A Review of the Evidence - Search CIMMYT repository

Genetic driftWhen farmers select ears of maize from their harvest to save as seed, they rarely use astatistically defensible sampling strategy. Often they select too few ears, or they select nonrandomlyby picking ears from the same field or from the same region within a field. Theseselection practices can lead to a phenomenon known as genetic drift. In small or nonrepresentativesamples, randomly distributed rare alleles may be lost. In the case of openpollinatedmaize populations, a randomly selected sample of 100 ears will preserve (with aprobability of 95%) rare alleles that occur with frequencies of only 5%, but alleles presentwith a frequency of 3% or less are likely to be lost. Genetic drift is linked to losses invariability and to decreases in the proportion of heterozygous individuals withinpopulations (Crossa, 1989).In her study of gene flows among maize landraces in Cuzalapa, Mexico, Louette (1994,1995) determined that 32% of the seed lots in the study area were constituted by shellingfewer than 40 ears. According to Crossa (1989, 1994), 40 ears is the minimum numberneeded to avoid problems of genetic drift associated with non-representative sampling, soit seems likely that these seed lots regularly suffer losses of rare alleles. Many of the seedlots constituted from less than 40 ears involved colored landraces, which tend to be grownin extremely small plots for specialized purposes. Quite unexpectedly, the level of geneticdiversity of colored landraces (measured by enzymatic polymorphism parameters) wasfound to be comparable to the level of genetic diversity of other landraces whose seed lotswere constituted from much larger numbers of ears. This suggests that whatever rarealleles are lost because of farmers’ non-representative seed selection practices are beingreplaced by alleles introduced as a result of gene flows between landraces planted inadjacent plots (Louette, 1995).MutationMutation can occur during meiosis when chromosomes become separated and are laterreassembled in different combinations bearing new collections of alleles. Essentially arandom event, mutation allows recombination of genetic material at each generation.No studies were found in the literature reporting specifically on the effect of mutation onthe rate of genetic change in maize. Mutation and random variation were implicated,however, in an interesting case study on seed production practices in Zambia. In the late1980s, agricultural officials in Zambia became concerned that seed of the highly popularsingle-cross hybrid SR52 had become highly contaminated. A trial conducted to compareseed lots obtained from different commercial producers confirmed that there was a highlevel of variability not only between seed lots of different producers, but even within seedlots from the same producer (Ristanovic et al., 1985). Across six locations, yields rangedfrom 5.7 t/ha to 6.9 t/ha, a difference of more than 21%. Further investigation revealed thatthe inbred lines used to produce SR52 had become highly contaminated. Thiscontamination was thought to have resulted from residual variation and mutation, ratherthan from accidental outcrossing.26