Principles of Plant Genetics and Breeding. George Acquaah

the concept of general worth of a plant.

      3 Discuss the concept of intuitive selection.

      4 Discuss the application of combining ability analysis in plant breeding.

      5 Discuss a method of estimating heritability of a trait.

5 Introductionto reproduction

      Purpose and expected outcomes

       Rudolph Camerarius is credited with establishing sexual differentiation, noting that male and female sex organs exist in the Plant Kingdom. Some species produce flowers while others do not. In flowering species, reproduction involves the union of gametes, following pollination. Plant breeders need to understand the mode of reproduction in order to manipulate plants effectively to develop new and improved ones for crop production. After studying this chapter, the student should be able to:

      1 Discuss the importance of the mode of reproduction to plant breeding.

      2 Distinguish between self‐pollination and cross‐pollination.

      3 Discuss the natural barriers that favor or hinder each of the modes of reproduction.

      4 Discuss the implications of mode of reproduction in schemes and strategies employed in plant breeding.

      5 Discuss the use of male sterility and self‐incompatibility in breeding.

      6 Discuss the natural mechanisms that favor allogamy.

      7 Discuss the genetic consequences of allogamy.

      8 Discuss the implications of allogamy in crop improvement.

5.1 Importance of mode of reproduction to plant breeding

      Plant breeders need to understand the reproductive systems of plants for the following key reasons:

       The genetic structure of plants depends on their mode of reproduction. Methods of breeding are generally selected such that the natural genetic structure of the species is retained in the cultivar. Otherwise, special efforts will be needed to maintain the newly developed cultivar in cultivation.

       In flowering species, artificial hybridization is needed to conduct genetic studies to understand the inheritance of traits of interest, and for transfer of genes of interest from one parent to another. To accomplish this, the breeder needs to understand thoroughly the floral biology and other factors associated with flowering in the species.

       Artificial hybridization requires an effective control of pollination so that only the desired pollen is allowed to be involved in the cross. To this end, the breeder needs to understand the reproductive behavior of the species. Pollination control is critical to the hybrid seed industry.

       The mode of reproduction also determines the procedures for multiplication and maintenance of cultivars developed by plant breeders.

5.2 Overview of reproductive options in plants

      Four broad and contrasting pairs of reproductive mechanisms or options occur in plants.

       Hermaphrodity versus unisexualityHermaphrodites have both male and female sexual organs and hence may be capable of self‐fertilization. On the other hand, unisexuals, having one kind of sexual organ, are compelled to cross‐fertilize. Each mode of reproduction has genetic consequences, hermaphrodity promoting a reduction in genetic variability, whereas unisexuality, through cross‐fertilization, promotes genetic variability.

       Self‐pollination versus cross‐pollinationHermaphrodites that are self‐fertile may be self‐pollinated or cross‐pollinated. In terms of pollen donation, a species may be autogamous (pollen comes from the same flower – selfing), or allogamous (pollen comes from a different flower). There are finer differences in these types. For example, there may be differences between the time of pollen shed and stigma receptivity.

       Self‐fertilization versus cross‐fertilizationJust because a flower is successfully pollinated does not necessarily mean fertilization would occur. The mechanism of self‐incompatibility causes some species to reject pollen from their own flowers, thereby promoting outcrossing.

       Sexuality versus asexualitySexually reproducing species are capable of providing seed through sexual means. Asexuality manifests in one of two ways – vegetative reproduction (in which no seed is produced), or agamospermy (in which seed is produced).

5.3 Types of reproduction

      Plants are generally classified into two groups based on mode of reproduction: sexually reproducing or asexually reproducing. Sexually reproducing plants produce seed as the primary propagule. Seed is produced after sexual union (fertilization) involving the fusion of sex cells or gametes. Gametes are products of meiosis and, consequently, seeds are genetically variable. Asexual or vegetative reproduction mode entails the use of any vegetative part of the plant for propagation. Some plants produce modified parts such as creeping stems (stolons or rhizomes), bulbs, or corms, which are used for their propagation. Asexual reproduction is also applied to the condition whereby seed is produced without fusion of gametes (called apomixis). It should be pointed out that some plants could be reproduced by either the sexual or asexual mode. However, for either ease of propagation or for product quality, one mode of reproduction, often the vegetative mode, is preferred. Such is the case in flowering species such as potato (propagated by tubers or stem cuttings) and the sugarcane (propagated by stem cuttings).

5.4 Sexual reproduction

      Sexual reproduction increases genetic diversity through the involvement of meiosis. Flowering plants dominate the terrestrial species. Whereas flowers are required, flowering plants may reproduce sexually or asexually.

      5.4.1 Sexual lifecycle of a plant (alternation of generation)

      The normal sexual lifecycle of a flowering plant may be simply described as consisting of events from first establishment to death (from seed to seed in seed‐bearing species). A flowering plant goes through two basic growth phases – vegetative and reproductive, the former preceding the latter. In the vegetative phase, the plant produces vegetative growth only (stem, branches, leaves, etc., as applicable). In the reproductive phase, flowers are produced. In some species, exposure to a certain environmental factor (e.g. temperature, photoperiod) is required to switch from vegetative to reproductive phase. The duration between phases varies among species and can be manipulated by modifying the growing environment.

In order for sexual reproduction to occur, two processes must occur in sexually reproducing species. The first process, meiosis, reduces the chromosome number of the diploid (2n) cell to the haploid (n) number. The second process, fertilization, unites the nuclei of two gametes, each with the haploid number of chromosomes, to form a diploid. In most plants, these processes divide the lifecycle of the plant into two distinct phases or generations, between which the plant alternates (called alternation of generation) (Figure 5.1). The first phase or generation, called the gametophyte generation, begins with a haploid spore produced by meiosis. Cells derived from the gametophyte by mitosis are haploid. The multicellular gametophyte produces gametes by mitosis. The sexual reproductive process unites the gametes to produce a zygote that begins the diploid sporophyte generation phase.

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