A Review of the Evidence - Search CIMMYT repository

accidental cross-pollination, random mutation, and gene segregation. Since both types ofselection pressure are highly variable, it is difficult to generalize about the rate of geneticchange; depending on the circumstances, the genotype of a landrace or improved OPV canchange significantly from one generation of plants to the next, or it can remain essentiallyunchanged across many generations of plants.In hybrids, by far the most important source of genetic changes is segregation — randomrecombination of alleles that occurs when seed is recycled. Key results of a simulationexercise designed to show the likely effects of inbreeding in maize hybrids appear to besupported by findings published in the empirical literature on seed recycling:♦ When maize hybrids are recycled, yield usually decreases significantly from the F1 to theF2 generation. Yield tends to stabilize in subsequent generations, however, and mayeventually begin to increase again if farmers are exerting selection pressure.♦ When maize hybrids are recycled, the size of the yield decrease observed between the F1and F2 generations depends in large part on the level of inbreeding of the originalparents. Generally speaking, the greater the degree of inbreeding in the parents, thegreater the degree of heterosis in the F1 generation, and the greater the yield declineobserved between the F1 and F2 generations. This relationship may be confounded byenvironmental factors, however.♦ The degree of inbreeding of the parents affects not only the size of the expected yielddecrease but also its variability. The greater the level of heterozygosity in F1 plants, thegreater the variability in inbreeding depression expected in F2 and F3 plants.♦ Whether or not advanced-generation hybrids outyield landraces and improved OPVsdepends on the original difference in yield and on the magnitude of the decline in yieldcaused by recycling. In some instances, recycled hybrids continue to outyield the othertypes of materials, which explains why hybrid recycling may make sense.♦ Recycling of hybrids may have little effect on qualitative traits such as kernel size andshape, grain texture, and pounding quality.The finding that seed recycling often leads to significant genetic changes in farmermaintainedcultivars suggests that there may be a need to reassess the categoriestraditionally used to classify maize varieties (e.g., landraces, improved OPVs, hybrids). Inaddition, the rapid rate of genetic change observed to take place in farmers’ fields hasimportant implications for research impacts assessment studies. Practical guidelines for usein estimating the returns to maize breeding research are presented in the appendix.v