Objective      To analyze the structure of 89K pathogenicity island (89K PAI) of Streptococcus suis serotype 2 (S. suis 2)

and to explore its evolutionary pathway.       Methods      BLAST was performed to obtain S. suis 2 89K PAI homologous sequences and to establish the homologous sequence database. The base composition pattern of 89K PAI was analyzed to find recombination hotspots

and 89K PAI was divided into several independent regions. The function prediction of each open reading frame (ORF) was performed to distinguish functional modules. Phylogenetic and co-linearity analysis of 89K PAI and its homologous sequences were performed to speculate the possible evolutionary pathway.       Results    S. suis 2 89K PAI was divided into four conserved regions

four main non-conserved regions and a Tn 916 transposon

showing a mosaic structure made up of heterologous sequences. The conserved regions were involved in 89K PAI lateral gene transfer and its maintenance in host bacteria genome. The conserved regions showed a congruent evolutionary pattern and in some bacterial genome the conserved regions were continuous.       Conclusion        The ancestor status of the conserved regions exists as a continuous entirety

and the non-conserved regions from diverse origins are inserted into the conserved regions by multiple lateral gene transfer and recombination

resulting in the mosaic structure of S. suis 2 89K PAI.