Single crossIf two elite lines are available that together possess all desired traits at adequate levels, one cross (single cross [A × B]) may be all that is needed in the breeding program.
Three‐way crossSometimes, for combining all desirable traits several cultivars or elite germplasm are required, since each pair may have some negative traits in common. In this case, multiple crosses may be required in order to have the opportunity of obtaining recombinants that combine all the desirable traits. The method of 3‐way crosses ([A × B] × C) may be used. If a 3‐way cross product will be the cultivar, it is important that especially the third parent (C) be adapted to the region of intended use, since it contributes more genes than each of the A and B parents.
Double crossA double cross is a cross of two single crosses ([A × B] × [C × D]). The method of successive crosses is time consuming. Further, the complex crosses such as double cross have a low frequency of yielding recombinants in the F2 that possess a significant number of desirable parental genes. When this method is selected, the number of targeted desirable traits should be small (at most about 10). The double‐cross hybrid is genetically more broad‐based than the single‐cross hybrid but is more time consuming to make.
Diallel crossA diallel cross is one in which each parent is crossed with every other parent in the set (complete diallel), yielding n − (n−1)/2 different combinations (where n = number of entries). This method entails making a large number of crosses. Sometimes, the partial diallel is used in which only certain parent combinations are made. The method is tedious to apply to self‐pollinated species. Generally, it is a crossing method for genetic studies, and less for the purpose of creating populations for breeding.
Figure 6.2 The basic types of crosses used by plant breeders. Some crosses are divergent (a) while others are convergent (b).
6.10.2 Convergent crosses
These are conservative methods of crossing plants. The primary goal of convergent crossing is to incorporate a specific trait into an existing cultivar without losing any of the existing desirable traits. Hence, one (or several) parents serve as a donor of specific genes and is usually involved in the cross only once. Subsequent crosses entail crossing the desirable parent (recurrent parent) repeatedly to the F1, in order to retrieve all the desirable traits. A commonly used convergent cross is the backcross (see Chapter 17).
6.11 Wide crosses
The first choice of parents for use in a breeding program is cultivars and experimental materials with desirable traits of interest. Most of the time, plant breeders make elite × elite crosses (they use adapted and improved materials). Even though genetic gains from such crosses may not always be dramatic, they are nonetheless significant enough to warrant the practice. After exhausting the variability in the elite germplasm as well as in the cultivated species, the breeder may look elsewhere, following the recommendation by Harlan and de Wet. These researchers proposed that the search for desired genes should start from among materials in the primary gene pool (related species), then proceed to the secondary gene pool, and if necessary, the tertiary gene pool. Crossing involving materials outside the cultivated species is collectively described as wide crosses. When the wide cross involves another species, it is called an interspecific cross (e.g., kale). When it involves a plant from another genus, it is called an intergeneric cross (e.g. wheat) (see Box 6.1). Crosses between crops with their wild progenitor species should not be considered wide crosses, despite the sometimes‐used different scientific names (barley, Hordeum vulgare, was derived from H. spontaneum; lettuce, Lactuca sativa, was derived from L. serriola). Genetically such “species” are fully compatible and behave genetically as an intraspecific cross (i.e. cross within the same species).
Bryan Kindiger
USDA‐ARS, Grazinglands Research Laboratory, 7207 West Cheyenne St., El Reno, OK 73036 USA
A historical review
Research in maize‐Tripsacum hybridization is extensive and encompasses a period of more than 60 years of collective research. A vast amount of literature exists on various facets of this type of hybridization ranging from agronomy, plant disease, cytogenetics, breeding, and genetic analysis. Consequently, no single article can cover all the research relevant to this topic. This report will not address all the various issues, but focus primarily on specific research and experiments which would perhaps be of value to a student interested in this topic. The interested student is encouraged to review the references below and follow the additional references cited by the various authors to obtain more information on this topic.
Interspecific hybrids, or hybrids generated between species, are utilized by plant breeders to discover and transfer genes from one plant to another plant of a related species which cannot be found in the particular species of interest. One of the most interesting instances of interspecific hybridization is that between maize (Zea mays L.) and its most distant relative gamagrass (Tripsacum spp.). Tripsacum L. is a perennial, warm season bunch grass found throughout most of the subtropical and tropical regions of the Western Hemisphere (de Wet et al. 1981, 1982) (Figure B6.1).
Figure B6.1 A stand of Tripsacum dactyloides (eastern gamagrass) in Woodward, County, OK, USA. Individual standing in the gamagrass is Dr. Victor Sokolov, Institute of Cytology and Genetics, Novosibirsk, Russia.
Approximately 16 species have been classified taxonomically in Tripsacum. The most common species is Tripsacum dactyloides L. that can be found growing in much of the USA, Mexico, Central America, and South America (de Wet et al. 1981, 1982). The most commonly studied maize‐Tripsacum interspecific hybrids are those generated between diploid maize (2n = 2x = 20) and tetraploid Tripsacum dactyloides (2n = 4x = 72). Regardless of their complete difference in chromosome number, plant phenotype, and environmental niche, hybrids are relatively easy to generate.
In 1939, Mangelsdorf and Reeves published their historical monograph “The Origin of Indian Corn and Its Relatives,” in which they discussed their research and views on the relationship of cultivated Zea mays to its distant cousins, teosinte (the closest relative of maize) and Tripsacum spp. (Mangelsdorf and Reeves 1939). Though these early views regarding the origins of maize and its relationship to teosinte and Tripsacum are controversial and are open to discussion and further investigations, the procedures for generating such interspecific hybridizations remains relatively unchanged.
