MBE Advance Access published online on June 25, 2009
Molecular Biology and Evolution, doi:10.1093/molbev/msp124
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Research Article |
Comparative evolutionary analysis of the major structural subunit of Vibrio vulnificus type IV pili
1 Department of Microbiology, University of Washington, Seattle, WA
2 Northwest Fisheries Science Center, National Marine Fisheries Service, NOAA, U.S. Department of Commerce, Seattle, WA
3 Department of Ecology and Evolution, SUNY at Stony Brook, NY
* Correspondence to: Evgeni V. Sokurenko [Department of Microbiology, room E309 HSB, UW 357242, Health Sciences Bldg, Room E309, 1959 NE Pacific St., Seattle, WA 98195, Phone: (206) 685-2162; Fax: (206) 543-8297; E-mail: evs{at}u.washington.edu]
Mark S. Strom [Resource Enhancement and Utilization Technologies Division, Northwest Fisheries Science Center, National Marine Fisheries Service, 2725 Montlake Blvd. E., Seattle, WA 98112, Phone: (206) 860-3377; Fax: (206) 860-3467; E-mail: mark.strom{at}noaa.gov]
Received for publication May 11, 2009. Revision received June 15, 2009. Accepted for publication June 17, 2009.
Type IV pili contribute to virulence in Vibrio vulnificus, the bacterium responsible for the majority of fatal seafood-related infections. Here we performed within- and between-species evolutionary analysis of the gene that encodes the major structural subunit of the pilus, pilA, by comparing it with pilD and gyrB, the genes encoding the type IV prepilin peptidase and β subunit of DNA gyrase respectively. While the diversity in pilD and gyrB is similar to each other and likely to have accumulated after speciation of V. vulnificus, pilA is several times more diverse at both nonsynonymous and synonymous levels. Also, in contrast to pilD and gyrB, there are virtually unrestricted and highly-localized horizontal movements of pilA alleles between the major phylogenetic groups of V. vulnificus. The frequent movement of pilA involves homologous recombination of the entire gene with no evidence for intragenic recombination between the alleles. We propose that pilA allelic diversity and horizontal movement is maintained in the population by both diversifying and frequency-dependent selection most likely to escape shellfish innate immunity defense or lytic phages. Other possibilities leading to such selection dynamics of V. vulnificus pilA could involve adaptation to diverse host populations or within-host compartments, or natural DNA uptake and transformation. We show that the history of nucleotide diversification in pilA predates V. vulnificus speciation and this diversification started at or before the time of the last common ancestor for V. vulnificus, V. parahaemolyticus and V. cholerae. At the same time, it appears that within the various pilA groups of V. vulnificus there is no positive selection for structural mutations and consequently no evidence for source-sink selection. In contrast, pilD has accumulated a number of apparently adaptive mutations in the regions encoding the membrane-spanning portions of the prepilin peptidase, possibly affecting fimbrial expression and/or function, and is being subjected to source-sink selection dynamics.
Key Words: type IV pili • adhesion • diversifying selection • recent adaptive evolution
Contributed equally