[OG bub]

Breeding Hybrid Varieties of Outcrossing Plants

Many species of cross-pollinated cultivated plants are seed-propagated, whereas other outcrossing species propagate themselves vegetatively as clones in nature and are also propagated as clones in cultivation. Breeding procedures appropriate to exclusively seed-propagated outcrossing crop species are the concern of this chapter; breeding procedures appropriate to species that are propagated in cultivation either exclusively or largely by vegetative means will be considered later.

In a population of seed-propagated randomly mating cross-pollinated plants every individual can be expected to be homozygous at many loci, but also heterozygous at many loci. As a consequence of the segregation and recombination that follows open outcrossing among genetically different plants, alleles of many loci are reshuffled and regrouped into vast numbers of multilocus allelic configurations each generation; an allele that is homozygous in one generation may become heterozygous in the next generation and vice versa. Analogously, alleles that are part of one multilocus configuration in one generation may be found in different multilocus combinations of alleles in subsequent generations. When many genetic loci are represented by more than one allele, it is likely that almost limitless numbers of genetic combinations will be present, and it is hence unlikely that any two individuals with precisely the same multilocus genotype will occur in successive generations in any large open-pollinated population. Under natural selection, whether in nature or in cultivation, only those individual alleles and those multilocus combinations of alleles that promote adaptedness and reproductive fitness in the particular shifting sets of environmental circumstances to which the population has been exposed are likely to have been favored and to have increased in frequency at the expense of alleles or combinations of alleles less favored in those particular sets of environments. In cultivation the shift toward better-adapted genotypes in the local environment can be accelerated by effective natural, farmer- and/or breeder-directed selection. Nevertheless, increase in yield is likely to be very slow even under intense selection, such as the selection that was practiced with U.S. corn in the open-pollinated period from about 1860 to 1930. Trueness to type in an open-pollinated population is a statistical feature of the population as a whole; it is not a characteristic of individual plants, which is often the case with pure-line populations. Hence, the constantly changing composition of open-pollinated populations virtually guarantees that such populations will be less efficient and low yielding than monogenotypic populations composed solely of numerous identical copies of truly exceptional genotypes that have been identified as superior through repeated testing by nature, by farmers, and/or by breeders. It is consequently, not surprising that a common goal in the breeding of seed-propagated outcrossing species has been the development of monogenotypic single-cross hybrid varieties. Single-cross hybrid varieties are composed of numerous identical copies of a genotype that has been identified by means of extensive testing programs as excelling in adaptedness, in yielding ability, and, likely, in product quality as well. Because the two inbred parents of such hybrids are homozygous at all loci, all single-cross F1 hybrid plants, even though they carry different alleles at many loci, have the advantage of being homogeneous for the same superior highly selected multilocus genotype. Such varieties also have the advantage, which they share with inbred line varieties of selfers and with vegetatively propagated single-clone varieties, of being precisely reproducible year after year. However, seed-propagated Fl hybrid varieties have a disadvantage, namely, that on reproducing sexually they segregate in an unmanageable manner so that very large numbers of different genotypes appear in the F2 generation and very few, if any, among these segregants perform as well as the single genotype of the Fl . As a consequence, yields of seed-propagated F2 populations resulting from segregation during reproduction in the F1 are nearly always at least 10%, and often more than 30%, lower in yielding ability than their parental monogenotypic F1 hybrid parents. Consequently, growers must obtain a new supply of Fl seed each generation. In high-input agricultures seed is inexpensive as compared with the gains attending the growing of hybrid varieties; hence, this disadvantage is relatively minor. Although some plant breeders regard the breeding of single-cross hybrids as only a highly specialized form of traditional open-pollinated breeding methods, most breeders consider single-cross methods to be such a great and strikingly successful departure from all previous methods as to justify classifying this method as a new and intrinsically different breeding procedure. The earliest ideas basic to breeding hybrid varieties date back to the 1870s and 1880s (Beal 1876-1882) and the first decade of the 1900s (Shull 1908, 1909). We turn now to a remarkable series of complexly intertwined developments that occurred in the first 50 years or more of the 1900s, developments featuring repeated cycles of population improvement within originally open-pollinated source populations with the goal of enhancing the value of the inbreds that could be extracted from the improved source populations. Thereafter test crosses to determine general combining ability (GCA) and/or specific combining ability (SCA) were added as crucial tests needed in the breeding of the superior inbreds to identify the best monogenotypic Fl hybrid varieties. Breeding procedures appropriate to seed-propagated outcrossing crop species, such as corn, will be described in this chapter, whereas procedures appropriate to producing superior clonal varieties and/or varieties that are propagated from planting mixtures (including mixtures of clones and/or various kinds of seed mixtures), will be described later. Before attempting any of the above tasks, however, it is appropriate to explore the development of the ideas on which population-improvement methods in outcrossing crops are based. It is fitting that this be done largely in terms of studies conducted with corn (Zea mays). No other species can be selfed and crossed so reliably or as inexpensively, and, as a result, it has been possible to investigate breeding methods in corn in ways and with precision unrivaled with any other outcrossing species.

No discourse concerning the development of methods of breeding hybrid varieties could be entire without reference to two nongenetic features that were essential to the rapidity with which the procedures basic to breeding hybrid varieties were ultimately adopted. The first feature has to do with plant breeders' rights. Breeding homozygous lines, open-pollinated varieties, and clonally reproduced varieties have, in general, not been rewarding for commercial breeders in the absence of legally established plant breeders' rights. Once released, such varieties can be propagated and sold by anyone without profit to the breeder. Hybrid varieties, in contrast, offer built-in economic protection; the breeder can retain control of the parental inbreds and sell only hybrid seed. This is because the next and later generations produced from F1 hybrids yield much less than the Fl hybrids; hence growers of hybrid varieties must, of necessity, return each year for Fl hybrid seed. Although hybrid varieties were first developed by publicly supported institutions, private breeding firms soon produced hybrids equal or superior to those of state-supported experiment stations, and hybrid corn rapidly became an eminently successful commercial operation. The resources put into producing hybrid corn by industry were quite substantial as compared with those available to state-supported plant breeding; moreover, seed of truly excellent hybrid varieties was being sold inexpensively. An additional feature was that hybrid corn was a major component in the passage from traditional agriculture to intensive technology-based agriculture. Many commercial hybrids were soon bred specifically for adaptedness to increasingly higher plant densities and increased applications of fertilizers. Hence, hybrid corn varieties were soon developed that were highly adapted wherever agricultural technologies were sufficiently advanced to benefit economically, and at present hybrid variety breeding methods are increasingly being extended to other crops.

[ethereal]

excellent info

[Open Eyes]

Good read but could use some simple editing to make use of paragraphs for ease of reading.