Evidence for horizontally transferred genes involved in the biosynthesis of vitamin B(1), B(5), and B(7) in Heterodera glycines

Abstract

Heterodera glycines is a nematode that is highly adapted to manipulate and parasitize plant hosts. The molecular players involved in these interactions have only recently begun to be identified. Here, the sequencing of the second stage juvenile transcriptome, followed by a bioinformatic screen for novel genes, identified seven new genes involved in biosynthesis and salvage of vitamins B₁, B₅, and B₇. With no confirmed reports in the literature, each of these biosynthesis pathways is believed to have been lost in multicellular animals. However, eukaryotic-like introns in the genomic sequences of the genes confirmed eukaryotic origin and nematode-specific splice leaders found on five of the cDNAs confirmed their nematode origin. Two of the genes were found to be flanked by known nematode sequences and quantitative polymerase chain reactions on individual nematodes showed similar and consistent amplification between the vitamin B biosynthesis genes and other known H. glycines genes. This further confirmed their presence in the nematode genome. Similarity to bacterial sequences at the amino acid level suggested a prokaryotic ancestry and phylogenetic analysis of the genes supported a likely horizontal gene transfer event, suggesting H. glycines re-appropriated the genes from the prokaryotic kingdom. This finding complements the previous discovery of a vitamin B₆ biosynthesis pathway within the nematode. However, unlike the complete vitamin B₆ pathway, many of these vitamin B pathways appear to be missing the initial enzymes required for full de novo biosynthesis, suggesting that initial substrates in the pathways are obtained exogenously. These partial vitamin B biosynthesis enzymes have recently been identified in other single-celled eukaryotic parasites and on rhizobia symbiosis plasmids, indicating that they may play an important role in host-parasite interactions and survival within the plant environment.

Keywords: Heterodera glycines; biosynthetic pathways; biotin; horizontal gene transfer; pantothenate; thiamin; vitamin B1; vitamin B5; vitamin B7.

Fig. 1

A. Graphical representation of the H. glycines thiM, thiE, and thiD genes. B. Graphical representation of the M. hapla 91-kb contig with panC, other predicted HGT genes, and gland specific genes.

Fig. 2

Prokaryotic subtrees of the vitamin B biosynthesis enzymes. aLRT branch supports are labeled at each node. Complete color coded trees with full species names can be found in the supplemental data; for clarity, a near complete THI4 tree is shown. H. glycines is marked with a large arrow, while other nematode species are marked with a small arrow. The log likelihood for each tree is also shown.