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Microorganisms are a goidmine for evolutionary genetics as their genomes can evolve at an extraordinary rate which results in some of the most extravagant adaptations in terms of genome structure and function as well as survival in the most unusual environments. One trend observed in several evolutionary scenarios is genome degeneration. It is most prominent in endosymbionts and obligate intracellular parasites and is a consequence of many constraints encountered in the intracellular environment. The process involves loss of many protein-coding genes, resulting in greater dependence on the host, and loss of non-coding DNA such as intergenic regions, which has a direct impact on regulation of genome function. I have chosen two evolutionarily distinct systems to analyze the stages and functional consequences of genome degeneration, namely the impact of genome compression on transcription in an obligate parasite Antonospora locustae (genus Microsporidia), and gene content in the mitochondrion of a diatom endosymbiont found in the dinoflagellate Durinskia baltica. I have successfully mapped transcriptional start and termination sites from 14 loci in Antonospora locustae, and cloned fragments of two genes that are part of the electron transport chain from the mitochondrion of the diatom endosymbiont in Durinskia baltica. My analysis reveals that transcription in A. locustae is always initiated immediately upstream of the open reading frame at a single point for every locus, whereas transcriptional termination can occur at several points for a single gene and, in some instances overlaps with a downstream reading frame. The identification ofNADH5 and ATPase9 from the mitochondrion of the endosymbiont in D. baltica is further evidence for the preservation of function in this enigmatic organelle.

Seventy-eight harbour porpoises, Phocoena phocoena, (33 females, 45 males) were obtained from the summer (June-September) cod fishery incidental-by-catch in the Gulf of St. Lawrence, and examined for the presence of cranial sinus nematodes. This is the first quantitative and in-depth study of the distribution of Stenurus minor (Kuhn, 1829) Baylis and Daubney, 1925 in the cranium of by-caught harbour porpoises. Stenurus minor was present in the cranial sinuses of all adult porpoises ($>$1yr, n = 66, x = 2362, range 87-8920) and absent in all young-of-the-year ($<$1yr, n = 12). Only fifth-stage worms were observed and these were equally distributed between the right and left side ot the skull (mean intensity = 1158 and 1213 in the left and right side, respectively). Female S. minor were predominant (1:1.8 sex ratio). Mean intensity of S. minor was similar among all infected porpoises, suggesting an annual loss and recruitment of this parasite. Parasite load had no apparent effect on porpoise body condition (measured as % blubber weight of carcass). No gross lesions associated with the presence of numerous S. minor in the cranial sinuses, were observed. There was an inverse relationship between the intensity of S. minor and mean worm-length, suggestive of a "crowding effect". Mean worm-length in lightly infected porpoises was 17.8 $ pm$ 0.2 mm and 16.1 $ pm$ 0.2 mm in heavily infected animals. Possible life-cycles and modes of transmission are examined in light of our findings. The absence of S. minor in porpoises less than 1 year old suggests heteroxenous transmission of this parasite, via an intermediate host in the food chain. Transmammary and or transplacental transmission is unlikely.

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By definition, parasites are expected to have fitness consequences for their hosts by reducing survival and fecundity. If such events are density dependent they may play a regulatory role in their host's dynamics. However, there are few studies in the wild that provide empirical evidence to support these suppositions. To understand the impact of parasites it is necessary to explore the interactions between parasite and host and the mechanisms that regulate nematode populations. The aims of this work are to: 1) identify the species specific patterns of infection; 2) investigate the interactions between and within nematode species and 3) examine the regulatory mechanisms that control nematode fecundity. The distribution of parasites between hosts and the variation between years, seasons, reindeer age and location are also examined. Nematode infections of Svalbard reindeer are dominated by two species: Ostertagia gruehneri and Marshallagia marshalli and their contrasting life-histories and population dynamics highlights the importance of investigating at the level of individual species. For 0. gruehneri, there is significant annual variation but no strong seasonal pattern in abundance. Susceptible calves do not acquire infection until their second summer. Egg output is highly seasonal with a peak in July and controlled through density dependent effects on worm development. In contrast M marshalli, shows a strong seasonal cycle which does not vary between years and the peak occurs in late winter suggesting winter transmission. Egg output is low and also confined to the winter months. The quantification of these traits is important in allowing parameterisation of models with data from the study system. In many studies parameters are estimated from studies of domestic host parasite systems and these may be inappropriate in this natural system. The role of immunity and arrested development and the relationship between transmission and environmental heterogeneity are discussed.

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No description available.