The reduction of inner (adaxial) pollen sacs (microsporangia = MS) is a common derived character for the three asteracean species Microseris bigelovii, M. pygmaeaand M. elegans. The genetic of this reduction was determined in the F6 inbred population of an interspecific cross between M. douglasiiwith 4 MS and M. bigelovii with 2 MS by the cosegregation with molecular markers (AFLPs). One major gene and four modifying genes could be mapped as quantitative trait loci (QTLs). Three of those modifying genes had only an effect on the homozygous recessive (2 MS) genotyp of the major gene. 2 MS were produced only if at least 5 alleles of those modifying genes determined the 2 MS phenotype. We isolated one plant with the heterozygous genotype of the major gene and the homoyzgous recessive 2 MS genotype in all the modifying genes. The 87 offspring of this plant showed a visible 3:1 segregation in normal fertile plants with 4 MS and small sterile plants with 2 MS in a more or less homozygous genetic background. Thus AFLP markers closely linked with the MS locus can be isolated. In order to test many markers in respect of their cosegregation with the MS locus, DNA bulks of 10 plants with the 2 MS phenotype and 10 plants with the 4 MS phenotype will be screened with about 1000 AFLP primer combinations. After having tested 444 Eco/ Mse primer combinations the MS locus could be mapped in an intervall of 4 Centimorgan between two flanking markers. Those markers will be transferred into codominant SCARs (Sequence Characterized Amplified Regions). Recombinants between those markers can be used to narrow the F7 population of about 2000 plants that segregate for the MS locus to about 160 plants for the fine mapping. With the isolation of markers closely linked to the MS gene (or the gene itself) we have the tools to explain the evolution of this diagnostic character in detail.

Key words: anther evolution, evolutionary genetics, Microseris, microsporangia