Although cassava is a major food crop, its scientific breeding began only recently compared with other crops. Significant progress has been achieved, particularly in Asia where cassava is used mainly for industrial processes and no major biotic constraints affect its productivity. Cassava breeding faces several limitations that need to be addressed. The heterozygous nature of the crop and parental lines used to generate new segregating progenies makes it difficult to identify parents with good breeding values. Breeding so far has been mainly based on a mass phenotypic recurrent selection. There is very little knowledge on the inheritance of traits of agronomic relevance. Several approaches have been taken to overcome the constraints in the current methodologies for the genetic improvement of cassava. Evaluations at early stages of selection allow for estimates of general combining ability effect or breeding values of parental lines. Inbreeding by sequential self-pollination facilitates the identification of useful recessive traits, either already present in the Manihot gene pool or induced by mutagenesis.

Cassava breeding: opportunities and challenges.

Mutations in plant breeding

Publisher Summary Grain yields for maize are expected to increase because of the combined effects of two items—improved cultural practices and management and increased genetic potential of the hybrids. The improvements of these items can be expected to contribute significantly to give higher state averages in the future. An important cause for yield gain is that the genetic potential of the most recent hybrids takes advantage of higher plant densities to produce more grain per unit field area. Perhaps further gains can be achieved by the development and evaluation of parental materials at higher plant densities to identify greater resistance to barrenness. Stalk lodging, however, is still a major problem and must be corrected if higher plant densities are to be used. Consequently, breeders must develop materials that have better disease resistance and stalk quality before hybrids can be planted at higher plant densities to produce more grain per unit area. Recurrent selection can be used effectively to improve stalk quality in breeding populations, but caution must be used in selection to avoid yield declines. Occasionally, germplasm with resistance for some pests may have to be introduced from unadapted sources, and this could cause temporary reductions for hybrid performance until unfavorable genes linked with the resistance genes are eliminated.

Genetic improvement of maize yields.

Referee: Ms. Peggy Wagoner, Rodale Institute, 611 Siegfriedale Road, Kutztown, PA 19530-9749 One-third of the planet's arable land has been lost to soil erosion in recent decades, and the pace of this degradation will increase as the limits of our food production capacity are stretched. The persistent problem of worldwide soil erosion has rekindled interest in perennial grain crops. All of our current grain crops are annuals; therefore, developing an array of new perennial grains - grasses, legumes, and others – will require a long-term commitment. Fortunately, many perennial species can be hybridized with related annual crops, allowing us to incorporate genes of domestication much more quickly than did our ancestors who first selected the genes. Some grain crops — including rye, rice, and sorghum — can be hybridized with close perennial relatives to establish new gene pools. Others, such as wheat, oat, maize, soybean, and sunflower, must be hybridized with more distantly related perennial species and gen...

Breeding Perennial Grain Crops

Publisher Summary This chapter discusses quantitative genetics, which deals with the inheritance of those differences among individuals that are expressed in terms of degree rather than kind The relevance of quantitative genetics to plant breeding lies in the fact that manipulation of genetic variability of quantitative traits through inbreeding, crossbreeding, and selection is the essential feature of any plant breeding program A primary objective of quantitative genetic research is an understanding of the genetic consequences of such manipulations The chapter focusses on reviewing and interrelating recent research results in areas of most significance to plant breeders, such as (1) kinds of genetic variability found, (2) effects of inbreeding and crossbreeding, (3) genotype-environmental interaction, and (4) selection methodology and response Inbreeding results from mating between related individuals The degree of inbreeding is measured by the inbreeding coefficient, which is the probability that two genes at a locus are identical by descent The implications of quantitative genetics to breeding methodology are also discussed in the chapter

Quantitative genetics--empirical results relevant to plant breeding

Although the irradiation treatment was detrimental to yield, the irradiated strain significantly outyielded the original variety and yielded only slightly less than the mass-selected control strain after four generations of selection. It is doubtful that irradiation of an already genetically variable population will prove to be particularly valuable as a breeding procedure; yet it is too early to draw definite conclusions. Additional generations of selection are needed. Because of its simplicity and apparent effectiveness, the use of mass selection of individual plants is suggested as a possible method of yield improvement in populations of corn segregating for genes controlling yield. Where the advantages of hybrids other than yield are desired, mass selection is suggested as a possible means of increasing the frequency of favorable genes in segregating populations prior to inbreeding. If over-dominance exists at any locus, mass selection will not be successful in altering gene frequency for that locus after an equilibrium point is reached. Some possible explanations are advanced for the apparent lack of success of early corn breeders using mass- selection and ear-to-row breeding techniques.

C. O. Gardner

Papers

An Evaluation of Effects of Mass Selection and Seed Irradiation With Thermal Neutrons on Yield of Corn1