A Review of the Evidence - Search CIMMYT repository

DiscussionEvery time a farmer recycles maize seed, as opposed to acquiring fresh seed whose geneticcomposition and degree of purity are reliably known, there is some risk that the latestgeneration of plants will differ from preceding generations in terms of its geneticcomposition. Various potential sources of genetic change have been discussed in thissection, and evidence has been presented from studies that have sought to document therate and extent of genetic change attributable to some of these sources.Generally speaking, the empirical evidence suggests that throughout much of thedeveloping world, maize cultivars are exposed to continual selection pressure. Geneticchanges can be induced intentionally by farmers through varietal management and seedselection practices, or they can occur inadvertently as a result of environmental selectionpressure, random mutation and gene segregation, unintentional contamination by foreignpollen, accidental seed mixing, and other factors. In the presence of seed recycling,intentional and unintentional selection pressure can lead to significant genetic changes,sometimes over relatively short periods.Genetic change in maize is frequently mentioned in the context of the so-called“contamination” of MVs, but landraces also are subject to selection pressure. Althoughlandraces are generally thought of as genetically stable and unchanging, we now know thateven the most ancient landraces receive constant infusions of new genes and that theircharacteristics are constantly changing. Recent empirical work has shown that landracesmay be subjected to several distinct types of intentional selection pressure. For example,Louette and Smale (1998) have shown that farmers in Cuzalapa, Mexico select seed forreplanting on the basis of yield components (e.g., ear size, ear shape, kernel number, kernelsize); selection based on these criteria tends to favor genotypes that are most productiveunder local conditions. At the same time, the Cuzalapa farmers also select seed based oncharacteristics of economic or cultural value in ways that protect perceived ideotypes. Thepresence of the second type of selection pressure helps explain how many differentlandraces can continue to coexist in conditions of heavy gene flow.Landraces are not the only types of maize cultivars that undergo a continual process ofgenetic change, however. Improved OPVs and hybrids also undergo frequent changes ingenetic composition as a result of seed recycling, which is much more common than isgenerally acknowledged.In improved OPVs, as in landraces, genetic change results from a combination ofintentional and unintentional selection pressure. Genetic change results not only fromfarmers’ deliberate efforts to select for desired characteristics, but also from unintentionalenvironmental pressures, accidental cross-pollination, random mutation, gene segregation,and so forth. Since both types of selection pressure are highly variable, it is difficult togeneralize about the rate of change occurring in improved OPVs; depending on thecircumstances, the genotype of an improved OPV can change significantly from onegeneration of plants to the next, or it can remain essentially the same across manygenerations of plants.48