Currencies of Mutualisms: Sources of Alkaloid Genes in Vertically Transmitted Epichloae

Abstract

The epichloae (Epichloë and Neotyphodium species), a monophyletic group of fungi in the family Clavicipitaceae, are systemic symbionts of cool-season grasses (Poaceae subfamily Poöideae). Most epichloae are vertically transmitted in seeds (endophytes), and most produce alkaloids that attack nervous systems of potential herbivores. These protective metabolites include ergot alkaloids and indole-diterpenes (tremorgens), which are active in vertebrate systems, and lolines and peramine, which are more specific against invertebrates. Several Epichloë species have been described which are sexual and capable of horizontal transmission, and most are vertically transmissible also. Asexual epichloae are mainly or exclusively vertically transmitted, and many are interspecific hybrids with genomic contributions from two or three ancestral Epichloë species. Here we employ genome-scale analyses to investigate the origins of biosynthesis gene clusters for ergot alkaloids (EAS), indole-diterpenes (IDT), and lolines (LOL) in 12 hybrid species. In each hybrid, the alkaloid-gene and housekeeping-gene relationships were congruent. Interestingly, hybrids frequently had alkaloid clusters that were rare in their sexual ancestors. Also, in those hybrids that had multiple EAS, IDT or LOL clusters, one cluster lacked some genes, usually for late pathway steps. Possible implications of these findings for the alkaloid profiles and endophyte ecology are discussed.

Keywords

• ergot alkaloids
• indole-diterpenes
• lolines
• Clavicipitaceae
• epichloae
• endophytes
• grasses
• Poaceae
• Poöideae
• symbiosis