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.

       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).