Population structure, the physical arrangement of related and unrelated individuals, can have profound effects on the apparent outcrossing rate (\(\hat t\)) in a plant population. However, detailed experimental investigations of the impact of population structure on \(\hat t\) are few. We compared the apparent outcrossing rates of experimental populations of grain sorghum with seed families spatially arranged in stratified and overdispersed treatments. Using alcohol dehydrogenase allozymes as genetic markers, \(\hat t\) was calculated for each of the treatments at two locations over three years. For all six comparisons, the overdispersed treatment yielded significantly larger apparent outcrossing estimates than the stratified treatment. In one case, the difference was over five-fold. Site-specific and time-specific differences were small compared to treatment differences. Whether natural structuring plays a role in altering the effective outcrossing rates of natural populations has been addressed by only a few descriptive studies; structuring effects appear to have an impact in only about half of such studies. The sample is still too small to make any generalizations.
Population structuring can also be of significance in plant breeding programs. Controllable variation in \(\hat t\) values of the magnitude reported herein may be useful in optimizing selection methods for quantitative characters in experimental plant breeding populations. Further work is under way to determine the effects of the variation in apparent outcrossing rates on genetic gains from selection.