Evidence for phenotypic assortative mating for flower colour in periwinkle

Two experiments were conducted with periwinkle, Catharanthus roseus, to det ermine the extent of natural outcrossing. Three white-flowered, fully self- fertile, monogenic recessive mutants, namely, dwarf, wavy leaf margin and c urved leaf were used, together with their parental white-flowered variety,' Nirmal', and a normal pink-flowered variety, PS-1. The extent of total outc rossing ranged from 43.4 to 79.3% among mutants. Outcrossing between white x white-flowered plants ranged from 28.3 to 65.3% and was two to four times greater than that between white x pink-flowered plants in the three mutant s. The extent of outcrossing between white x pink-flowered plants was simil ar (12.2-15.1%) in all mutants and also similar to that in the normal white -flowered variety,'Nirmal' (11.4%), where white x white flower outcrossing could not be estimated. There were no large differences in the number of se eds per fruit, percentage fruit set and germination percentage of seeds obt ained from self, white x white and white x pink Rower crosses made in the g lasshouse. There were also no significant differences in the number of flow ers produced by the genotypes used in the study. The observed higher freque ncy of white x white flower matings compared with white x pink flower matin gs appeared to be due to the constancy of flower colour exhibited by the bu tterfly pollinators Pachliopta hector and Catopsilia pyranthae during their flower visits. Observations made on the occurrence of natural self-pollina tion revealed that automatic self-pollination did not occur in periwinkle.