THE INHERITANCE OF TRISTYLY IN DECODON-VERTICILLATUS (LYTHRACEAE)

The inheritance of tristyly is generally thought to be governed by two diallelic loci, S and M, with complete dominance and epistasis betwee n the loci. Significant departures from the two-locus model have, howe ver, been recently reported in species of Oxalis. This study presents a genetic analysis of tristyly in the eastern North American monotypic perennial Decodon verticillatus (L.) Ell. (Lythraceae). Crosses condu cted on plants from six natural populations in Ontario resulted in pat terns of segregation consistent with the two-locus model. The L morph is homozygous at both loci (ssmm). Crosses among F1 progeny identified two F1 individuals of the M morph homozygous for the dominant allele at the M locus (ssMM) and an F1 individual of the S morph homozygous f or the dominant allele at the S locus (SSmm). Crosses between these ge notypes demonstrated epistasis of the S locus over the M locus and pro duced putative double heterozygotes (Sm/sM). Five of these F2 genotype s were both self-fertilized and crossed as pollen donors to individual s of the L morph. Segregation in F3 and backcrossed progenies confirme d the two-locus model and provided no evidence for linkage between the loci. The results support the model of inheritance established for di stantly related Lythrum salicaria, suggesting that the genetic control of tristyly may be uniform throughout the Lythraceae.