Principles of Plant Genetics and Breeding. George Acquaah

emasculationIn species such as alfalfa, the raceme is immersed in 57% ethanol for 10 seconds and then rinsed in water for a few seconds.

       Commercial gametocidesThese are chemicals designed to kill the anthers (e.g. sodium methyl arsenate).

      If pollination will not immediately follow emasculation, the flowers should be covered to exclude contaminating pollen from elsewhere. Once properly pollinated, the flower should be tagged for identification.

6.7 Pollination

      Success of pollination depends on pollen maturity, quality (freshness), and timing of pollination, among other factors.

      6.7.1 Pollen collection and storage

      In some species (e.g. soybean) pollination immediately follows emasculation. In this case, there is no need for storage. Fresh pollen gives the best success of crossing. Good pollen flowers may be picked and placed in a Petri dish or some suitable container for use. In some species, mechanical vibrations may be used to collect pollen. Pollen is most copious at peak anthesis. Generally, pollen loses viability quickly. However, in some species, pollen may be stored at a cool temperature and appropriate humidity for the species for an extended period of time.

      6.7.2 Application of pollen

      Commonly, pollen is applied directly to the stigma by using a fine brush or dusting off the pollen onto the stigma directly from the flower of the pollen source (e.g. the staminal column may be used as brush). Sometimes, objects such as cotton bulbs or toothpicks are used to deposit pollen on the stigma. In some flowers, pollen deposition is made without direct contact with the stigma. Instead, pollen may be injected or dusted into a sack covering the emasculated inflorescence and agitated to distribute the pollen over the inflorescence. A key precaution against contamination during pollination is for the operators to disinfect their hands and tools between pollinations, when different varieties are involved. It is critical to tag the pollinated flower for identification at the time of harvesting.

      6.7.3 Tagging after pollination

      After depositing the desired pollen, it is critical to identify the flowers that were pollinated with an appropriate tag or label. The information on the label should include the date of emasculation, date of pollination, name of seed parent, and name of pollen parent. The tag should be attached to the pedicel of the emasculated flower, not the branch.

6.8 Number of F1 crosses to make

      There are practical factors to consider in deciding on the number of crosses to make for a breeding project. These include the ease of making the crosses from the standpoint of floral biology, and the constraints of resources (labor, equipment, facilities, and funds). It will be easier to make more crosses in species in which emasculation is not needed (e.g. monoecious and dioecious species) than in bisexual species. Some breeders make a small number of carefully planned crosses, while others make even thousands of cross combinations.

Generally, a few hundred cross combinations per crop per year would be adequate for most purposes, for species in which the F₁ is not the commercial product. More crosses may be needed for species in which hybrids are commonly produced for the purpose of discovering heterotic combinations. As will be discussed next, breeding programs that go beyond the F₁ usually require very large F₂ populations. Regarding the number of flowers per cross combination, there is variation according to fecundity. Species such as tomato may need only one or two crosses, since each fruit contains over 100 seeds. Plants that tiller also produce large numbers of seed. Each crop species has its own reproduction rate, which may be huge (e.g. tobacco: 1000s of seeds produced per plant, 100s per bowl) or relatively small (e.g. pea: about 100 per plant, about 2–5 per pod).

6.9 Genetic issues in hybridization

      Because hybridization involves combining two sets of genes in a new genetic matrix through the meiotic process, it is accompanied by a variety of genetic‐based effects.

      6.9.1 Immediate effect

      The immediate effect of hybridization is the assembly of two different genomes into a newly created individual. Several genetic consequences may result from such union of diverse genomes, some of which may be desirable, whereas others may not be desirable. The key ones are as follows:

       Expression of recessive lethal geneCrossing may bring together recessive lethal genes (that were in the heterozygous state) into the expressible homozygous state. The resulting hybrid may die or lose vigor. By the same token, hybridization can also mask the expression of a recessive allele by creating a heterozygous locus. Individuals carry a certain genetic load (or genetic burden), representing the average number of recessive lethal genes carried in the heterozygous condition by an individual in a population. Selfing or inbreeding predisposes an individual to having deleterious recessive alleles that were protected in the heterozygous state to becoming expressed in the homozygous recessive form.

       Hybrid necrosisEspecially the crossing of parents that are somewhat distantly related (but still the same crop species), may result in the phenomenon of hybrid necrosis. Interactions between pairs of genes in both parents may work out unfavorably to the physiology of the plant. This phenomenon has been reported in wheat and rye, but also in Arabidopsis.

       HeterosisGenes in the newly constituted hybrid may complement each other to enhance the vigor of the hybrid. The phenomenon of hybrid vigor (heterosis) is exploited in hybrid seed development (see Chapter 19).

       Transgressive segregationHybrids have features that may represent an average of the parental features, or a bias toward the features of one parent, or even new features that are unlike either parent (transgressive segregates). When the parents “nick” in a cross, transgressive segregates with performance superseding either parent is likely to occur in the segregating population.

       Genome‐plastome incompatibilityPlastomes (the genetic material found in plastids such as in chloroplasts) and genomes in most genera function to form normal plants, regardless of the taxonomical distances between the plastid and nuclear genomes. However, in some genera, plastomes and genomes, having co‐evolved to a significant degree, are only compatible within a specific combination.

      6.9.2 Subsequent effect

      The subsequent effect of hybridization, which is often the reason for hybridizing parents by breeders, occurs in the F₂ and later generations. By selfing the F₁ hybrid, the parental genes are reorganized into new genetic matrices in the offspring. This occurs through the process of meiosis, a nuclear division process that occurs in flowering plants. Contrasting alleles segregate and subsequently recombine in the next generation to generate new variability. Furthermore, the phenomenon of crossing over that leads to the physical exchange of parts of chromatids from homologous chromosomes provides an opportunity for recombination of linked genes, also leading to the generation of new variation.

      6.9.3 Gene recombination in the F₂

      The goal of crossing for generating variability for selection is to produce a large number of gene recombinations from the parents used in the cross. In hybrid seed programs, the F₁ is the end product for commercial use. However, in other crosses, the F₂ and subsequent generations are evaluated to select genotypes that represent the most desirable recombination of parental genes. The F₂ generation has the largest number of different gene combinations of any generation following a cross. The critical question in plant breeding is the size of F₂ population to generate in order to have the chance of including that ideal recombinant this is homozygous for all the desirable genes in the parent. Three factors determine the number of gene recombination that would