QTL mapping reveals specific genes for the evolutionary reduction of microsporangia in Microseris (Asteraceae)

Where closely related plant species with basic and derived characters can be crossed and produce fertile offspring, the genetics of character evolution can be inferred from the segregation patterns in hybrid offspring. Three species of Microseris (Compositae) share a reduction from the usual 4 to 2 pollen sacs per anther by the suppression of the adaxial microsporangia (MS), as a common derived character. These species can be crossed with a species with 4 MS, The segregation of MS numbers from such a hybrid was analysed by co-segregation with genetically mapped molecular markers (AFLPs). Five loci specifically affecting the MS number were found. The homozygous recessive genotype for a main gene is necessary but not sufficient to reduce MS numbers. Three of the four modifiers interact only in this homozygous recessive background to reduce the number to 2. This suggests that mutation of the main gene was followed by selection of the modifiers to stabilize the new phenotype. Since the plants are nearly completely self-fertilising in nature, the loss of two MS will not reduce reproductive success. The selection pressure needed to assemble such a complicated system for the complete absence of the two adaxial MS needs to be explained.