North American study on essential derivation in maize:: inbreds developed without and with selection from F2 populations

An essentially derived variety largely retains the characteristics of a parental or ancestral variety. A consensus has not been reached regarding the threshold for declaring essential derivation in maize (Zea mays L.), partly because benchmark data are lacking. Our objective in this study, commissioned by the American Seed Trade Association, was to determine the range of paren tal contribution among maize inbreds developed without and with selection. Seed companies in North America contributed existing proprietary data on the molecular marker similarity of 100 or more families, developed without selection from F-2 populations, with each of their parents. The companies also sent us seed samples of elite inbreds, developed with selection from F-2 populations, for analysis using 60 RFLP marker loci and 20 SSR marker loci. Among the families developed without selection, the average parental contribution was close to the expected value of 0.50 for F-2 populations. Specific families received up to 79% of their alleles from one parent. Although selection tended to increase the frequency of such transgressive segregants, it did not necessarily increase the maximum parental contribution in an F-2 population Parental contributions were consistent between the elite inbreds and their early-generation families. We conclude that inbreds with 70% to nearly 80% of their genome derived from one parent can be obtained from an F,population. Further empirical data would be valuable particularly for backcross populations, which were not available in this study.