Principles of Plant Genetics and Breeding. George Acquaah

for interspecific gene transfer. Advances in Agronomy 43: 199–240.

      5 Stalker, H.T. (1980). Utilization of wild species for crop improvement. Advances in Agronomy 33: 111–147.

      6 Stoskopf, N.C. (1993). Plant Breeding: Theory and Practice. Boulder, CO: Westview Press.

      7 Zohary, D. (1973). Gene‐pools for plant breeding. In: Agricultural Genetics (ed. R. Moav). New York: Wiley.

Internet resources

      http://csf.colorado.edu/perma/stse/isolate.htm – Basic isolation practices for reducing or eliminating natural cross‐pollination in field crossing.

      http://www.actahort.org/books/200/200_3.htm – Application of wide crosses in tomato improvement.

Outcomes assessment

      Part A

      Please answer the following questions true or false:

      1 A hybrid is a product of unidentical parents.

      2 Emasculation is undertaken to make a flower female.

      3 An intergeneric cross occurs between two species.

      4 Wheat is a product of a wide cross.

      5 Bridge crosses are used to facilitate crosses between two parents of identical ploidy levels.

      Part B

      Please answer the following questions:

      1 What is hybridization?

      2 What are wide crosses?

      3 Give three specific reasons why wide crosses may be undertaken.

      4 Explain the phenomenon of linkage drag.

      5 Give examples of major crops that arose through wide crosses.

      Part C

      Please write a brief essay on each of the following topics:

      1 Discuss the basic steps in artificial hybridization.

      2 Discuss the challenges of wide crosses.

      3 Discuss the techniques used for overcoming the challenges to wide crosses.

      4 Discuss the technique of bridge crossing.

      5 Discuss the genetic issues in hybridization.

7 Clonal propagation and in vitro culture

      Purpose and expected outcomes

       Conventional plant breeding entails sexual recombination between plants to obtain novel and useful combinations of traits in the resulting new product that can be reproduced more or less “true to type.” Clonal propagation on the other hand does not entail recombination, but rather the asexual multiplication of plants such that uniformity and identity are preserved intact. Plant propagators and horticulturists commonly use clonal propagation in their operations. Sometimes, plant breeders need a large number of clones for genetic studies. Modern plant breeding tools such as biotechnology include the use of techniques for generating clones and for genetic modification under aseptic conditions.

      After completing this chapter, the student should be able to discuss:

      1 Micropropagation and its applications in breeding.

      2 The importance of cell and tissue culture in plant breeding.

      3 The characteristics of asexual propagation that have breeding implications.

      4 Apomixis and the breeding of apomictic species.

      5 The advantages and limitations of clonal propagation.

7.1 What is a clone?

      In biology, a clone is an organism whose genetic information is identical to that of the parent organism (progenitor) from which it was created. In plants, a clone means a genetically uniform plant material derived from a single individual and propagated exclusively by vegetative (non‐seed) methods. Clones are not produced via recombination and meiosis, but from replication and mitosis. Plant breeders generally make a cross to obtain from true seeds a segregating progeny that will be followed by cycles of clonal reproduction, during which the superior types will be maintained, and the inferior types discarded. There is therefore no further genetic segregation during these cycles of clonal reproduction (in contrast to sexual progeny obtained from crosses between homozygous autogamous parents and allogamous populations).

7.2 Clones, inbred lines, and pure lines

As previously discussed, plants may be naturally sexually or asexually propagated. Further, sexually propagated species may be self‐fertilized or cross‐fertilized. These natural modes of reproduction have implications in the genetic structure and constitution of plants and breeding implications as already discussed. Plant breeders are able to manipulate the natural reproductive systems of species to develop plants that have atypical genetic constitution. The terms pure line, inbred line, and clone are applied to materials developed by plant breeders to connote sameness of genetic constitution in some fashion. However, there are some significant distinctions among them.

       Pure linesThese genotypes are developed as cultivars of self‐pollinated crops for direct use by farmers. As products of repeated selfing of single plants, pure lines are homogeneous and homozygous and can be naturally maintained by selfing.

       Inbred linesThese are genotypes that are developed to be used as parents in the production of hybrid cultivars and synthetic cultivars in the breeding of cross‐pollinated species. They are not meant for direct release for use by farmers. They are homogenous and homozygous, just like pure lines. However, unlike pure lines, they need to be artificially maintained because they are produced by forced selfing (not natural selfing) of naturally cross‐pollinated species.

       ClonesClones are identical copies of a genotype. Together, they are phenotypically homogeneous. However, individually, they are highly heterozygous. Asexually or clonally propagated plants produce genetically identical progeny.

7.3 Categories of clonally propagated species based on economic use

      Clonally propagated species may be divided into several broad categories on the basis of economic use:

       Those cultivated for a vegetative product – Important species vegetatively cultivated for a vegetative product include sweet potatoes, yams, cassava, sugarcane, and Irish potatoes. These species tend to exhibit certain reproductive abnormalities. For example, flowering is reduced, and so is fertility. Some species such as potatoes have cytoplasmic male sterility. Sometimes, flowering is retarded (e.g. by chemicals) in production (e.g. in sugarcane).

       Those cultivated for a fruit – Plants in this category include fruit trees and cane fruits. Examples include apple, pear, grape, strawberry, and banana.

       Those cultivated for floral products – Plant in this category include tulips and many cut flower species.

7.4 Categories of clonally propagated species for breeding purposes

      For breeding purposes, vegetatively propagated crops may be grouped into four types based on