Genes, Mendel and

Meiosis

Why are Genetics Important?

Key to plants being able to survive (evolve)

changes in environment is genetic variation.

Plant breeders use this genetic variation to

breed new cultivars.

This genetic variation is due to changes in

the genetic code of a gene or the sequence

that controls expression of the gene.

A gene is a DNA sequence coding for a

single polypeptide, t-RNA or r-RNA

DNA RNA protein

nucleic nucleic amino

acids acids acids

So changing the nucleic acid sequence of DNA can result in changing the amino acid sequence of a protein

Wild type Mutant

Herbicide

binding site

Herbicide

can‟t bind

Promoter region Gene Sequence Terminator region

xAA aa

Aa

AaAaAA aa

“Law of segregation” – A pair of alleles for a given gene (trait) separate or segregate in the gametes equally.

“Law of independent assortment” –Allelic pairs of genes for two traits will behave independently of each other.

Genetic modification of crop plants.Increase productivity.Have better end-use

quality.Can be produced with

fewer input costs, with greater profit.

Self-pollinator Out-pollinator

• Tolerant to inbreeding

• Few deleterious recessive alleles

• “Closed” flowers

• Little heterosis

• Intolerant to

inbreeding

• Many deleterious

recessive alleles

• „Open” flowers.

• High heterosis

Reproduction through plant parts

Reproduction through apomixis

A-sexual Reproduction

Types of Cultivar

Pure line.

Out breeding populations.

Clones.

Hybrids.

Breeding Objectives

Genetic Variation

Selection

•People

•Politics

•Economics

What factors are

to consider in

setting Breeding

Objectives?

Increase yield of product harvested

over a specified area.

Increase the inherent quality of the

end product.

Reduce the cost of producing the end

product (while maintaining yield and

quality).

Yield

Increase yield of product per area.

Increase the region of adaptation.

Increase

harvestability

Improve

Storability

Visual appearance

Uniformity

Identify or create

genetic variability

Select for desirable

recombinants

Plant Breeding Operations

Artificial Hybridization

High Yield

/Disease

Susceptible

Low Yield

/Disease

Resistantx

High Yield &

Disease

Resistant

Wide Crossing

Interspecific and

Intergeneric Hybridization

Triticale

2n=6x=42

AABBDDRR

or AABBRR

Rye

2n=2x=14

RR

Wheat

2n=6x=42

AABBDD

or AABB

Induced Mutation

Alkylating agents:

• Most commonly used is Ethyl-methane-

sulphonate (EMS)

Radiation:

• X-rays used to be most common

source.

• Gamma rays, now most favors source.

Mutagens are indiscriminant

agents.• Clean up and stabilize mutants.

• Get rid of undesirable mutants.

Selection of desirable mutants.• Rapid screen necessary.

• In vitro screening.

[4x = 44][2x = 22] Colchcine Autotetraploid

[4x = 44]

Seedless Watermelon

[2x = 22] x

[2x = 22][3x = 22] xMale Sterile

Identify or create

genetic variability

Select for desirable

recombinants

Plant Breeding Operations

Parents TT x tt

F1 Tt

F2

1:TT 2:Tt 1:tt

3 Tall : 1 Short

Parents TT x tt

F1 Tt

F2

Frequn

TT

¼

Tt

½

tt

¼

Parents TT x tt

F1 Tt

F2

Frequn

TT

¼

Tt

½

tt

¼

F3

Frequn

TT

¼

TT1/8

Tt

¼

tt1/8

tt

¼

3/8 TT 2/8 Tt 3/8 tt

Parents TT x tt

F1 Tt

F2

Frequn

TT

¼

Tt

½

tt

¼X X

Parents TT x tt

F1 Tt

F2

Frequn

TT

¼

Tt

½

tt

¼

F3

Frequn

tt

¼

X X

Genetically

Fixed

Parents TT x tt

F1 Tt

F2

Frequn

TT

¼

Tt

½

tt

¼X

Parents TT x tt

F1 Tt

F2

Frequn

TT

¼

Tt

½

tt

¼

F3

Frequn

TT

¼

TT1/8

Tt

¼

tt1/8

3/6 TT 2/6 Tt 1/6 tt

X

Segregation

Quantitative Genetics

0

200

400

600

800

1000

1200

1400

500 510 520 530 540 550 560 570 580 590 600 610 620

Normal Distribution

Genotype ?

Phenotype.• Dominance (allelic interactions).

• Epistasis (non-allelic interactions).

Environment.

A virus that parasitizes

bacteria. Bacteriophase DNA

passes into the bacteria cell

and hence can replicate.

Produced by bacteria as a

defense mechanism against

phages. Enzymes act like

scissors by cutting phage

DNA at specific sites.

P1

P2

P1 P2 F2

Molecular Markers

If a particular trait is difficult

to assess, then find an easily

assessable trait that is closely

linked to the difficult one.

P1

P2

P1 P2 F2

Molecular Markers

R S R S S R R R S R

Marker assisted selection.

◦ Difficult to evaluate characters.

◦ Quantitative Trait Loci (QTL‟s)

DNA finger printing to identify genotypes (or cultivars).

◦ To secure proprietary ownership.

◦ Select parents with known genetic distance.

