Study On The Horizontal Gene Transfer And Evolution Between Oomycetes And Land Plants

Oomycetes is a diverse group of deeply branching eukaryotic microorganisms that includes pathogens of plants, insects, crustaceans, fish, vertebrate animals, and microbes. The diseases caused by these pathogens are notoriously difficult to manage on thousands of crop and ornamental plants, resulting in tens of billions of dollars of losses annually. Because of their filamentous growth habit, oomycetes had been traditionally classified in the kingdom of fungi. In fact, modern molecular and biochemical analyses suggest that oomycetes have little taxonomic affinity with filamentous fungi, and are very distant in evolutionary terms from fungi, animals and plants. Rather, they are more closely related to brown algae and diatoms in the stramenopiles, one of several less well studied eukaryotic kingdoms.Horizontal gene transfer (HGT) describes the transmission of genetic material across species boundaries and is an important evolutionary phenomenon in prokaryotes. HGTs in eukaryotes are not as much as those in prokaryotes, but after sequencing of lots of eukaryotes, we find the role of HGT in eukaryotic evolutionary history is very important too. It has been reported that land plants can get genetic material across species by HGT. But HGT between land plants and oomycetes is still unexplored. As pathogens of plants for a long time, we assumed there were HGTs between oomycetes and land plants. And the research on this will help people to realize this important pathogen of plant further and offer methods for controlling diseases causing by oomycetes.We mainly tried to study on the HGT in oomycetes, including HGT with plants and bacteria. First, we downloaded the proteomes and genomes of six oomycetes species and constructed pan-genome by OrthoMCL. Pan-genome had96788genes clustered into14831gene families. Second we used one sequence of every gene family to compare with those of NCBI nr datebase then identified572genes that show the highest similarity to corresponding genes from land plants and694genes that show the highest similarity to corresponding genes from bacteria. We constructed a phylogeny for all1266genes identified and all homolog groups available from the NCBI nr datebase and used these to define17candidate HGT events.2HGTs were oomycetes-to-land plants transfers, and1was land plants-to-oomycetes HGT.3other HGT events were also found, but the direction of these transfer events was difficult to confirm. In addition,11HGT events were found between bacteria and oomycetes. All the11HGTs were bacteria-to-oomycetes.Putative functional annotation of the HGT candidate genes suggests that one transfer between plant and oomycetes has the function which can increase the expression of defense genes(including itself) in Arabidopsis. And another bacteria-to-oomycetes transfer can encode a zeta toxin which can directly lead to programmed bacteria cell death. The zeta toxins are a family of proteins that are normally present within various pathogenic bacteria and can mysteriously trigger suicide when the cells undergo stress. The mechanism underlying this programmed bacterial cell death is that zeta toxins convert a compound required for bacterial cell wall synthesis into a poison that kills bacteria from within. In the future it may be possible to hijack this mechanism for bacterial defense and to design drugs that mimic these toxins.Our study suggests that genetic exchange between plants and oomycetes is exceedingly rare, but has occurred during their evolutionary history and might add important metabolic traits to both oomycetes and plant lineages. HGT between oomycetes and bacteria is a continuing process, might add important traits for oomycetes infecting bacteria too.