Genetic Control of Floral Morph and Petal Pigmentation in Linum grandiflorum Desf., a Heterostylous Flax

Two types of floral morph, called thrum and pin, occur in heterostylous species. The thrum and pin flowers have shorter and longer styles, respectively. Heterostyly in Linum has been studied since Darwin's era. The floral morph, self-incompatibility, and related phenotypes have been well characterized using a natural population, but genetic analysis using a segregated population has not been reported, and the mode of inheritance of heterostyly in Linum remains to be investigated. We prepared a segregated population by crossing thrum and pin flowers of Linum grandiflorum Desf. and investigated style and stamen lengths. On the basis of the style to stamen length ratio, the population could be divided into thrum and pin clearly at a ratio of 1:1. Style length of pins was 1.6 times longer than that of thrums. To investigate the factor regulating the difference in style length, we further measured the style cell length of thrums and pins. The style cell length of pins was longer than that of thrums, whose rate was comparable to the rate of the style length ratio between thrum and pin flowers. These findings indicate that the floral morph of heterostyly in Linum is controlled by a single diallelic locus, and that a difference in the cell expansion rate caused thrum and pin morphs in the style, such as in a typical heterostylous species. PCR genotyping showed that TSS1, an S candidate gene reported previously, cosegregated completely with the thrum phenotype, indicating strong linkage between the S locus and TSS1. Furthermore, three flower colors (red, pink, and white) were observed at a 1:2:1 ratio, and no white thrum flowers or red pin flowers were found in this population. These flower color phenotypes could also be controlled by a diallelic locus, whose two alleles (R and r) would be incompletely dominant, and therefore, may be linked to the S locus.