Cytological information (mainly in interspecific hybrids).

Saturated gene mapping.

Possible to transfer single genes from other species and non-plants into plant.

Have transgenes expressed and to function successfully.

Bypass natural barriers which limit sexual gene transfer.

Allow breeders to utilize gene from completely unrelated species.

Create new variability beyond that currently available in germplasm.

Check functionality of tranformed plants

Select cells that have been transformed

Regenerate whole plants from single transformed cells

Develop a suitable construct

Develop a mechanism to transfer the gene into the target plant

Find a desirable gene

At present plant transformation is limited to

transforming single genes.

Techniques can only be applied to genes that

have been identified and cloned.

Identification of suitable promoters for the

genes that are to be introduced.

Transformation is still largely uncontrolled and

many thousands of plants need to be screened

to select ones with few deleterious effects.

Which pests and diseases affect crops

Effect of plant pests and diseases

Types of plant resistance

Mechanism for pest and disease

resistance

Pest management systems.

Air borne

fungi

Soil borne

fungi

Bacteria

Viruses

Eelworms

Insects

Other, Incl

Mammals

Reduce useable yield.

•All diseases and pests.

Reduce end-use quality and

storability.

•Most crops, especially fruits and

vegetables.

Susceptible Host

Pathogen

Favorable

environment

No

disease

No

disease

No

disease

No

disease

No

disease

No

disease

Disease

Vertical resistanceControlled by a single gene.

Results in distinct resistance classes.

Resistance is usually absolute (yes or no).

Horizontal resistance.Controlled by multiple genes.

Results in continuously variable levels of resistance.

Usually resistance is not absolute.

Relationship between

resistance genes and

virulent genes is called

Locks & Keys

Locks (dominant resistance genes)

can only be opened with the right

keys (recessive virulent genes).

Plant Genotype Pest genotype Plant response

aabb Any virulent gene Susceptible

A_bb No virulent genes Resistant

A_bb a‟a‟B‟B‟ Susceptible

A_B_ a‟a‟B‟B‟ Resistant

A_B_ a‟a‟b‟b‟ Susceptible

Easy to manipulate genetically.

Identification of resistant phenotypes is easier.

Race specific.

Resistance tends not to be durable for some disease types.

Advantages

Disadvantages

Horizontal resistance more durable than vertical resistance.

Ability to control a wide spectrum of races.

New pathotypes have difficulty overcoming all resistance loci.

Advantages

Probability of combining all (or many) resistance alleles into a single genotype are low.

Disadvantages

Resistance due to lack of infection.• Hypersensitivity

• Mechanical

Resistance due to lack of spread

after infection.• Antibiosis: Plant resistance that reduces,

survival, growth, development, or

reproduction of pests feeding on the plant.

• Antixenosis: Plant resistance that reduces

pest preference or acceptance of the plant.

Escape: Plant morphology avoids disease.Tolerance: Plant “resistance” that

results in a plant suffering less injury or yield loss than a susceptible plant when both are equally infested.

Using genes from Bacillus

thuringiensis (B.t.).

What is a weed?

Yield loss as they compete for:

• Interceptable light.

• Water.

• Nutrients.

Harbor Pests:

• Over winter insects, host to diseases

and cause infection.

Reduce Crop quality

• Weed seed contamination.

Annual weeds:• Complete life cycle in one year. Relatively

easy to control. Seeds can remain dormant for many years.

Biennial weeds:• Germinant in the spring of one year, live

vegetatively through winter and flower the following spring.

Perennial weeds:• Most difficult to control when established.

Mechanical:

• Non-selective herbicidal cultivation.

• Inter-row cultivation.

• Hand weeding.

Cultural:

• Inter-cropping.

Biological:

• Insects.

Chemical.

Group Description

1

Foliar, monocots, ACCase (Acetyl CoA Carboxytase) inhibitors,

binds to ACCase and disrupts fatty acid synthesis, which leads to

membrane degeneration (i.e. Hoelon, Assure II).

2Foliar and soil, dicots, ALS (Acetolactate synthase) inhibitors, binds

to ALS and disrupts synthesis of branched amino acids (i.e. Beyond).

3Soil applied, mainly dicots, Tubulin inhibitors, interferes with cell

division (Treflan).

4Foliar, mainly dicots , synthetic auxins, upsets plant growth regulator

balance by mimicking an increase of auxins (i.e. 2,4-D).

5,6&7Foliar and soil, mainly dicots, binds to a pigment in photosystem II

and disrupts photosynthesis (Triazine, Sencor).

9

Foliar, nonselective, EPES inhibitor, binds to EPES synthase and

disrupts pathway, which is responsible for producing the precursors of

aromatic amino acids (i.e. Roundup).

Wheat JGG

Hydrid

Biological control:• Encourage natural predators and parasites.• Biopesticides.

Cultural control:• Resistant cultivars; trap crops; intercropping.• Cultivation & tillage; crop rotation, timing.

Mechanical & Physical control:• Screens; traps.

Reproductive & Genetic control:• Introduce harmful pest genes; mass release of sterile

insects.

Chemical control:• Pesticides used in an appropriate manner; hormones.

Next Class

Test #3

Wednesday, November 18th

2010

10:30-11:20