Reduction from the normal 4 pollen sacs (microsporangia) to 2 microsporangia (MS) per anther has occured independently in about 50 angiosperm families and thus is a prime example for the parallel evolution of a potentially diagnostic character (homoplasy). Within the asteracean genus Microseris the reduction of the inner (adaxial) MS is a synapomorphy for three diploid annual species. The cosegregation of MS numbers with genetically mapped molecular markers (AFLPs) was analyzed in the F6 inbred population of an interspecific cross between tetrasporangiate (4MS) M. douglasii and disporangiate (2MS) M. bigelovii. One major gene and four modifying genes specifically affecting the expression of the inner MS were detected. The dominant (4 MS) allele of the major gene was epistatic over three of the modifiers. The modifiers could stabilize a 2 MS phenotype only in the homozygous recessive (2 MS) genotype of the major gene. 2 MS were produced if at least 5 alleles of those modifying genes determined the 2 MS phenotype. Homozygosity for the 4 MS allele in only one of the modifying genes in an otherwise 2MS genetic background produced an unstable phenotype with variable average MS numbers in different plants. However more than two alleles from M. douglasii (4 MS) led to the production of 4 MS in most flowers of one capitulum. This non-linear ("canalized") relationship between gene dosis and expression of MS and the observation of genotypes with variable expression of inner MS suggests how the disporangiate phenotype has evolved from a stable tetrasporangiate phenotype with a minimum of potentially maladapted intermediate phenotypes.

Key words: diagnostic characters, Microseris, microsporangia, organ numbers, QTL