Diversity and composition of the seed-associated mycobiome of silver birch (Betula pendula Roth) in Sweden

Silver birch ( Betula pendula Roth) frequently produces large proportions of empty seeds lacking a viable embryo, contributing to low germination rates and reduced seed quality. To investigate whether fungi may be associated with seed developmental status, we characterised the diversity and composition of the seed-associated mycobiome of filled (embryo present) and empty (embryo absent) seeds collected in Sweden. Seeds were analysed using complementary culture-based isolation with multilocus phylogenetic identification and high-throughput ITS2 amplicon sequencing. In total, 12 fungal genera were isolated in culture, while amplicon sequencing detected 97 genera. Fungal diversity and community composition differed significantly between empty and filled seeds, depending on the analytical approach. Empty seeds harboured higher fungal diversity than filled seeds in the amplicon sequencing dataset, whereas the relative abundance of major fungal functional guilds did not differ significantly between seed types. Several genera containing known or putative plant pathogens, including Alternaria , Penicillium and Fusarium , were detected. However, the observed patterns do not support a primary role of fungi in embryo abortion. This study provides the first molecular characterisation of the seed-associated mycobiome of silver birch in Sweden and establishes a baseline for future investigations of seed health and seed-fungus interactions in birch.