What does the floral morphology of the holoparasitic family Mitrastemonaceae (Ericales) tell us about its systematic position?

Mitrastemonaceae are a small angiosperm fam-ily comprising only one genus of two species with a disjunct distribution: Mitrastemon matudae Ya-mam. occurs from Mexico across Central Americato Colombia and M. yamamotoi Makino is knownfrom India across Southeast Asia to Japan. They areholoparasites on roots of Fagaceae and as endo-phytes, they only become visible outside the hosttissue and above ground when flowering. They pro-duce cream-yellow- to whitish-pink-coloured flow-ers from the third or fourth year on after the initialhost penetration and are reported to be pollinatedby a wide range of insects and a bird species. Thegenus Mitrastemon was traditionally thought to berelated to Rafflesiaceae (now placed in Malpighia-les) based on their holoparasitic and endophytic lifeform. A phylogenetic placement within Ericales hasonly relatively recently been suggested based onmolecular data. However, the phylogenetic positionamong Ericalean lineages remains largely elusive. Apossible sister-group relationship with Lecythidace-ae was recently proposed but received only weakstatistical support. For this study, we conducted adetailed investigation of the floral structure of bothspecies of Mitrastemonaceae using modern techniques including scanning electron microscopy, mi-crotome sectioning, and X-ray computed tomography and re-evaluated the floral morphologicalliterature on the family. We compare and discuss ourfindings with respect to floral morphological diversi-ty observed in other ericalean lineages and providea morphology-based hypothesis for the systematicposition of Mitrastemonaceae within the order.