We report a case of gene fusion and horizontal gene transfer (HGT), two noncanonical molecular evolutionary processes whose functional significance remains obscure. Diffusion of products between enzymes can be a limiting step in metabolic pathways, a problem that can be mitigated in part by the physical association of enzymes that act in adjacent steps of the pathway. A manual search of the 1000 Fungal Genomes Project revealed 10 species within the large Pezizomycotina (Ascomycota) containing a single gene that encodes both aconitate hydratase (aconitase family) and isocitrate/isopropylmalate dehydrogenase (IsoDH), two enzymes that are adjacent in the citric acid cycle and other pathways. Surprisingly, the species containing this fusion were found to be distantly related, a finding that could be due to either: a) repeated independent fusion of these two genes; or b) a single gene fusion event and repeated horizontal transfer of the fused gene.
To distinguish between these possibilities, we reconstructed phylogenetic trees for each of the enzymes separately and compared these gene trees to the consensus species tree. Our results strongly support a grouping of the fused genes, implying multiple cases of HGT. Expanding this search to other fungal groups revealed additional species harboring aconitase-IsoDH fusions in a pattern that suggested additional HGT events, but also suggested a fusion event independent from those within Pezizomycotina. The striking propensity for HGT of these fused genes suggests that they impart a fitness advantage, likely through increased efficiency of metabolic pathways, revealing the ways that HGT can disseminate useful molecular innovations throughout the tree of life. .