Methods of Crop Improvement and Production of Hybrid Seeds

April 7, 2023 Agric4Profits

There are three general methods of crop improvements and production of hybrid seeds namely: Introduction, Selection, and Hybridization.

1. Introduction

This is the act of importing crop variety from other countries to be used as foundation stock in breeding programme. Crop introduction from other countries may be of superior productivity and they often provide better foundation stock for breeding.

Foreign varieties may possess resistance to some diseases or insects pests or may have some useful characteristics that can be transferred to adapted varies by hybridisation.

Collection of exotic and domestic wild and cultivated species and strains of various crop plants are being screened for disease and insect resistance, and other characteristics that might be useful in breeding.

When some variety, strain or inbred line is found to be outstanding for some desired character such as disease resistance, heterosis capacity or cytoplasmic sterility, it is likely to be used by most plant breeders.

This results in narrow genetic base, which could be disastrous when a new or minor race of parasite multiplies and almost destroys previously resistant varieties or hybrids that are widely grown.

2. Selection

Mass selection is a quick method of purifying or improving mixed or un-adapted crop varieties. It is done by selecting a large number of plants of the desired characteristics and then increasing the progeny. It serves to eliminate undesirable types.

2a. Pure-line or pedigree selection

Pure-line pedigree selection or individual plant selection consists of growing individual progenies of each selected plant so that their performances can be observed, compared and recorded. Only a few superior strains among the numerous original selections are saved for advanced testing.

In crops that are partly or largely crossed-fertilized selection must be repeated until the strains appear to be uniform. Also cross-fertilized crops selected for propagation must be self-pollinated under controlled conditions. This may be done by covering the floral parts to exclude foreign pollen or by hand pollinating, or both.

Pure-line selection offers a quick means of segregating desired types from mixed varieties. In cross-pollinated crops or when the original variety and the selections differ by several genetic factors, several (five or more) generations of selection may be required to purify the strains so that each would continue to breed true thereafter while being tested for yield.

The major limitations of this method are the partial inbreeding that reduces yield as well as the lack of complete control of the pollen parentage of the seed ears.

2b. Pure-line concept

The modern methods of handling, testing, and increasing individual selections of self-fertilized crops had as their basis the pure-line concept proposed in 1903 by W. L. Johansson who worked with beans.

According to this concept variations in the progeny of a single plant of a self-fertilized species are not heritable but are due to environmental effects. Failure to recognise this principle before resulted in some futile works in continuously selecting self-fertilized crops such as wheat and barley.

When homozygous self-fertilized plants are selected, the progeny is pure for all its characters until outcrossing or mutations occur. Pure-line comprises the descendants of one or more individuals of like germinal constitution that have undergone no germinal change.

3. Hybridization

Hybridization is the only effective means of combining the desirable characters of two crop varieties.

The first step in breeding by hybridisation is to choose parents that can supply the important character or characters that a good standard variety lacks. It is important to have in mind definite characteristics.

The seeds that develops from the cross-pollinated flowers, when planted, produce plants of the first filial of F₁ generation.

These plants should be all alike, only the dominant and mutual recessive characters being expressed. In the second F₂ generation, the plants break up or segregate into all possible combinations of the dominant and recessive characters of the two parents.

Plants of the types desired are selected, these and several subsequent generations are handled as previously described under selection methods.

Reselection continues thereafter until the desired strains are uniform usually for three to six generations. However, additional true breeding plants of the desired recombination would be obtained in later generations from segregating F₂ lines.

3a. Inbreeding of Cross-fertilized crops

Before making hybrids in cross-fertilized crops, it may be desirable to select and inbreed the varieties for several generations or until they are reasonably pure for the characters desired. Without such inbreeding, the hybrids that are obtained from cross-fertilized crops cannot be reproduced.

Artificial self-pollination in a normally cross-fertilized crop leads to segregation into pure uniform (homozygous) lines. There is often rapid reduction in vigor when self-pollination is practiced. When appropriate lines of self-pollinated plants are intercrossed there is usually a restoration of vigor, a fact that is used in hybrid corn production.

In addition, certain abnormalities such as sterility, poor chlorophyll development, dwarf habit, lethal seedlings, and susceptibility to diseases may appear as result of inbreeding.

3b. Back–crossing

First generation plants are backcrossed with the good variety. Thereafter in each segregating generation a number of plants approaching the desired new recombination are selected and backcrossed with the good commercial parent.

Backcrossing is a useful method of breeding when it is desired to add only one or two new characters to an otherwise desirable variety. In this method the good variety is crossed with one having the other character desired.

Usually, with repeated rigid selection, the improved type is recovered in approximately five backcrosses.

3c. Bulk Propagation of Hybrid Material

In the bulk method of handling hybrid material the entire population from a cross is grown and harvested in mass each year.

Successive crops of seed are sown for five to ten years to allow natural selection for characters, such as cold resistance or insect resistance, to take place.

By this time the poorly adapted strains are largely eliminated, while the remaining strains are mostly true breeding (or homozygous). Plants are then selected, and the progenies tested as previously described.

Production of Hybrid Seed

Commercial hybrid seed of corn can be produced by detasseling the seed-parent rows in a cross-pollination field, or by use of cytoplasmic male-sterile. Pearl millet hybrids have been produced by planting a mixture of four selected lines.

They can also be produced by clipping (topping) the upper part of the heads on the seed-parent plants before they shed pollen, but after the stigmas are exerted or by pollinating cytoplasmic male sterile lines.

Hybrid seeds of crops that naturally are wholly or largely self-pollinated can be produced economically only by the use of male-sterile lines. Plants with cytoplasmic male-sterility have been discovered or developed in many crops.

A related strain that carries the recessive sterile-producer gene is used as a pollinator to maintain or to increase the male-sterile lines. An unrelated variety or strain that carries the dominant fertility-restorer gene is used as a pollen parent to produce commercial hybrid seed.

In summary, the Mendel’s laws of inheritance and how these laws now form the basis for all studies of heredity or genetics. Crop improvement has recorded tremendous progress in the last century due to advances in genetic engineering, genetic mapping and cloning.

The traditional methods of crop improvement e.g. selection, introduction and hybridization as well as production of hybrid seeds were described in this unit. The breeding of more productive types is the only stable means of increasing crop yield; therefore improved varieties are fast replacing inferior local varieties.