Abstract

The soybean cyst nematode (SCN), Heterodera glycines is an obligate plant parasite that can cause devastating crop losses. To aide in the study of this pathogen, the SCN genome and the transcriptome of second stage juveniles and eggs were shotgun sequenced. A bioinformatic screen of the data revealed nine genes involved in the de novo biosynthesis and salvage of the B vitamins: B₁, B₅, B₆, and B₇. Each of these de novo pathways are believed to have been lost in most animals and their similarity to bacterial homologues made them candidates for horizontal gene transfer (HGT). Each gene had eukaryotic-like introns and poly-A signals confirming that they came from a eukaryote. Four genes had nematode-specific splice leaders attached to their cDNA which confirmed that they came from nematodes. Quantitative polymerase chain reaction on individual nematodes showed similar amplification between the vitamin B biosynthesis genes and other known nematode genes, this further confirmed their presence in the nematode genome. The similarity to bacterial homologues on the amino acid and nucleotide level suggested prokaryotic ancestry and phylogenetic analysis of the genes supported a likely HGT event from bacteria to SCN. HGT was further supported by gene synteny of the two genes HgSNZ and HgSNO. These two genes also were shown to form a complete, functional pathway for vitamin B₆ biosynthesis, representing the first example the HGT of a complete functional pathway from bacteria into nematodes. The other pathways appear to be missing the initial enzymes for complete de novo biosynthesis of the vitamins, suggesting that the initial substrates are provided by the host. These partial biosynthesis pathways have been found in other single-celled parasites and rhizobia indicating an important role in parasitism and survival within the plant environment.