Die Evolution von Mykoheterotrophie - auf Spurensuche in der Mykorrhiza der Polygalaceae (Kreuzblumengewächse)

The Polygalaceae provide a unique opportunity to shed light on the structural mycorrhizal changes along the morphological gradient from large trees to small achlorophyllous, mycoheterotrophic herbs. This study suggests an incremental morphological progression of the root system as well as the arbuscular mycorrhizal structures in Polygalaceae from allorhizic, intensively branched root systems with less complex mycorrhizal colonisation patterns conserved in the woody species, to scarcely branched, homorhizic root systems with distinctive mycorrhizal morphotypes in the herbaceous lifeforms. Microscopic analyses of the mycorrhizal structures allowed the distinction of 9 progressive modifications of the presumably ancestral mycorrhiza in the woody species, which accumulate in various combinations and degrees in the herbs. Due to the combination of some of these progressive traits, some photosynthetic herbs from various clades already resemble the highly complex mycorrhiza in the mycoheterotrophic Polygalaceae Epirixanthes spp. (Imhof 2007). On the basis of these results a new model on the origin of mycoheterotrophy is suggested. Additionally the fungal constitutens of this mycorrhiza are taxonomically characterized on basis of their 18S RNA and the phylogenetic relations between the fungal symbionts are analysed to further reveal the evolution of specificity for distinct fungal lineages in the course of evolution of mycoheterotrophy in this plant family.