Meristem geometry and heritable variation in numbers of florets and involucral bracts in Microseris pygmaea (Asteraceae, Lactuceae)

Strains A92 and C96 of the Chilean annual, Microseris pygmaea, represent the genetically most divergent biotypes within the species. They differ, among other characters, in the numbers of florets and inner and outer phyllaries (involucral bracts) per head. Higher numbers in A92 are mainly the consequence of smaller primordia while the capitulum buds in both strains are about equal in size. Disruptive selection from a large F2 of the hybrid between A92 and C96 has produced lines with transgressive numbers of parts with non-parental correlations among numbers of parts. The early development of the capitula was studied in F4 families derived from two F2 plants selected for extreme organ number phenotypes. Differences in organ numbers among these families are primarily due to differences in capitulum size which did not differ significantly between the parental strains. Values for other parameters such as floret primordium size, height of the zone in which inner phyllaries arise, and difference in size between peripheral and central florets, did not significantly contribute to the observed differences in organ numbers. The multiple gene differences suggested by the transgressive segregation in the F2 can be partly traced back to various primary effects on meristem differentiation.