By sequencing the transcriptomes of seven Pristionchus species as well as three closely related diplogastrid nematodes (Diplogasteroides magnus, Koerneria sudhausi and Acrostichus species), the authors were able to leverage a well-established phylogenetic relationship between the ten species to conduct a series of hypothesis-driven inquiries at the species, genus and family levels of evolutionary relationships [8]. The focus of this work is on a family of cellulase genes thought to have been acquired from an ancestral slime-mold species in an ancient HGT event. The cellulases were identified in all seven Pristionchus species and in K. sudhausi, but were not found in the remaining two species. Assays confirmed a perfect correlation between the presence of the gene in the transcriptomic data and demonstrable cellulase activity, thus satisfying the first criteria of functional activity of a horizontally acquired gene.
A comparison of the nematode cellulase genes with the cellulase genes in slime molds indicates that amelioration, the process by which the foreign gene assumes the identity of its host, has taken place. Notably, intron size and number in the genes reflect the general gene-structure patterns found within each species. This satisfies the second criterion that the genes be integrated into the host genome after a transfer event. Innovatively, Mayer et al. [8] build on the current phylogenetic framework to address the last two features of successful HGT, namely longevity and selective pressure. The 12 Pristionchus cellulase ESTs that contained the carbohydrate-binding module at their carboxyl terminus were used in a phylogenetic reconstruction of the genes' history. For 11 of the 12 genes, the topology of the phylogeny was identical with that of the species tree. The observed concordance of the gene history with the species history implies that the gene transfer must have survived multiple speciation events and points to it being ancient. This is a strong indicator of longevity of the cellulases following acquisition by an ancestral species.