Horizontal Gene Transfer (HGT) goes against what we typically consider the normal transfer of genetic material from parent to offspring. HGT involves the transfer of genetic material from one organism to another. Within the bacteria, whose mode of survival typically depends on phagocytosis, there is a fairly amount of HGT. Events of HGT have been rarely observed in Eukaryotes because numerous barriers exist to prevent foreign nucleotides from entering a cell’s nucleus. Some of these barriers in the Fungi include a substantial cell wall made of chitin, multiple cell and nuclear membranes to cross, and the secretion of metabolic enzymes to the outside of the cells and subsequent uptake of the nutrients. Despite these barriers, there is now evidence of multiple occurrences of HGT in the fungi.
In a recent article published in the journal Current Biology, Jason Slot and Antonis Rokas, both of Vanderbilt University, provided evidence of HGT in two Ascomycete clades. In this study, the authors identified a 23-gene cluster from the genus Aspergillus which relocated to the genus Podospora. Genes that are in this cluster synthesize the toxic compound, Sterigmatocystin, which is a precursor to aflatoxins, noted for their production in Aspergillus. Both genera are located in the subphylum Pezizomycotina, so each clade is not distantly related, but HGT was observed using different methods.
While it’s easy to observe genetic material passed from generation to generation, recognizing HGT is a little more difficult. The main way the researchers have identified HGT is using phylogenetic methods to identify gene clusters whose homology cannot be explained by lineage alone.
Thomas Richards points out in his commentary on the Slot & Rokas paper (also in Current Biology), that because fungi do not phagotrophically consume their food they are less likely to incur HGT event. There are two notable hypotheses to why we do see HGT in the fungi. First, many secondary pathway genes in Eukaryotes are encoded in gene clusters, and the fungi have a fair amount of these clusters. Gene clusters, which are more functional in a natural selection sense, are therefore more likely to persist upon transmission, as opposed to individual genes. Data from HGT studies in fungi support this hypothesis. Second, fungi are naturally, from the basis of their biology and natural history, intimately tied to other organisms, and fulfill roles as saprobes, pathogens, or symbionts. This close intimacy increases the opportunity for genes to transfer from one organism to another. Data suggests that this hypothesis is true also, as many of the recorded instances of HGT in fungi have been observed in organisms with overlapping environments.