MBE Advance Access published online on March 19, 2004
Molecular Biology and Evolution, doi:10.1093/molbev/msh124
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2004; all rights reserved
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1 Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102, USA
* To whom correspondence should be addressed. E-mail: hjohanne{at}nature.berkeley.edu.
In this study we investigate the possibility of selection acting on the proline-rich antigen (PRA) gene in natural populations of the two human pathogens Coccidioides immitis, Coccidioides posadasii and three of their close relatives, Chrysosporium lucknowense, Chrysosporium queenslandicum and Uncinocarpus reesii. We addressed the following questions: Is diversifying selection acting on PRA in the pathogenic species as a result of avoidance of the host's immune system, and has adaptation to a pathogenic life style lead to positive directional selection and increased rate of evolution in PRA between the species? For these purposes, we amplified and sequenced from 40 individuals belonging to the five species, the entire coding region of the PRA gene, as well as partial sequences from the coding region of each of the three housekeeping genes glyderaldehyde-3-phosphate dehydrogenase, glutamine synthetase A and hexokinase A. We used likelihood-based methods to compare models of different types of selective pressure among codons to analyze the mode of evolution of the genes, and found that the PRA gene evolves under positive selection but the investigated parts of the housekeeping genes evolve primarily under purifying selection. We found a very low level of intraspecific variability and no evidence of diversifying selection, suggesting that the increased rate of evolution in the PRA gene is not a result of avoidance of the host's immune system. Neither did likelihood-based analyses suggest that selection was stronger on the branch separating pathogenic and non-pathogenic species. Instead, we suggest that positive selection act on PRA as a consequence of spore cell-wall morphogenesis unique to each species. Key Words:
Coccidioides, positive selection, proline-rich antigen, vaccine, paml, antigen
© 2004 Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2004; all rights reserved.
Original Articles
Positive Directional Selection in the Proline-Rich Antigen (PRA) Gene among the Human Pathogenic Fungi Coccidioides immitis, C. posadasii and Their Closest Relatives
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