Volatile Insensitivity and Localized Alkaloids Support Commensalism in the Oxytropis sericea–Alternaria oxytropis Association

The endophytic fungus Alternaria oxytropis is vertically transmitted through silky crazyweed ( Oxytropis sericea Nutt.) and produces the neurotoxin swainsonine, yet its influence on host chemical signaling remains unclear. Here we characterize the floral and foliar volatile organic compound (VOC) profiles of endophyte-infected (E+) and endophyte-free (E-) O. sericea using dynamic headspace GC-MS on greenhouse- and field-grown plants – the first characterization of volatile profiles in this species. Endophyte infection had a negligible effect on VOC composition, explaining less than 1% of compositional variance in both greenhouse and field settings, with no compound differing consistently by infection status. Seven volatiles were reliably detected across both environments. The dominant axes of variation (PCA) instead reflected differences among individuals and between size-matched E+/E- plant across years. These accounted for most compositional variance (72.6% greenhouse, 64.9% field), and VOC profiles differed markedly between collection years, likely reflecting fine-scale environmental and physiological variation. Ancillary nectar assays and pollinator observations revealed no consistent differences between E + and E- plants, and seed set was unaffected regardless of endophyte status. Swainsonine in floral organs was roughly an order of magnitude lower than in leaves and near-trace in nectar, indicating that any localized chemical defense is likely decoupled from scent chemistry. Together, these findings position the Oxytropis-Alternaria association toward the commensal end of the symbiotic continuum: the fungus maintains vertical transmission while the host shows no detectable effect on volatile signaling, pollinator attraction, or reproductive output, despite the resources required to sustain fungal alkaloid biosynthesis. This system offers a tractable model for understanding how endophyte associations persist in the absence of measurable host benefit.