Transposable elements are DNA sequences present in all the large phylogenetic groups, both capable of changing position within the genome and constituting a significant part of eukaryotic genomes. The mariner family of transposons is one of the few which occurs in a wide variety of taxonomic groups, including freshwater planarians. Nevertheless, so far only five planarian species have been reported to carry mariner-like elements (MLEs), although several different species have been investigated. Regarding the number of copies of MLEs, Girardia tigrina is the only planarian species in which this has been evaluated, with an estimation of 8,000 copies of the element per haploid genome. Preliminary results obtained in our laboratory demonstrated that MLE is found in a large number of different species of planarians, including terrestrial. With this in mind, the aim was to evaluate the occurrence and estimate the number of MLE copies in different planarian species collected in south Brazil. Twenty-eight individuals from 15 planarian species were analyzed. By using PCR and the hybridization of nucleic acids, it was found that MLE was present in all the analyzed species, the number of copies being high, probably over 10³ per haploid genome.

Several mollicute genome projects are underway, offering unique opportunities to study genes and metabolic pathways on a genome-wide scale. Here, we have analyzed the conservation and diversity of purine and pyrimidine metabolism in mycoplasmas. An evaluation of discrepancies between genomic analysis and enzymatic data revealed interesting aspects about these organisms. We found important examples in which enzyme activity was reported without the annotation of a corresponding gene. An interesting example concerns phosphopentomutase. In Mollicutes, we have identified CDSs orthologous to sequences recently identified as new phosphopentomutases in archaeobacteria that are structurally related to phosphomannomutases. It is suggested that these sequences could replace the function of phosphopentomutases in mollicutes lacking the canonical phosphopentomutase gene (deoB). Also, the activity of 5'-nucleotidase was reported in mollicutes that do not possess any CDS related to ushA. Hypothetical proteins exhibiting domains similar to newly characterized 5' nucleotidases in Escherichia coli are proposed as possible CDSs related to this enzymatic activity in Mollicutes. Based on our analysis, the reductive genome evolution of Mollicutes does not appear to result in a minimum set of genes nor a minimum set of metabolic functions shared by all mollicute species.