|Guzman, Peter - IOWA STATE UNIVERSITY|
Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 4, 1999
Publication Date: N/A
Interpretive Summary: Genetic variation (or equivalently genetic diversity) is the raw material of a plant improvement program. Once a breeding population of plants has been formed the initial level of genetic variation has been established. Plant breeders are then concerned with utilizing (selecting) the useful variation to produce cultivars and also with maintaining the variation of the population for future improvement. The maintenance of genetic variation is influenced primarily by the size of the sample that breeders keep. There has been a great deal of theoretical work done on the effect of sample size on the maintenance of genetic variation, but little in the way of empirical studies. This study documents the change in genetic variance after five rounds of selection where 5, 10, 20, or 30 individuals were kept to initiate the next round of selection. Theory predicts substantial loss of variation when only 5 individuals are kept whereas very ylittle loss of variation is expected when 30 are kept. After five rounds of selection there was no change in genetic variation for grain yield from what existed in the population in which selection was initiated. This suggests that the genetic control of grain yield is complex and probably involves the interactions of many genes. Because there was no loss in genetic variation, breeders can design short-term (15 to 20 years) breeding programs that involve keeping fewer individuals. This allows for increased efficiency in design of breeding programs, greater selection intensities, and hence greater response to selection. This research will be of interest to breeders and crop scientists who design breeding programs.
Technical Abstract: Establishing adequate effective population size in maize (Zea mays L.) recurrent selection programs is important due to random genetic drift and inbreeding depression. The objectives of this study were to (i) evaluate the performance of the BS11 Cycle 0 (C0) and the BS11 Cycle 5 (C5) populations from four S1-progeny selection programs each with a different effective population size (5, 10, 20, or 30) but with a common selection intensity of 20%, and (ii) compare the additive genetic variance among the C0 and C5 populations. Five cycles of selection were conducted by intermating 5, 10, 20, or 30 lines. One hundred-thirty C5 S1 lines from each of the selected populations (i.e. C5-5, C5-10, C5-20, and C5-30) and 100 C0 S1 lines were topcrossed to BS11 C0. The resulting half-sib progenies were evaluated at five environments in a replication within sets randomized incomplete block design. The four selection programs resulted in na significant increase in grain yield, reduced grain moisture, and root an stalk lodging. For yield, the 10-S1 program showed the highest gain cycle-1 of 0.16 Mg ha-1 followed by the 30-S1 program with 0.13 Mg ha-1 cycle-1. The 5-S1 program had a higher gain cycle-1 than the 20-S1 program. The additive genetic variance for yield did not change significantly. Heritability for yield was highest for C5-20, but there were no significant differences among populations. These results suggest little to no advantage of using larger effective population sizes to maintain genetic variability for short-term recurrent selection.