EVOLUTION OF MICROSPORANGIUM NUMBERS IN MICROSERIS (ASTERACEAE, LACTUCEAE)

The genus Microseris contains species with disporangiate stamens and s pecies with tetrasporangiate stamens. We determined the number of micr osporangia per stamen in serial sections of heads from all 13 species of Microseris, its close relative Uropappus lindleyi, and the two allo polyploid species of Stebbinsoseris. Four Microseris species, three di ploid and one tetraploid, have two microsporangia per stamen; all othe r species investigated have four. The most parsimonious assumption is that the disporangiate condition is derived and arose once in the evol ution of Microseris. The inheritance of the number of microsporangia p er stamen in crosses between M. bigelovii (disporangiate) and M. dougl asii (tetrasporangiate) was determined. Segregation of microsporangium number per stamen in F2s derived from these crosses is quantitative r ather 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 te trasporangiate stamens in the Fl and in most F2 plants. The observed p attern of inheritance suggests a major gene with dominance and quantit ative modifiers.