Helitron Transposable Elements In Hirsutella Minnesotensis

Helitrons are rolling-circle transposable elements and widespread in eukaryotic,but most prevalent among plant and animal genomes.Helitrons often insert nearby or even within a gene,may cause deleterious effects by disrupting the protein coding sequence or altering the gene expression or regulation.Another important thing is Helitron can capture the flanking gene and driven them expansion,so Helitrons plays an important role in the evolution of genomes,genes and phenotypes.Soybean cyst nematode(SCN),Heterodera glycines,is one of the most destructive pests in soybean-growing regions allover the world.Hirsutella minnesotensis(Hypocreales,Ascomycota)is a dominant endoparasitic fungus by using conidia that adhere to and penetrate the secondary stage juveniles of soybean cyst nematode and have potential as a biological control agent.In this study,we comprehensively survey autonomous and non-autonomous Helitrons in H.minnesotensis according to RepHel transposases,to reveal the structural and encoding characteristics of Helitrons and analyzes its implications for the evolution of genes and genomes.The results of this study are as follows:Phylogenetic analysis of RepHel transposases showed that there are two groups of autonomous Helitrons elements: HmHeli1 and HmHeli2 in H.minnesotensis genome;the HmHeli1 not only has conserved terminal sequence and structure,but also encodes highly similar or even identical RepHel transposase,indicating that it expanded recently and is still active in the genome.HmHeli1 has the same structure and encoding characteristics with the Helitron2 element: there are two conserved sequences "TCAG" and "TATTTT" at the 5?and 3? ends respectively,a GC-rich hairpin near the 3?-end and asymmetric terminal inverted repeats making two ends pairing with each other.The transposase encoded by HmHeli1 contains a zinc finger,a Rep and a Helicase domain,but does not have the endonuclease domain.Based on the structural features of HmHeli1,we ab initio predicted the HmHeli1 in H.Minnesotensis genome.We found 35 autonomous and 59 non-autonomous HmHeli1 s components with distinct terminals,accounting for 0.59% of the genome.The analysis of RepeatMasker showed that the abundance of HmHeli1 s and HmHeli1 fragments in the genome was upto 2.47%.A few HmHeli1 elements have multiple 5' ends,and individual HmHeli1 elements are tandemly arrayed replication products.HmHeli1s are preferentially inserted into the gene-rich region even within the functional genes in the same direction as genes.Furthermore,there is an AT-rich motif existed at the target site.HmHeli1 does not have ability to capture genes actively,but can accept the insertion of other transposable elements passively,which may be related to the conserved and complementary ends.There are 8 different RepHel transposons Clusters in fungi.The transposases encoded by HmHeli1 elements belong to Cluster1,suggests that Helitron of Cluster1 has similar structural feature with Hm Heli1 elements;Cluster2 transposases are belonging to the same class with canonical RepHel transposases in plants and animals,so Helitrons in Cluster2 are similar to canonical Helitrons in animals and plant.The autonomous HmHeli1 elements are mainly distributed in Hypocreales.Sordariales has Helitron2 elements with clear terminal as well,but its terminal sequence is different from HmHeli1 elements.Evidence of high sequence identity and discontinuous distribution suggested that HmHeli1 elements could have been transferred into these species in part by horizontal transfers.In conclusion,HmHeli1 is non-canonical Helitrons that are characterized by asymmetric terminal inverted repeats,show hallmarks of recent activity and may still be active which not only played an important role in the evolution of H.minnesotensis genome and other Hypocreales genome,but also has the potential to be developed as a universal genetic analysis tool for biological research.