The genetic basis of flowering time in F, progeny derived from a cross of Mimulus guttatus was studied using seven unlinked isozyme markers. Each marker represented a specific chromosomal segment; its cosegregation with flowering time was used to characterize genes affecting flowering time. Three of the seven chromosomal segments were found to show significant, partially dominant effects on the expression of flowering time. These effects were relatively small, with segments marked by Me, 6Pgd2,

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... and Tpi individually explaining only 2.3, 2.5, and 9.2 percent of total phenotypic variance, respectively. No significant pairwise epistasis was observed between any marked chromosomal segments. The results of this study indicate that the quantitative variation of flowering time in M. gutsatus has a partial genetic basis, likely to consist of multiple genes of relatively small, partially dominant effects, with the potential to respond to natural selection. Yong-Bi Fu, Biotechnology Laboratory, University of British Columbia, # 237-61 74 University Blvd. Vancouver, B.C. V6T I23 Canada Flowering time, a life history trait, has exhibited a considerable variation in natural populations