Horizontal Gene Transfer in Pristionchus

The acquisition of genes from non-parental organisms occurs through a process called ‘horizontal gene transfer’. This phenomenon is frequently observed in prokaryotes and thought to be an important driving force for evolution. For example, bacteria can acquire antibiotic resistant genes from other bacteria, thus giving them an advantage to survive in hospitable environments. In contrast, horizontal gene transfer was long thought not to be frequently found in eukaryotes. However, work over the last decade revealed that an important exception to this is observed in the phylum nematoda where horizontal gene transfer has been repeatedly demonstrated. In particular, a small group of nematodes are parasitic to plants, and the genes encoding proteins to invade and digest plants have been shown to originate from bacteria and fungi. Interestingly, with the publication of the genome of the free-living nematode Pristionchus pacificus, we have found that this nematode also has horizontally acquired genes encoding for the enzyme cellulase (Dieterich et al. 2008). In nature, cellulose is the most abundant organic material produced by plants, although Pristionchus does not feed on plants themselves. Thus, it is a mystery why the cellulase genes were acquired in Pristionchus and subsequently expanded through gene duplications. Specifically, new evidence shows that in all Pristionchus spp. cellulase genes were attained and duplicated, which suggests these genes are under positive selection during evolution. For example, in our standard lab strain P. pacificus PS312, there are 8 copies of the cellulase genes in the genome and previous studies have shown that these cellulases are enzymatically active (Mayer et al. 2011).

Using genomics, genetics and state of the art genome-editing tools such as CRISPR, we are systematically studying the function of cellulase within the Pristionchus’ ecological niche. The available toolkit makes Pristionchus a unique system to study the function and ecological significance of horizontally acquired cellulases. This might also provide important insight for related genes independently acquired in plant parasitic nematodes. Furthermore, with new genomes coming out every day, and more and more horizontal gene transfer events are being detected, we hope that using the powerful Pristionchus evolutionary system, we will provide new insights into this important evolutionary phenomenon.

Selected References

Dieterich, C., Clifton, S. W. , Schuster, L. N. , Chinwalla, A., Delehaunty, K., Dinkelacker, I., Fulton, L., et al. (2008): The Pristionchus Pacificus Genome Provides a Unique Perspective on Nematode Lifestyle and Parasitism. Nature Genetics 40 (10): 1193–98.
Mayer, W. E., Schuster, L.N. , Bartelmes, G., Dieterich, C., Sommer, R. J. (2011):  Horizontal Gene Transfer of Microbial Cellulases into Nematode Genomes Is Associated with Functional Assimilation and Gene Turnover. BMC Evolutionary Biology 11 (January): 13.
Rödelsperger, C., Meyer, J. M. , Prabh, N., Lanz, C., Bemm, F., Sommer, R. J.  (2017): Single-Molecule Sequencing Reveals the Chromosome-Scale Genomic Architecture of the Nematode Model Organism Pristionchus Pacificus. Cell Reports 21 (3): 834–44.