## 4 Conclusions  
The ability to produce AHL signals is very common in the genus _Burkholderia_. The genes of _Burkholderia_ QS systems follow at least 11 typical topological arrangements (10 are common to topological arrangements in other Proteobacteria). Some species, like _B. mallei_ and _B. pseudomallei_ and certain members of the BCC group, contain multiple AHL QS systems that usually do not operate independently from each other but form a complex hierarchical regulatory network. QS genes in _Burkholderia_ are usually located on chromosome 2 where most genes related to virulence and secretion systems are found [14]. This is in agreement with the finding that QS genes regulate pathogenesis and virulence in many species of _Burkholderia_. The chemical structure of the AHL signals, the local topology of the QS genes and the location of QS systems within the chromosomes show a degree of conservation throughout the entire genus. The fact that some AHL QS systems, which produce OHC8 and OHC10, can be associated with different local topologies in different species possibly indicates that QS topologies evolved before the _Burkholderia_ genus separated into the current taxonomical groups. We speculate that the AHL signals appeared before the appearance of the genus and the subsequent rearrangements of the ancestral QS systems lead to the local and chromosomal topologies seen in current species. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Speces** & **QS system** & **ID** & **Gene topology** & **Chr. No. ** & **Major AHL** & **Comments** \\ \hline \multirow{4}{*}{_B. glumae_} & _tofR/tofM/tof_ & M1 &  
The conserved patterns of AHL QS gene neighborhoods is at apparent variance with the general view that gene arrangements in prokaryotes are evolutionarily volatile and may change substantially even on short evolutionary scales when gene sequences diverge minimally [68]. On the other hand, it is generally believed that genome architecture is not a straightforward product of adaptation but rather is determined by an intricate balance between selection pressure, recombination as well as the activity of selfish elements [68]. Seen from this evolutionary perspective we speculate that the appearance of a novel QS system may cause a major change in the lifestyle of a bacterial species, for instance it allows the species to colonize a new nutrient-source. Consequently, there will be selection pressure against losing the new system by genetic rearrangements. On the other hand, continuously present genetic rearrangements may add new elements to the system, such as the regulation of new genes. A point of interest is the number of AHL QS genes in _Burkholderia_. [PERSON] and associates argued that the number of bacterial signaling genes is relatively higher in bacteria living in more complex environments, and relatively lower in those living in more sheltered habitats [69,70]. While the original statement referred to two-component systems, a similar general tendency might also be expected with respect to AHL QS systems. The tendency is much less clear, however. Firstly, the number of AHL systems per genome is considerably lower than that of two-component systems. Secondly, the number of AHL systems and solo _luxR_ genes is the highest in _Rhizobia_ and the _pseudomallei_ group of _Burkholderia_, but the other two clades of _Burkholderia_ have a low to average number of QS genes. As such, we speculate that the number of AHL genes can be better understood in terms of precise molecular mechanisms connected with the lifestyle of QS bacteria rather than in terms of niche characteristics. It must be noted however that _Burkholderia_ also contain at least one other QS system which utilizes another type of signaling molecule which is called Burkholderia Diffusible Signaling Factor (BDSF) [71, 72, 73]. The newly identified QS system, just like the AHL system, is very widespread among the _Burkholderia_ genus [74,75].