Horizontální genový transfer mezi Silene latifolia a jejím bakteriálním patogenem Ralstonia solanacearum

Bc. Martina Talianová

Master's thesis

Horizontal gene transfer from a bacterial pathogen Ralstonia solanacearum to its putative host plant Silene latifolia

Abstract:

Although HGT is considered as an important evolutionary process in prokaryotic organisms, it is assumed to occur rarely among eukaryotes. In the diploma thesis, I report the case of horizontal gene transfer involving pathogenic plant bacterium Ralstonia solanacearum and plant species belonging to the genus Silene. The analysed sequence MK14 is a part of the horizontally transferred fragment from R. solanacearum. It consists of a part of the genes sulfate adenylyltransferase and uroporphyrinogen- III C-methyltransferase. The bigger part of MK14 is created by sulfate adenylyltransferase. When translated to the protein sequence, it is not disrupted by termination codons, the only termination codon corresponds to that one at the end of sulfate adenylyltransferase in R. solanacearum. This indicates that MK14 is not degenerated. RT-PCR showed that MK14 is expressed, and thus it either did not have enough time to degenerate or it probably provides any adaptive function to be maintained. It is assumed that it could play a role in the internal mechanism of the plant defence against the pathogen Ralstonia solanacearum. Further, MK14 is present in both dioecious species (S. latifolia, S. diclinis, S. dioica) in males and females, and in hermaphroditic species (S. viscosa, S. noctiflora, S. zawadzskii). Phylogenetic analysis showed a discrepancy in clustering hermaphroditic species with dioecious species. The presence of hermaphroditic S. viscosa within the cluster of dioecious species may indicate that MK14 was transferred from R. solanacearum to the autosomal ancestor of the sex chromosomes (in the hermaphroditic progenitor of Silene species). Subsequently, the autosomal ancestor of the X chromosome (containing the sequence MK14) evolved into the X chromosome in dioecious species while in S. viscosa it remained conserved in the original form. The MK14 might be also secondarily introduced to the common ancestor of S. zawadzskii and S. noctiflora from the proto-Y chromosome of S. diclinis. Very low silent and nonsilent divergence values for the MK14 sequence with the "proto-X" and "proto-Y" origin indicate selective pressures on the RNA-level for the sequence to stay conserved. A comparison of the whole genome of R. solanacearum against the database of plants revealed that the HGT between this pathogen and Silene species might not be an exceptional case. Together with the previous results, it is possible to hypothesize that R. solanacearum possess a mechanism which enables the transfer of its genetic materials to the plant host cells. These results are unique, because to date, there are only few known cases of HGT from bacteria to plants.

Keywords

horizontal gene transfer, nucleotide substitution models, adaptive evolution, phylogenetic analysis

Language used: English

Date on which the thesis was submitted / produced: 25. 5. 2006

Thesis defence

• Date of defence: 15. 6. 2006
• Supervisor: RNDr. Bohuslav Janoušek, Ph.D.

Masaryk University

Faculty of Science

Master programme / field:
Biology / Mathematical Biology