The harmful effects of close inbreeding have been noticed for many centuries (34, 35, 165). With the rise of Mendelian genetics, it was realized that the main genetic consequence of inbreeding is homozygosis (165, Ch. 2). Two main theories were early proposed to account for inbreeding depression and its converse, heterosis (the increase in vigor observed in an F1 between two inbred lines). These are the overdominance and partial dominance hypotheses, discussed in more detail below. Research into this question has continued up to the present, and this is one of the topics that we discuss. Darwin (35, 36) was the first to point out that the evident adaptations of many plants for ensuring outcrossing could be understood in terms of the selective advantage of avoiding inbreeding depression. We review the evidence that the evolution of breeding systems of animals and plants has been significantly influenced by the occurrence of inbreeding depression. In order to do this, we consider the contemporary genetic theory of inbreeding depression and heterosis, and the experimental data concerning the strength of inbreeding depression. We emphasize data and theory relevant to natural, rather than domesticated, populations as we are chiefly concerned to evaluate the evolutionary significance of inbreeding depression. We do not attempt to give a complete bibliography of this very extensive field but try to concentrate on what seem to be the most significant findings in relation to this aim.

https://www.jstor.org/stable/pdf/2097132.pdf

Inbreeding depression and its evolutionary consequences

DNA markers have enormous potential to improve the efficiency and precision of conventional plant breeding via marker-assisted selection (MAS). The large number of quantitative trait loci (QTLs) mapping studies for diverse crops species have provided an abundance of DNA marker–trait associations. In this review, we present an overview of the advantages of MAS and its most widely used applications in plant breeding, providing examples from cereal crops. We also consider reasons why MAS has had only a small impact on plant breeding so far and suggest ways in which the potential of MAS can be realized. Finally, we discuss reasons why the greater adoption of MAS in the future is inevitable, although the extent of its use will depend on available resources, especially for orphan crops, and may be delayed in less-developed countries. Achieving a substantial impact on crop improvement by MAS represents the great challenge for agricultural scientists in the next few decades.

/pdf/marker-assisted-selection-an-approach-for-precision-plant-1be57ryhno.pdf

Marker-assisted selection: an approach for precision plant breeding in the twenty-first century

Breeders have traditionally improved plant varieties by selecting on the basis of phenotype. Now restriction fragment length polymorphism (RFLP) linkage maps are being constructed for most major crop plants and these maps provide a more direct method for selecting desirable genes via their linkage to easily detectable RFLP markers. The integration of RFLP techniques into plant breeding promises to: (1) Expedite the movement of desirable genes among varieties, (2) Allow the transfer of novel genes from related wild species, (3) Make possible the analysis of complex polygenic characters as ensembles of single Mendelian factors, and (4) Establish genetic relationships between sexually incompatible crop plants. In the future, high density RFLP maps may also make it possible to clone genes whose products are unknown, such as genes for disease resistance or stress tolerance.

RFLP Mapping in Plant Breeding: New Tools for an Old Science

A method for DNA reassembly after random fragmentation, and its application to mutagenesis of nucleic acid sequences by in vitro or in vivo recombination is described. In particular, a method for the production of nucleic acid fragments or polynucleotides encoding mutant proteins is described. The present invention also relates to a method of repeated cycles of mutagenesis, shuffling and selection which allow for the directed molecular evolution in vitro or in vivo of proteins.

Methods for generating polynucleotides having desired characteristics by iterative selection and recombination

In the mid-1980s, the development of abundant molecular markers, appropriate statistical pro- cedures, and user-friendly computer software that implemented these statistical procedures permitted the detection of molecular markers associated with quantitative trait loci (QTL) for complex traits. Marker-assisted selection was then proposed as a means of exploiting mark- ers linked to QTL to develop improved cultivars. But while thousands of marker-trait associations have been reported for many traits in different plant species, far fewer examples of success- fully exploiting mapped QTL have been reported in the literature. Key lessons learned from apply- ing markers in plant breeding include the follow- ing: (i) the purpose of detecting QTL should be clearly defi ned before embarking on QTL map- ping; (ii) procedures for marker-based selection depend on the number of QTL; (iii) estimates of QTL effects for complex traits are often incon- sistent; and (iv) gain per unit cost and time rather than gain per cycle should be considered. Future applications for complex traits will likely focus on predictive methodologies for marker-based selection before phenotyping and for marker- based selection without QTL mapping. These applications will take advantage of cheaper costs of genotyping than of phenotyping.

/pdf/molecular-markers-and-selection-for-complex-traits-in-plants-2z2qfex7a3.pdf

Allard

Papers

Principles of plant breeding