Recombination of 16S rDNA genes were previously identified in some other bacteria [42–44]. In actinomycetes, the occurrence of short rDNA segments with high number of non-random variations was attributed to the lateral transfer as the most parsimonious
PRIMA-1MET explanation . Later, Gogarten et al.  suggested that, analogously to an entire bacterial genome, 16S rDNA possesses a mosaic character originated by LGT, respectively by transfer of gene subunits. As bacterial genomes often carry more than one rRNA operon, intragenomic heterogeneity of the rDNA copies is occasionally found to blur the phylogenetic picture [47–50]. Although there is no direct information on the number of rRNA gene copies in Arsenophonus genomes, Stewart and Cavanaugh  showed bacterial genomes to encode in average five rRNA operons. The most closely related bacterium of which the complete genome has recently been sequenced, Proteus mirabilis, carries seven copies [GenBank: AM942759]. Arsenophonus-focused studies indicate that two different forms of the rRNA operon are present in its genome, as is typical
for Enterobacteraceae [23, 52]. Furthermore, Šorfová et al.  suggest that the variability among individual copies may cause the incongruence observed between triatomines and their Arsenophonus lineages. They point out that this process Selleck EX527 could, in principle, explain an otherwise problematic observation: in some hosts, such as triatomines or some homopterans, the hosts and the Arsenophonus bacteria create reciprocally
monophyletic clusters but do not show any cospeciation pattern. In the symbionts of grain weevils, divergence between rRNA sequences within a genome was shown sometimes to exceed divergence of orthologous copies from symbionts from different hosts; this unusual situation was hypothesized to reflect loss of recombinational repair mechanisms from these symbiont genomes . Estimates of the divergence time With the present incomplete knowledge of the Arsenophonus genome, it is difficult to assess whether and how deeply rRNA heterogeneity affects phylogenetic reconstruction. Trying to find alternative solution, out Šorfová et al.  attempted to use the estimation of divergence times as a guide for deciding between different coevolutionary scenarios. They used the Escherichia-Salmonella divergence [54, 55] as a calibration point for calculating the divergence time among various Arsenophonus lineages from triatomine bugs. Applying the ACY-1215 order Multdiv method , they placed the ancestor of triatomine-associated symbionts into a broad range of approx. 15 – 40 mya and concluded that this estimate is compatible or even exceeds the age estimates available for the tribe triatomine (according to Gaunt and Miles ). Here, we took advantage of a new age-estimate for closely related bacteria, namely the louse-associated symbionts of the genus Riesia .