Fig Macula as a Key Multifunctional Structure Mediating the Fig-Fig Wasp Mutualism

Plant-insect mutualisms often drive the evolution of adaptive morphological and physiological traits, enabling ecological specialization and diversification. Fig trees (Ficus spp., Moraceae) and their pollinating wasps (Agaonidae) are engaged in a brood-site pollination mutualism that exemplifies such adaptive specializations. This study investigates the morphological and ecological roles of maculae, characterized as distinct-pigmented regions on the fig surface, in the mutualistic interaction between Ficus citrifolia and fig wasps. Through morphological analyses using light and electron microscopy, we demonstrated that maculae concentrate numerous stomata and exhibit secretory activity. This activity is evidenced by the exudation of a sugary-like solution and by the presence of epidermal and subepidermal cells with features consistent with sugar- and terpene-secreting cells, such as abundant starch reserves, numerous mitochondria, plastids containing osmiophilic droplets, a Golgi complex with dilated cisternae, oil bodies, and extensive endoplasmic reticulum. Histochemical tests confirmed a terpenic-sugary secretion in the macula cells. We demonstrated that non-pollinating fig wasps avoid ovipositing through macular regions. This behavior may reflect a selective pressure to prevent structural damage to maculae caused by ovipositor insertion, thus preserving their functional integrity. Temperature measurements revealed that figs are up to 10% cooler on average than the ambient air. Therefore, our findings suggest that fig maculae are multifunctional structures, simultaneously performing the roles of extrafloral nectaries, gas exchange, and thermal regulation, which are crucial for maintaining suitable internal conditions for wasp larval development. These results provide novel insights into previously underexplored plant adaptations supporting specialized brood-site pollination mutualisms.