Genomic insights into the evolution of secondary metabolism of Escovopsis and its allies, specialized fungal symbionts of fungus-farming ants

Abstract

The metabolic intimacy of symbiosis often demands the work of specialists. Natural products and defensive secondary metabolites can drive specificity by ensuring infection and propagation across host generations. But in contrast to bacteria, little is known about the diversity and distribution of natural product biosynthetic pathways among fungi and how they evolve to facilitate symbiosis and adaptation to their host environment. In this study, we define the secondary metabolism of Escovopsis and closely related genera, members of which are specialized, diverse ascomycete fungi best known as mycoparasites of the fungal cultivars grown by fungus-growing ants. We ask how the gain and loss of various biosynthetic pathways corresponds to divergent lifestyles. Long-read sequencing allowed us to define the chromosomal features of representative Escovopsis strains, revealing highly reduced genomes (21.4-38.3 Mb) composed of 7-8 chromosomes. Escovopsis genomes are highly co-linear, with genes localizing not only in the same chromosome, but also in the same order. Macrosynteny is high within Escovopsis clades, and decreases with increasing phylogenetic distance, while maintaining a high degree of mesosynteny. To explore the evolutionary history of biosynthetic pathways in this group of symbionts relative to their encoding lineages, we performed an ancestral state reconstruction analysis, which revealed that, while many secondary metabolites are shared with non-ant associated sordariomycetes, 56 pathways are unique to the symbiotic genera. Reflecting adaptation to diverging ant agricultural systems, we observe that the stepwise acquisition of these pathways mirrors the ecological radiations of attine ants and the dynamic recruitment and replacement of their fungal cultivars. As different clades encode characteristic combinations of biosynthetic gene clusters, these delineating profiles provide important insights into the possible mechanisms underlying specificity between these symbionts and their hosts. Collectively, our findings shed light on the evolutionary dynamic nature of secondary metabolism in Escovopsis and its allies, reflecting adaptation of the symbionts to an ancient agricultural system.