Doctoral Dissertations

Spring 1994

Dissertation

Genetics

Degree Name

Doctor of Philosophy

Thomas D Kocher

Abstract

The causative agent of Lyme disease, Borrelia burgdorferi, is a spirochete transmitted among vertebrate hosts by Ixodid ticks. The outer surface of bacteria plays an important role in the ability to establish an infection in its host. The possibility that positive selection acts to accelerate the evolution of the outer surface protein genes was examined by comparing the DNA sequences of the OspA/B operon of B. burgdorferi isolates from North America and abroad. The Mean d$\rm\sb s$ values were two-fold higher than mean d$\rm\sb N$ values. Central regions of the OspB gene show an excess of silent mutations, while end regions are constrained by biased base composition or selective constraints. Osp proteins are under little purifying selection for the maintenance of primary structure.

Local genic variation within separate spirochetal populations along the Maine coast was investigated. Locations were expected to contain single strains adapted to their unique set of conditions. Nine unique OspB sequences were observed among 110 isolates. Six strains were characterized as having a unique OspB amino acid sequence, and five of which were found at multiple sites. It seems unlikely that the five strains are associated with a unique host.

Arbitrarily Primed Polymerase Chain Reaction (AP-PCR) genomic fingerprints of several B. burgdorferi isolates were generated to further characterize those members within each OspB type. Microgeographic heterogeneity was observed. The genetic relatedness within OspB types varied from 31.8% to 100%. Three isolates from the same rat contained different fingerprints. Hosts may be infected by a large number of strains. Some fingerprints were shared among different OspB types, which suggests that genetic exchange of plasmids may be occurring between spirochetes.

Researchers have recently been questioning the validity of Ixodes dammini as a species distinct from I. scapularis. Nucleotide sequences of the mitochondrial 16S rRNA gene for several Ixodes species were generated to investigate the genetic relatedness within this genus. Samples of I. dammini showed a one percent sequence difference compared to I. scapularis. This difference was approximately half that characteristic of other interspecific differences with the I. ricinus complex, but significantly more than typical levels of intraspecific variation.

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