CHAPTER 1: Read over chapter one. We will refer to material from this chapter throughout the semester.

CHAPTER 2: The nature of variation of plants.

Humans have selected for the properties they desired in plants. Selection has been both unconscious and purposeful. We want to understand why and how selection works and how we have been able to manipulate it to our advantage.

Artificial versus natural selection (pp. 26-27).
With either type, there must be heritable variation upon which selection can work.
It is presumed that everyone has had an introduction to genes and alleles. If not, reading the appropriate section in one of the introductory botany or biology books will be helpful.

Genes and alleles
The plant nucleus contains chromosomes which contain most of the genes.
Many important crop characters appear to be controlled by one gene.
A duplicate set of chromosomes occurs in each cell of the plant (diploid number).
These are homologous chromosomes. But, the same form of the gene may not occur in both. Different alleles. Homozygous and heterozygous. Dominant versus recessive.

In populations, there are often individuals with lots of different alleles. Sometimes mutations create new allelic combinations.
Mutations that are better for the plant or for man are quite rare. If there is no immigration, the only source of new alleles in a population is mutation. For these to be established in a population of plants, sexual reproduction must occur.
Although we usually look at one gene, in practice, thousands are being crossed each time. Inbreeding and sexual reproduction

The most common type of breeding system involves fusion of gametes from different individuals. This is called outcrossing.
When these events occur within one flower, it is called inbreeding. Outcrossing may be obligatory. Self-incompatibility. Non-synchronous release of pollen and receptive stigmas. Many weeds self-fertile.
In nature, variation is often great and the game strategy appears to be maximum variation. Outcrossing helps to accomplish this.
Inbreeding leads to the production of homozygous individuals. Outbreeding generally leads to greater heterzygosity.
Crosses between one of the offspring and the parents (or parental types) is called back crossing. The process of making crosses between sibs is called sib-sib crossing. Both lead to the production of inbred, homozygous lines. These produce uniform stands of selected genotypes.
Genotype versus phenotype. Look these up if you don't remember them. Hybrid vigor (heterosis) is important in crop plants.
Another reason that self-compatibility is good is that in many crops it is hard to get good fertilization.
Asexual reproduction
Another method that is especially useful for humans. The plants produced are genetically identical to the parents. All resultant plants are part of a clone.
Grafting is an asexual technique that is widely used. Other sources of variation in plants Mutation is the original source of genetic variation.
Sexual recombination is the mechanism for maintaining it. "Crossing over" is another important effect in making new combinations of genetic material. Deletions. Duplications. Inversions. Pieces in new genetic backgrounds. Sometimes small pieces get lost in shuffle.

Sometimes plants end up with more than the diploid number of chromosomes. Often occurs when some of the reproductive cells don't divide properly.
Produce polyploids. Diploid, tetraploid, hexaploid, triploid etc. Many crop plants involve polyploidy in their formation.
See table p. 25. Many types of polyploids are sterile. Polyploids are usually larger than diploids etc. This process often occurs by chance and humans have taken advantage of the rare plants in which it occurred.

Geographic variation in plants

Humans usually want uniform plants, but in nature just the opposite is favored. Variable offspring tend to have better survival.
Humans provide a more or less uniform environment and enough care to make sure crops survive. In nature, gene frequencies vary in populations of plants over geographic distance.
At some point, limits in how much the plant can tolerate are reached and this helps to define the range of a plant.
These same things are true for crop plants, but people can help the crop avoid some otherwise limiting factors.
Originally, crop plants may have been restricted by oceans, mountains, and deserts. Humans have transported many of them where they would not have gone naturally.

Read: "Beyond Traditional Methods", p. 28-32.
There are various kinds of culture methods including tissue culture, cell culture, hairy root cultures. Genes can be introduced by "shooting them in", by protoplast fusion, or other molecular methods.

Plant species
Plant species are not defined well by reproductive barriers as in many groups of animals. Polyploids are often derived from diploid ancestors.
Many hybrids occur between related plant species. Botanists tend to consider a species to be a group of populations that are derived from a single ancestor and which can be distinguished morphologically from other groups of populations.
Often cultivated plants have been given different names than their wild progenitors. One of the big problems is deciding what are the wild progenitors of cultivated plants.
Evolutionary relationships. How do we know how closely related plants are?
Plant names
The species name has two parts. The generic or genus name and the specific epithet. This system goes back to Linnaeus.
Every plant has only one correct name. The names are Latin and should be underlined or italicized when written.
Problems with common names. Genus is a collection of related species. The family is a group of related genera. These are the units most botanists use to organize taxonomic relationships. Most families are intuitively easy to recognize.
The meaning of some common Latin names (p. 33) is helpful for remembering them.

Variation in Plants

natural selectiongenotype
artificial selectionphenotype
genehybrid vigor
alleleasexual reproduction
homologous chromosomeclone
tetraploidcrossing over
populationgene frequency
immigrationspecific epithet
hybrid backcross
sib-sib cross

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Revised January 2005

© David S. Seigler, Integrative Biology 363, Plants and Their Uses, Department of Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA. 217-333-7577.