Marina Marcet-Houben and Toni Gabaldon
Trends in Genetics Vol. 26 No. 1 2010
Horizontal gene transfer between bacteria is widely studied for its numerous consequences including increased antibiotic resistance, virulence and transfer of metabolic pathways. What is less often considered is horizontal gene transfer from a prokaryotic donor to a eukaryotic recipient. Although this is known to happen, its mechanisms and effects are poorly understood. This study searches for genes of bacterial origin in the fungal kingdom to attempt to gain a broader understanding of the frequency of gene transfer from prokaryotes to fungus and what its evolutionary implications might be.
This study was conducted using whole genomes of over 60 different fungi and over 600 genomes from prokaryotic and other eukaryotic organisms. Using a conservative detection method they found 713 genes over 53 genomes that were acquired from prokaryotic sources. This is an indication of the frequency of horizontal gene transfer to eukaryotes, but also represents a novel way to search for horizontal gene transfer events using whole genomes. However, the number of transfer events is difficult to estimate due to gene loss and multiple genes being transferred in a single event.
The distribution of horizontal gene transfer events over the different fungal clades is not even and suggests factors that could aid or hinder transfer. Identification of traits in eukaryotic species that make them good candidates for horizontal gene transfer could have far reaching implications. Several of the genes that were observed to transfer were analyzed and their possible evolutionary advantages discussed. The first to be discussed was the arsenic detoxification pathway. It appears that the specific types of yeast mentioned have the machinery to reduce arsenate to arsenite but a bacterial reductase successfully transferred and replaced the standard yeast reductase. In another example, to convert between optical isomers of amino acids a racemase is necessary. Several different types were found to be transferred into two different members of the yeast family and a rotifer from a bacterial species. This could lead to the ability to use new sources of amino acids. Bacterial catalases were also found to be transferred to pathogenic fungal species. These catalases help protect pathogens from host reactive oxygen defense mechanisms. Finally, the transfer of a functional bacterial metabolic pathway was found in Aspergillus species. What is remarkable about this transfer is that the three genes that make up this pathway appear to have moved as two units, with two of the fused into a single gene, instead of three separate genes as they are found in the donor.
This study has a well thought out approach to searching for horizontal gene transfer events between prokaryotes and eukaryotes that could allow insight into the evolutionary history of many different species with unique abilities. It will be interesting to see if this type of horizontal gene transfer has played a larger role in the evolution of eukaryotes than previously thought.
University of Idaho