| Bacterial fruit blotch of melons, caused by Acidovorax citrulli, is a world-spared disease that causes serious damages on the plants of cucurbits worldwide.Due to the great genetic diversity of A. citrulli, it is difficult to select resistant cultivar. Here, we analysised the genetic diversity of 118 A. citrulli strains collected in China and abroad with the method of pulsed field gel electrophoresis (PFGE) as well as multilocus sequence typing (MLST). The results are listed as follows:(1) The results of PFGE electrophoresis band analysis indicated that there were 19 electrophoresis bands, ranking from 200~2000 kb. 118 A.citrulli strains could be classified into 9 banding patterns. The analysis of Ntsys 2.1 indicated that all A.citrulli strains could be grouped into two subgroups: group 1 included 73 A.citrulli strains, 67 of which were isolated from melon. Group 2 included 45 A. citrulli strains, 42 of which were isolated from watermelon. Group 1 includes strains mainly from melon and group 2 includes strains mainly from wartermelon. These data indicated that there were genetic differentiation on hosts of A. citrulli. Of 112 A. citrulli strains selected in China, 71 strains, 64 of which were selected from Xinjiang province and Inner Mongolia, belonged to group 1 and 41 strains, selected in diverse provinces in China, belonged to group 2. The relationship between genetic diversity and geographical origin was not clear. Typical strain Fc247 and strain Fc520, selected from the United States, belonged to group 1 and all the other strains selected abroad belonged to group 2. PFGE had high repeatability and specificity, and the result could be easily identified. It had reliable result by identifying large DNA fragments.(2) MLST analysis of 118 A. citrulli, carried out using 7 house-keeping genes (adk, gltA, glyA, pilT, spi, ugpB, and glnA), indicated that there were 73 sequence types (ST) out of 118 A. citrulli strains. Analysis of eBURST V3 indicated that all 118 A. citrulli strains could be grouped into 3 units: group 1, group 2 and a singleton. There were 53 STs (82 A. citrulli strains) in group 1 and 19 STs (35 A. citrulli strains) in group 2. Watermelon strain Fc380, selected from the United States, was grouped as a singleton. From the view of host range, 68 of the 82 A. citrulli strains in group 1 were isolated from melon and the rest 14 A. citrulli strains were isolated from watermelon. 2 of the 35 A. citrulli strains in group 2 were isolated from melon and the rest 33 A. citrulli strains were isolated from watermelon. This result tallied closely with the PFGE analysis, indicating that A. citrulli from group 1 were mainly selected from melon and A. citrulli from group 2 were mainly selected from watermelon. The difference between PFGE and MLST analysis was that 10 watermelon strains from Beijing were grouped in group 1 based on MLST, suggesting that these 10 watermelon strains had a close kinship with melon. From the point of geographical origin, core type ST45 from group 1 included 3 A. citrulli strains, all of which were selected from Xinjiang province, suggesting that all other A. citrulli strains in group 1 had very close relationship with the Xinjiang strains. Core type ST5 from group 2 included 11 A. citrulli strains, which were selected from Beijing, Xinjiang, Hainan, Heilongjiang and Taiwan, suggesting that A. citrulli strains of group 2 distributed diversely. Typical strain Fc247 and strain Fc520 (both from the United States) were grouped in group 1. Fc356 (from India), Fc376 (from the United States) and Fc491 (from Japan) were grouped in group 2. Fc380 (from the United States) was grouped as a singleton. The results of MLST analysis could be listed in the official site of MLST (http://www.mlst.net/) and facilitate the follow-up study. It could also help understand the epidemiology and population structures of bacteria.
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