To love and to drought: a cohesive relationship between desiccation-tolerant Cecidomyiidae galls and the poikilohydric Pleopeltis minima (Polypodiaceae)

The structure and function of fern galls are poorly studied, especially those occurring in poikilohydric ferns such as Pleopeltis minima (Polypodiaceae), which hosts globoid Cecidomyiidae galls on fronds. We expected that anatomical and histochemical features related to poikilohydry would be expressed in galls, potentially enhancing features that support gall inducer survival. We examined non-galled pinnae (NGP) and galled pinnae (GP) of P. minima from anatomical, histochemical, and histometrical perspectives. The NGP are covered on the abaxial surface by peltate scales with unlignified, pectin-thickened cell walls on the shield, which contain mucilage and polyphenols. The GP are covered by similar but larger scales on the adaxial and abaxial surfaces. This feature, potentiated in galls, is related to water absorption and reduction of water loss. The NGP mesophyll is dorsiventral, with parenchyma cells featuring thickened pectocellulosic walls. In galls, an outer storage tissue also has pectin-rich, thickened walls that also contain mucilage, increasing hydrophilic capacity. Nutritive cells surrounding the larval chamber accumulate metabolites that nourish the gall inducer, along with phenolics, which may mitigate free radical damage. Pleopeltis minima galls exhibit adaptations tied to poikilohydry, making them a valuable model for studying the physiological and biochemical aspects of galls in poikilohydric ferns.