PLANT-BREEDING PROGRESS AND GENETIC DIVERSITY FROM DE-NOVO VARIATION AND ELEVATED EPISTASIS

Breeding programs in major crops normally restrict the use of parents to those improved for a variety of traits. Gain from utilizing these g ood x good crosses appears to be high, and improvements are sufficient to encourage continued breeding within narrow gene pools even though each cycle is expected to lead to reduced genetic variability. These f inely tuned programs have gradually limited the amount of new diversit y introduced into the breeding gene pool. This breeding strategy has l ed to a genetic gap where there is a large difference in the favorable gene frequency between the improved and unimproved lines and to a nar rowing of genetic diversity within elite gene pools. At the same time, evidence has accumulated in plant breeding programs and long-term sel ection experiments in several organisms that the genome is more plasti c and amenable to selection than previously assumed. In the barley (Ho rdeum vulgare L.) case study reported here, incremental genetic gains were made for several traits in what appears, based on pedigree analys is, to be a narrow gene pool. Given this situation, we call for an exa mination of the generally held belief that the variation on which sele ction is based in elite gene pools is provided almost exclusively from the original parents. Classical and molecular genetic analyses have s hown that many mechanisms exist to generate variation de novo, such as gene amplification and transposable elements. Accordingly, we put for ward the hypothesis that newly generated variation makes an important contribution. We also hypothesize that gene interaction, epistasis, is more important than commonly viewed and that it arises from de novo g enerated diversity as well as the original diversity.