| Cytospora canker are among the most important forest diseases,causing devastating economic losses and ecological damage.Among them,poplar canker disease caused by Cytospora chrysosperma has become an important threat to the growth of poplar.C.chrysosperma with a broad host range,are able to infect more than seven different plant genus,such as Ailanthus altissima.However,Valsa mali and Valsa pyri mainly infected apple and pear trees and caused canker disease,respectively.Until now,the whole genome of C.chrysosperma,V.mali and V.pyri have been sequenced,assembled and annotated successively.Comparative genomics of Cytospora species with distinct host specificity,will provide leads to study host range determinants in the Cytospora and provide a stepping stone to indicating the molecular mechanism of the interaction between Cytospora spp.and their hosts,and controlling the Cytospora canker diseases.To characterize genomic variation among different Cytospora species,we sequenced and annotated the draft genome of Cytospora piceae X.L.Fan,which was described as a new Cytospora species on Picea crassifolia causing cankers,by Pac Bio sequencing technology.The bioinformatics and comparative genomics tools were applied to determine the shared and unique gene families of four Cytospora species.In addition,the genomic evolution of 10 fungi in the order of Diaporthales was analyzed,and the expanded or contracted gene families related to host range determinants during the evolutionary process were indicated.The main results and conclusions are as follows:(1)The assembled genome size of C.piceae was 39.25 Mb,with a GC content of51.79%.The genome contains 21 contigs and a total of 10835 protein coding genes were annotated.The phylogenetic tree based on single-copy orthologue genes showed that C.piceae was closely related to C.chrysosperma.(2)The results of comparative genomic analysis showed that: GC content distribution analysis indicated repeat-induced point mutation activity in all Cytospora species,and C.piceae had the strongest repeat-induced point mutation activity.The carbohydrate active enzymes of all four Cytospora spp.was similar in number.Among plant cell wall degrading enzymes,the auxiliary activity family 3 and 7 related to lignin degradation expanded significantly,while the auxiliary activity family 5,the key enzymes for lignin degradation,was absent in Cytospora spp.The number of genes in the glycoside hydrolase family 28 and8 of key enzymes for pectin degradation in the C.piceae and C.chrysosperma genome was similar to that of V.mali and V.pyri.C.piceae and other three Cytospora species all had more major facilitator superfamily transporters and fewer ATP-binding cassette family transporters.In addition,C.piceae contained more Drug: H+ Antiporter-2,Pleiotropic Drug Resistance and Multidrug Resistance Exporter transporters,while V.mali contained less Drug: H+ Antiporter-1 transporters.Gene Ontology functional classification indicated that the genes of all Cytospora species concentrated on hydrolase activity,V.mali has the highest number of the genes of this class,and the biological processes were mainly related to carbohydrate metabolism,pectin catabolic and oxidation reduction processes.Among the secondary metabolism core genes,C.piceae had fewer polyketide synthases genes than V.mali and V.pyri.Among the four C.piceae specific secondary metabolites genes,three were nonribosomal peptide synthases genes.Therefore,the carbohydrate active enzymes of four Cytospora species were similar in numbers,which showed strong pectin degradation ability.A complex pattern of presence or absence of nonribosomal peptide synthases genes in the secondary metabolites core genes,and the expanded multidrug transporters of four Cytospora species were observed,which indicated that they are likely to play an important role in host selection of Cytospora species.(3)An evolutionary analysis of 10 fungi from the order Diaporthales,including the genus Cytospora,revealed that they experienced extensive gene loss events during evolution.Moreover,C.chrysosperma with a wide range of hosts lost more genes than V.mali and V.pyri with a relatively narrow range of hosts.C.chrysosperma extended more gene families related to "secondary metabolite biosynthesis,transport and catabolism",The extended gene families in V.mali and V.pyri were mostly related to "carbohydrate transport and metabolism". |
