GENETIC-VARIATION FOR MERISTIC CHARACTERS OF THE CAPITULA OF MICROSERIS-PYGMAEA (ASTERACEAE, LACTUCEAE)

The small Chilean annual lactucean, Microseris pygmaea, seems to have originated from a single episode of long-distance dispersal from the west coast of North America. The genetic differentiation of the species in South America is being analyzed in a hybrid between two inbred strains representing the geographically and morphologically most diverse biotypes. Here, total number of achenes per head, number of outer and inner involucral bracts, and relative number of various achene morphs have been studied. All meristic characters are influenced by the genetically determined difference in capitulum size between the parental strains, which, moreover, is subject to considerable plastic modification. The dependence of the meristic characters on capitulum size is not linear but shows numerical canalization to Fibonacci numbers (e.g. 13 inner involucral bracts) or their ratios. Comparisons of capitula of the parental strains in the region of size overlap, regression of absolute or relative F3 offspring numbers on those of the F2 parents and artifical selection based on the phenotypic extremes in an F2 of 300 plants are used to demonstrate size-independent heritable variation for the number of outer phyllaries and the proportion of peripheral, hairy achenes. Violet spotting on the periperal achenes is transmitted as a single-gene dominant trait.