The peculiar post-senescence in globoid stem galls triggered by fungi and arboreal ants on Eremanthus erythropappus (DC.) McLeisch (Asteraceae)

Abstract

• The globoid stem galls under senescence can still react to biotic stimuli. • Ant movement in and out of the galls stimulate plant cell reactions. • The post-gall senescence reveals a reassumption of plant cell cycles. • The gall-ant-fungi interaction promotes a particular case of post-gall senescence. • Empty galls enter senescence for they close after the escape of the galling insect. Eremanthus erythropappus (DC.) McLeisch (Asteraceae), popularly known as “candeia”, is a common species in Minas Gerais state – Brazil and hosts six gall morphotypes. Our focus relies on the diagnosis of how one of these morphotypes, i.e., globoid stem galls induced by Neolasioptera sp. (Diptera: Cecidomyiidae) in the senescent stage can still react to the biotic stimuli of ants and fungi. These senescent globoid galls can be found in two forms: (1) empty galls characterized by the exit channel scar closed by plant tissue development, and (2) ant-occupied galls characterized by open exit channels, and the gall chamber occupied by ants and fungi. We hypothesize that the anatomical profile of senescent galls on E. erythropappus may favour the formation of peculiar ant-nest galls. The anatomical analyses revealed that phellogen activity along the exit channel completely closes the empty-galls after the escape of the galling Neolasioptera sp. The ant-occupied galls host seven different genera of ants, whose movement in and out of the gall may prevent the closing of the exit channel and allow the invasion of opportunistic fungal hyphae. The gall-ant-fungi interaction relies on the ants mechanically maintaining the opening of the exit channel previously dug by the galling Neolasioptera , while the fungi stimulate the metabolism of the cells in gall tissues. This interaction between E. erythropappus , fungi and ants, is responsible for the tissue responses and the maintenance of the peculiar ant-nest galls.