pollinated plants, pedigree selection is effective, but this does not work with cross-pollinating species.

e) Breeding Strategies

- Pedigree selection

 

- Mass selection

- Backcross breeding

4. Genetic diversity is necessary for crop improvement

a) Often a limited number of cultivars, or closely related cultivars are used in agricultural production.

5. Sources of Genetic Variation

a) Greatest genetic variation occurs at the center of origin.

 

b) Sports

c) Creating genetic variation

- Mutation breeding

6. Inbreeding vs. Heterosis

a) In some plants, inbreeding is associated with reduced vigor.

- self-pollination or crossing among related individuals

- ~50% of heterozygous loci become homozygous with each generation.

- With continued inbreeding, all loci homozygous

inbred line, pure line cultivar, pedigree

b) Heterosis or hybrid vigor

- Restoration of heterozygous condition

- In some species, hybrids exhibit superior performance compared to each parent.

- Genetic basis for hybrid vigor not completely understood.

- Use of hybrids is a common practice.

ex. corn, tomatoes, petunia, etc.

7. Many crop plants have extra chromosomes

a) polyploid - having more than the normal 2N chromosomes

3N = triploid

4N= tetraploid

 

=>In some cases, polyploid plants exhibit more desirable traits, and are more vigorous.

 

ex. Some apple (Winesap) and pear cultivars are 3N.

 

Tart cherries are 4N.

Cultivated strawberries are 8N.

b) aneuploidy -

c) amphidiploids -

ex. Brassica species such as rape and rutabaga

8. Take Home Messages

a) Many important horticultural attributes are quantitative traits, which are difficult to evaluate and study.

b) Genetic variation is a prerequisite for crop improvement through breeding.

c) Horticulturists have used a variety of different strategies for improving plants.