EVOLUTION OF MICROSPORANGIUM NUMBERS IN MICROSERIS (ASTERACEAE, LACTUCEAE)

The genus Microseris contains species with disporangiate stamens and species with tetrasporangiate stamens. We determined the number of microsporangia per stamen in serial sections of heads from all 13 species of Microseris, its close relative Uropappus lindleyi, and the two allopolyploid species of Stebbinsoseris. Four Microseris species, three diploid and one tetraploid, have two microsporangia per stamen; all other species investigated have four. The most parsimonious assumption is that the disporangiate condition is derived and arose once in the evolution of Microseris. The inheritance of the number of microsporangia per stamen in crosses between M. bigelovii (disporangiate) and M. douglasii (tetrasporangiate) was determined. Segregation of microsporangium number per stamen in F2s derived from these crosses is quantitative rather than Mendelian. The average number of microsporangia per stamen in the F2 plants ranges from 2.0 to 4.0. There is a predominance of tetrasporangiate stamens in the Fl and in most F2 plants. The observed pattern of inheritance suggests a major gene with dominance and quantitative modifiers.