Survival and reproduction in the face of changing environments is a key driving force during evolution. Living organisms have evolved various strategies to cope with continuously changing environments. One fundamental strategy is Horizontal Gene Transfer (HGT), where genes are transferred from one species to another. Once the organism obtains the new gene(s), it can rapidly exploit its new traits which might confer an advantage. Although HGT events are prevalent in unicellular organisms, recent whole-genome sequencing efforts have exposed extensive gene transfer events from prokaryotes to metazoans. This type of gene transfer is particularly crucial for symbiotic organisms that occupy new niches, where survival requires acquisition of new genes. For example, plant parasite nematodes (e.g., bacteriovorous) are capable of hydrolyzing hemi-cellulose, a function that is thought to have been acquired from bacteria. While HGT among prokaryotes is well characterized, the sequence of events leading to HGT from prokaryotes to metazoans is still elusive. Thus far, mechanistic studies of HGT in metazoans have been hindered by its rare incidence, and the fact that symbiotic organisms are difficult to maintain over long in-lab evolutionary studies.
To study HGT in the lab, we combine the powerful genetics of E. coli (the donor) and C. elegans (the recipient). Specifically, we attempt to induce HGT by feeding unc-119 mutant worms with E. coli that contain the unc-119 rescuing gene on a plasmid. In order to overcome extremely rare incidents of HGT, we culture large numbers of worms with the bacteria for multiple generations. In our initial attempt, we cultured 4×106 unc-119 mutants with E. coli for 10 generations (screening 4×107 worms in total) and isolated several worms with partial rescue of the Unc-119 phenotype. This partial rescue was not detected in the worm’s progeny suggesting that if HGT occurred, it was not transmitted via the germline to the next generation. In light of the high phenotypic variation of the unc-119 mutants, we added an additional and independent marker to ensure genuine HGT. In subsequent experiments we introduced a plasmid containing both unc-119 (+) rescue gene and an unc-122::gfp reporter gene that is expressed in the worm’s coelomocytes. We repeated the in-lab HGT experiment but did not detect unc-122::gfp signal in the unc-119-rescued worms. This suggests that bona-fide HGT did not occur.
These initial attempts indicate that HGT between E. coli and C. elegans is not easily induced under standard laboratory conditions and realistic time frames (weeks- months). It is possible that HGT events are more prone to occur when stress conditions arise. We therefore plan to repeat the in-lab evolution experiments when worms are subjected to various stress conditions (i.e., inducing worms into dauer state). In addition, E. coli might not be the natural bacteria that C. elegans encounters in nature. We therefore started to screen for bacterial species that are more likely to induce HGT in C. elegans.
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Obviously, this would create more work for you but I would be interested to know whether the efficiency of HGT differs from one direction to the other.
One way to investigate that question might be to develop culture conditions that are fine for worms but chemically inhospitable to your E. coli, causing them to grow slowly or not at all or to display some easily-scored stress phenotype. HGT of an appropriate metabolic gene from the worm to the bacteria could allow the latter to degrade the chemical challenge and establish a WT phenotype. Conveniently, any such genetic change in the bacteria would be “in the germline” to be cloned and analyzed at your convenience. You would then be able to demonstrate that the genetic difference was due to HGT of a worm sequence.