Year

2008

Degree Name

Doctor of Philosophy

Department

School of Biological Sciences - Faculty of Science

Abstract

Repetitive sequences often display a high level of homogeneity within species and can diverge rapidly from homologous copies found in other species. Neither this concerted evolution of repeated sequences nor the non-Mendelian processes of DNA turnover that spread these within a genome are controversial. What remains unclear is the significance of these genomic changes on the subsequent spread of variation within populations. In this thesis, the first ribosomal internal transcribed spacer (ITS1) was examined in the Anopheles punctulatus group to determine what generates intraindividual variation and to determine the significance of turnover in the divergence of An. farauti s.s. populations. Multiple length variants were detected within individuals by PCR and Southern hybridization. A high level of interspecific variation was found among 84 cloned isolates, yet several internal repeats are shared among these species. Much of the length variation is generated by a multicopy 5'-subrepeat, which was identified as a duplication of a conserved ITS2 region. The variation present in an An. farauti s.s. colony indicated that restrictions to interchromosomal recombination are unlikely. Instead, it is the presence of multiple units of turnover that appear to play a key role in the maintenance of intraindividual variation. A potentially recombinant data set of 36 sequences from an experimental mating between two An. farauti s.s. genotypes was analysed by recombination detection methods. No significant evidence was found for recombination, which suggests that substitutions are more important in generating variation. Bayesian phylogenetic analysis of the ITS1 using a data set of 187 sequences from 70 individuals showed that variation in this spacer is congruent with the geographic origin of the An. farauti s.s. samples. Remarkably, intraspecific concerted evolution occurs despite the level of intraindividual variation, while the interpopulation divergence demonstrates a high rate of spread of variation within populations. A quantitative analysis of intraindividual length variants in 14 of these populations was performed with 125 individuals using densitometry and Southern hybridization. Population-specific banding profiles were seen within individuals and the relative proportions of the different length variants within each population were homogeneous among a majority of the individuals. Two temporal samples from a colony also showed the relative abundance of these can change rapidly after just 150 generations. Thus ITS1 variants are spread rapidly in populations with a low level of interindividual variation, which indicates that a high rate of turnover is a significant factor in generating the allopatric divergence of An. farauti s.s.

02Whole.pdf (6086 kB)
03Appendices.pdf (1435 kB)

Share

COinS