SA Graan Maart 2015

Maart 2015

FOCUS

Seed

Special

in crop production

rom the beginning of time, producers have always manipu-

lated plant and animal genetic material (DNA), whether

directly or indirectly.

This is the process that brought about the domestication of animals

as well as plants by intentionally selecting for better individual per-

formers. Producers then fully exploited the natural variation towards

producing even better crops, trees and animals.

In modern days, the role played by breeders has not deviated from

that played by early producers. Plant breeders use variation that

exists within each crop to produce superior varieties. Producers

gain access to the varieties and plant them in large numbers for eco-

nomic purposes.

Conventional breeders rely on the use of classical genetic principles

based on the phenotype or physical characteristics of the organism

concerned. They have used this approach for many years to intro-

duce desirable traits into crop varieties.

In a conventional cross, each parent donates half of its genetic mate-

rial to make up the progeny. However, there is usually a transfer of

favourable as well as unfavourable traits. These unfavourable traits

have to be removed through successive generations. In each genera-

tion the progeny would have to be tested for all the desired traits to

make sure they are not lost.

This is really a lengthy process as many generations would then

be required before the combination of all desired traits is found.

Not only is the process time consuming, it is also very expensive.

The adoption of biotechnology makes conventional breeding far

more efficient.

Modern agricultural biotechnology involves a number of tools that

breeders utilise to understand and manipulate the genetic make-up

of organisms for use in the production and processing of agricultural

products. We have tools available that can be used in the increasing

and stabilisation of yields; tools that can be used to improve resis-

tance to pests, diseases and lately to abiotic stresses and lastly tools

to also enhance the nutritional content of foods.

Very importantly, we have tools that generate unique genetic finger-

prints that both the breeders and producers can use. These finger-

prints can be used by breeders to check if there is any infringement

with the plant breeder’s rights and likewise to attain genetic purity in

their breeding material. The fingerprints are also used in the mainte-

nance of the genetic resources that the ARC has.

These tools are collectively called DNA molecular markers. The

markers are segments of DNA that we use as indicators for the

presence of the target genes. Keep in mind that all traits that pro-

ducers and breeders are interested in, are controlled by the genes;

singularly or as a group of genes.

Breeders cannot depend on phenotypic or visual selection alone.

There are certain important traits that the breeder cannot easily

select for without the intervention of molecular tools. For example,

in breeding for lines with an improved resistance to a particular

disease it may not be easy to visually detect the presence of more

than one gene in a line.

Some genes, although present in a line, may not all be expressed,

thus the breeder may throw out a potential line. However, with the

use of molecular marker tools we are able to accurately tag the

genes. Markers are completely independent of the environment and

thus the detection of one or more genes will not be affected. Another

advantage of using markers is that the detection can be done at the

seedling stage without having to waste time and effort for the plant

to mature.

An example in

Figure 1

is where we had crossed a disease resist-

ant bean plant with a susceptible one. We attained the F1 genera-

tion, self-pollinated it to obtain the F2 generation that showed a

segregating pattern. We were able to help the breeder pick up resist-

ant plants (R) that emulated the donor parent, pick up susceptible

plants (S) that emulated the recipient parent and the interme-

diate plants (I) that showed various disease reactions. The interme-

diate plants clearly possessed segments from both parents and

were segregating for the disease.

Another example in

Figure 2

is where we generated genetic fin-

gerprints for use by the breeders. One breeder had to investigate

whether the three samples used in making crosses were in fact simi-

lar or different. To the breeder’s surprise, the analysis revealed that

samples 1 and 2 were similar and sample 3 completely differed from

the rest.

The breeder was now in a far better position to make informed deci-

sions about future crosses to be made.

LEBOGANG MADUBANYA,

ARC-Grain Crops Institute

Figure 1: An example of marker-assisted selection in a segregating

population.

R= resistant, S= susceptible and I= intermediate

In modern days, the role

played by breeders has not

deviated from that played by

early producers.

“