Metabolic and Elemental Stoichiometric Analyses in Ambrosia Fungi Reveal Lineage-Specific Strategies in Symbiotic...

The ambrosia fungus-beetle symbiosis is an obligate nutritional association, yet the specific metabolic contributions of fungal partners to beetle nutrition remain incompletely characterised. We compared several ambrosia fungal species with closely related non-ambrosia species to investigate their metabolic capabilities, carbon-nitrogen stoichiometry and uric acid catabolism. Ambrosia fungi displayed distinct but lineage-specific metabolic features that were largely influenced by phylogeny. Although both groups shared overarching metabolic similarities, ambrosia fungi such as Ambrosiella roeperi (Ceratocystidaceae) and Irpex subulatus (Irpicaceae) exhibited enriched amino acid biosynthesis pathways, especially for L-histidine and L-phenylalanine. Formaldehyde assimilation pathway was detected in ambrosia fungi, specifically A. roeperi (Ceratocystidaceae) and Harringtonia lauricola (Ophiostomataceae), reflecting strategies for coping with volatile environments of stressed host trees. Elemental stoichiometry revealed elevated C:N ratios in ambrosia fungi, particularly A. roeperi and I. subulatus, despite no significant differences in carbon or nitrogen when assessed independently. However, nitrogen content was not significantly enhanced, suggesting that nitrogen provisioning may not be conserved as an adaptive trait in ambrosia symbiosis. Similarly, fungal growth on uric acid as the sole nitrogen source did not differ between groups, indicating that nitrogen provisioning through recycling of beetle wastes may reflect lineage-specific patterns rather than a convergent trait across ambrosia fungi.