O.H. (1981). Nature and mechanism of cytoplasmic male sterility in plants: a review. Hort Science 16: 482–487.13 Pelletier, G. and Budar, F. (2007). The molecular biology of cytoplasmically inherited male sterility and prospects for its engineering. Current Opinion in Biotechnology 18: 121–125.
14 Schnable, P.S. and Wise, R.P. (1998). The molecular basis of cytoplasmic male sterility and fertility restoration. Trends in Plant Science 3: 175–180.
15 Stephens, J.C. and Holland, P.F. (1954). Cytoplasmic male sterility for hybrid sorghum seed production. Agronomy Journal 46: 20–23.
16 Stern, K.R. (1997). Introductory Plant Biology, 7e. Wm. C Brown Publishers.
17 Zhang, X., Wang, X., Jiang, P., and Zhu, W. (2010). Inheritance of fertility restoration for cytoplasmic male sterility in a new Gossypium barbadense restorer. Agricultural Sciences in China 9 (4): 472–480.
Outcomes assessment
Part A
Please answer the following questions true or false:
1 Biennial plants complete their lifecycle in two growing seasons.
2 A staminate flower is a complete flower.
3 Self‐pollination promotes heterozygosity of the sporophyte.
4 The union of egg and sperm is called fertilization.
5 A branched raceme is called a panicle.
6 The carpel is also called the androecium.
Part B
Please answer the following questions:
1 Plants reproduce by one of two modes, …………………….. or …………………
2 Distinguish between monoecy and dioecy.
3 …………………… is the transfer of pollen grain from the anther to the stigma of a flower.
4 What is self‐incompatibility?
5 Distinguish between heteromorphic self‐incompatibility and homomorphic self‐incompatibility.
6 What is apomixis?
7 Distinguish between apospory and displospory as mechanisms of apomixis
Part C
Please write a brief essay on the following topics:
1 Discuss the genetic and breeding implications of self‐pollination.
2 Discuss the genetic and breeding implications of cross‐pollination.
3 Fertilization does not always follow pollination. Explain.
4 Discuss the constraints of sexual biology in plant breeding.
5 Discuss how cytoplasmic male sterility (CMS) is used in a breeding program.
6 Discuss how genetic male sterility is used in a breeding program.
Purpose and expected outcomes
One of the principal techniques of plant breeding is artificial mating (crossing) of selected parents to produce new individuals that combine the desirable characteristics of the parents. This technology is restricted to sexually reproducing species that are compatible. However, in the quest for new desirable genes, plant breeders sometimes attempt to mate individuals that are biologically distantly related. In such interspecific crosses pre‐ and post‐fertilization barriers may occur. It is important for the breeder to understand the problems associated with making a cross, and how barriers to crossing, where they exist, can be overcome. After studying this chapter, the student should be able to:
1 Define sexual hybridization and discuss its genetic consequences.
2 Define a wide cross and discuss its objectives and consequences.
3 Discuss the challenges to wide crosses and techniques for overcoming them.
6.1 Concept of gene transfer and hybridization
Crop improvement typically involves the transfer of genes from one source or genetic background to another, or combining genes from different sources that complement each other, with the hope that the new cultivar will combine the best of both parents, while being distinct from both. When a plant breeder has decided on the combination of traits that he wishes to be incorporated in the new cultivar to be developed, the next crucial step is to find one or more sources of the appropriate gene(s) for such characters. In flowering species, the conventional method of gene transfer or gene combination is by crossing or sexual hybridization. This procedure causes genes from the two parents to be assembled into a new genetic matrix. It follows that if parents are not genetically compatible, gene transfer by sexual means cannot occur at all, or at best, may be fraught with complications. The product of hybridization is called a hybrid.
Sexual hybridization can occur naturally through agents of pollination. Even though self‐pollinating species may be casually viewed as “self‐hybridizing,” the term hybridization is reserved for crossing between unidentical parents (the degree of divergence is variable). Artificial sexual hybridization is the most common conventional method of generating a segregating population for selection in breeding flowering species. In some breeding programs, the hybrid (F1) is the final product of plant breeding (see hybrid breeding in Chapter 19). However, in most situations, the F1 is selfed (to give an F2) to generate recombinants (as a result of recombination of the parental genomes) or a segregating population, in which selection is practiced. In clonally propagated crops the F1 usually segregates sufficiently, and its clonally produced descendants will be submitted to selection without further crossing or selfing.
The tools of modern biotechnology now enable the breeder to transfer genes by circumventing the sexual process (i.e. without crossing). More significantly, gene transfer can transcend natural reproductive or genetic barriers. Transfers can occur between unrelated plants and even between plants and animals (by genetic transformation, see Chapter 24).
6.2 Applications of crossing in plant breeding
Sometimes, crossing is done for specific purposes, within the general framework of generating variability. Hybridization precedes certain methods of selection in plant breeding to generate general variability.
Gene transferSometimes, only a specific gene (or a few) needs to be incorporated into an adapted cultivar. Crossing is used for the gene transfer process, followed by additional strategic crossing to retrieve the desirable genes of the adapted cultivar (see backcrossing in
