Genomic analysis of the fresh water mollusc Biomphalaria glabrata to understand host: Parasite interactions

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The fresh water mollusc Biomphalaria glabrata is the intermediate host for the trematode parasite Schistosoma mansoni, this parasite is responsible for the human disease Schistosomiasis. The significance of B. glabrata in the transmission of schistosomiasis is such that it has been selected for complete genome sequencing. The Biomphalaria glabrata embryonic cell line is an important resource for researchers investigating the interaction between the snail and parasite. The genome of the Bge cells was analysed at the chromosomal level, using DAPI karyotyping. The karyotype revealed extensive aneuploidy, whereby a modal chromosome complement of 63 and 67 was observed in two isolates of the Bge cells, which exceeds B. glabrata’s 2n=36 chromosome number. Indeed, in addition to characterising the Bge cell chromosomes, a method was established for mapping single copy B. glabrata genes onto the chromosomes from the Bge cells using fluorescence in situ hybridisation. Despite the Bge cell’s inherent aneuploidy, the four genes mapped onto diploid homologous chromosomes. This methodology will be an important resource for the genome sequencing consortium. The interphase nucleus is an organised organelle, whereby chromosomes and gene loci have been shown to be located non-randomly and hence it is hypothesised that the organisation of the interphase nucleus is pertinent to the function of the genome. Since there is no data on how the genes of the snail genome behaves in interphase, it was assessed in the Bge cells line. Again, this is important for the sequencing initiative, but also for evolutionary biology. Radially distributed chromosome territories were observed in the nuclei of the Bge cells. The territory position was organised according to territory size, with small chromosome territories positioned towards the interior and large territories intermediately located. In addition, four B. glabrata genes were positioned non-randomly in the interphase nuclei of the Bge cells, again emphasising organised positioning of the genome. With co-culture of S. mansoni miracidia with the Bge cells there is up regulation of specific genes known to be involved in the host response to parasite. These genes are dramatically relocated within the interphase nuclei, implying that these are specific parasite induced nuclear events. An analysis of the genomic distribution of specific histone modified chromatin in the interphase nuclei of B. glabrata, revealed different nuclear distribution of modified chromatin. Indeed, a statistically significant difference in these patterns was observed between juvenile and adult snails, indicating developmental differences in the organisation of the snails’ genome. These differences maybe relevant to the snails’ resistance/susceptibility to the parasite