Evolutionary dynamics of insertion sequences in Helicobacter pylori.

Area of Science:

• Microbial genomics and evolution
• Bacterial genetics and pathogenesis

Background:

• Prokaryotic insertion sequence (IS) elements exhibit parasitic and symbiotic behaviors within microbial gene pools.
• Helicobacter pylori, a gastric pathogen, harbors diverse genetic elements, including IS605 and IS607.

Purpose of the Study:

• To investigate the evolutionary history and population genetics of IS605 and IS607 in Helicobacter pylori.
• To determine the roles of homologous recombination and adaptive selection in the evolution of these IS elements.

Main Methods:

• Screening of 488 H. pylori isolates from diverse geographic locations for the presence of IS605 and IS607.
• Nucleotide sequence analysis of IS605 (n=42) and IS607 (n=44) populations.
• Employing split decomposition and homoplasy tests to assess homologous recombination; analyzing nonsynonymous to synonymous substitution ratios to detect diversifying selection.

Main Results:

• IS605 and IS607 were found in 18% and 14% of isolates, respectively.
• IS605 populations showed blurred geographic structure due to recombination (homoplasy ratio, 0.56), while IS607 populations exhibited strong geographic structure with less recombination (homoplasy ratio, 0.2).
• Diversifying selection was detected in IS605 orfA (transposase gene) codons and in both IS605 and IS607 orfB codons (gipA homolog).

Conclusions:

• The evolution of IS605 and IS607 reflects selection for transposition activity and host-interaction functions.
• Similarities in the genetic structures of IS elements and H. pylori populations, coupled with evidence of adaptive evolution, suggest coevolution between these mobile elements and their bacterial host.
• These findings provide insights into the dynamic interplay between mobile genetic elements and bacterial genome evolution.