Principles of Plant Genetics and Breeding. George Acquaah

may condition a specific disease or pest resistance, or may be a product quality trait, flower shape, or color novelty in ornamentals, among other traits. In some species such as sugarcane, cotton, sorghum, and potato, hybrid vigor is known to have accompanied certain crosses.

       New character expressionNovelty is highly desirable in the ornamental industry. Combining genomes from diverse backgrounds may trigger a complementary gene action or even introduce a few genes that could produce previously unobserved phenotypes that may be superior to the parental expression of both qualitative and quantitative traits.

       Creation of new alloploidsWide crosses often produce sterile hybrids. The genome of such hybrids can be doubled to create a new fertile alloploid species (a polyploid with the genomes of different species), such as triticale, which is a synthetic species which consists of the genomes of (mostly) tetraploid wheat and rye.

       Scientific studiesCytogenetic studies following a wide cross may be used to understand the phylogenic relationships between the species involved.

       Curiosity and esthetic valueWide crosses may produce unique products of ornamental value, which can be useful to the horticultural industry. Sometimes just being curious is a good enough reason to try new things.

      6.11.2 Selected success with wide crosses

      Developing commercial cultivars with genes introduced from the wild can be an expensive and long process (see prebreeding in Chapter 8). Some linkages with genes of the wild donor need to be broken. In tomato, it took 12 years to break the linkage between nematode resistance and undesirable fruit characteristics. Nonetheless, some significant successes have been accomplished through wide crosses.

       Natural wide crossesNatural wide crosses have been determined by scientists to be the origin of numerous modern‐day plants of economic importance. Ornamentals such as irises, cannas, dahlias, roses, and violets are among the list of such species. In tree crops, apples, cherries, and grapes are believed to have originated as natural wide crosses, and so are field crops such as wheat, tobacco, and cotton, as well as horticultural crops like strawberry and sweet potatoes. Most natural wide cross products of economic value to modern society are used as ornamentals and are usually propagated vegetatively. This led G.L. Stebbins to observe that wide crosses may be more valuable in vegetatively propagated species than in seed‐propagated species.

       Synthetic (artificial) wide crosses.Apart from natural occurrences, plant breeders over the years have introgressed desirable genes into adapted cultivars from sources as close as wild progenitors to distant ones such as different genera. Practical applications of wide crosses may be grouped into three categories as follows:Gene transfer between species with the same chromosome numberWide crosses between two tomato species, Lycopersicon pimpinellifolium × L. esculentum, have been conducted to transfer resistance genes to diseases such as leaf mold and Fusarium wilt. Gene transfers in which both parents have identical chromosome numbers is often without complications beyond minor ones (e.g. about 10 percent reduction in pollen fertility). It is estimated that nearly all commercially produced tomatoes anywhere in the world carry resistance to Fusarium that derived from a wild source.Gene transfer between species with different number of chromosomesCommon wheat is a polypoid (an allohexaploid with a genomic formula of AABBDD). It has 21 pairs of chromosomes. There is diploid wheat, einkorn (Triticum monococcum), with seven pairs of chromosomes and a genomic formula of AA. It should be pointed out that later studies of the origin of the A genome showed that the diploid component of the Triticum genus is comprised of two distinct biological species, T. monococcum and T. urartu. The A in breadwheat is believed to be from T. urartu. There are several tetraploid wheats (AABB) such as emmer wheat (T. dicoccum). Transfer of genes from species of lower ploidy to common wheat is possible (but not always the reverse). Stem rust resistance is one such gene transfer that was successful.Gene transfer between two generaCommon wheat comprises three genomes of which one (DD) is from the genus Aegilops (A. tauschii). Consequently, gene transfers have been conducted between Triticum and Aegilops (e.g. for genes that confer resistance to leaf rust). Other important donors of resistance are Secale cereal (rye) and Agropyron sp.Developing new species via wide crossingA species is defined as a population of individuals capable of interbreeding freely with one another but which, because of geographic, reproductive, or other barriers, do not in nature interbreed with members of other species. One of the long‐term “collaborative” breeding efforts is the development of the triticale (X Triticosecale Wittmack). The first successful cross, albeit sterile, is traced back to 1876; the first fertile triticale was produced in 1891. The development of this new species occurred over a century, during which numerous scientists modified the procedure to reach its current status where the crop is commercially viable. Triticale is a wide cross between Triticum (wheat) and Secale (rye), hence triticale (a contraction of the two names). It is predominantly a self‐fertilizing crop. The breeding of triticale is discussed in Chapter 17.

6.12 Issue of reproductive isolation barriers

Hybridization is often conducted routinely without any problems when individuals from the same species are involved, provided there are no fertility regulating mechanisms operating. Even when such mechanisms exist, hybridization can be successfully conducted by providing appropriate pollen sources. Sometimes, plant breeders are compelled to introduce desired genes from distant relatives or other more or less related species. Crossing plants from two different species or sometimes even plants from two genera is challenging and has limited success. Often, the breeder needs to use additional techniques (e.g. embryo rescue) to intervene at some point in the process in order to obtain a mature hybrid plant. Reproductive isolation barriers may be classified into three categories (Table 6.2). These barriers maintain the genetic integrity of the species by excluding gene transfer from outside species. Some barriers occur before fertilization, some after fertilization. These barriers vary in degree of difficulty to overcome through breeding manipulations.

       Spatial isolationSpatial isolation mechanisms are usually easy to overcome. Plants that have been geographically isolated may differ only in photoperiod response. In this case, the breeder can cross the plants under a controlled environment (e.g. greenhouse) by manipulating the growing environment to provide the proper duration of day length needed to induce flowering.

       Pre‐fertilization reproductive barrierThese barriers occur between parents in a cross. Crops such as wheat have different types that are ecologically isolated. There are spring wheat types and winter wheat types. Flowering can be synchronized between the two groups by, for example, vernalization (a cold temperature treatment that exposes plants to about 3–4 °C) of the winter wheat to induce flowering (normally accomplished by exposure to the winter conditions). Mechanical isolation may take the form of differences in floral morphology that prohibit the same pollinating agent (insect) from pollinating different species. A more serious barrier to gene transfer is gametic incompatibility, whereby fertilization is prevented. This mechanism is a kind of self‐incompatibility (see Chapter 4). The mechanism is controlled by a complex of multiple allelic system of S‐genes that prohibit gametic union. The breeder has no control over this barrier.

       Post‐fertilization reproductive barriersThese barriers occur between hybrids. After fertilization, various hindrances to proper development of the embryo (hybrid) may arise, sometimes resulting in abortion of the embryo, or even formation of a haploid (rather than a diploid). The breeder may use embryo rescue techniques to remove the embryo and culture it to a full plant. Should the embryo develop naturally, the resulting plant may be unusable as a parent in future breeding endeavors because of a condition called hybrid weakness. This condition is caused by factors such as disharmony between the united genomes. Some hybrid plants may fail to flower because of hybrid sterility (F1 sterility) resulting from meiotic abnormalities. On some occasions, the hybrid weakness and infertility manifest in the F2 and later generations (called hybrid breakdown).