Conserved signature indels

Conserved signature inserts and deletions (CSIs) in protein sequences provide an important category of molecular markers for understanding phylogenetic relationships. CSIs, brought about by rare genetic changes, provide useful phylogenetic markers that are generally of defined size and they are flanked on both sides by conserved regions to ensure their reliability. While indels can be arbitrary inserts or deletions, CSIs are defined as only those protein indels that are present within conserved regions of the protein.

The CSIs that are restricted to a particular clade or group of species, generally provide good phylogenetic markers of common evolutionary descent. Due to the rarity and highly specific nature of such changes, it is less likely that they could arise independently by either convergent or parallel evolution (i.e. homoplasy) or synapomorphy. Other confounding factors such as differences in evolutionary rates at different sites or among different species also generally do not affect the interpretation of a CSI. By determining the presence or absence of CSIs in an out-group species, one can infer whether the ancestral form of the CSI was an insert or deletion and this can be used to develop a rooted phylogenetic relationship among organisms.

Most CSIs that have been identified have been found to exhibit high predictive value and they generally retain the specificity for the originally identified clades of species. Therefore, based upon their presence or absence, it should be possible to identify both known and even previously unknown species belonging to these groups in different environments.

Group specific CSIs are commonly shared by different species belonging to a particular Taxon(e.g. genus, family, class, order, phylum) but they are not present in other groups. These CSIs were most likely introduced in an ancestor of the group of species before the members of the taxa diverged. They provide molecular means for distinguishing members of a particular taxon from all other organisms.

Figure 1 shows an example of 5aa CSI found in all species belonging to the taxon X. This is a distinctive characteristic of this taxon as it is not found in any other species. This signature was likely introduced in a common ancestor of the species from this taxon. Similarly other group-specific signatures (not shown) could be shared by either A1 and A2 or B1 and B2, etc., or even by X1 and X2 or by X3 and X4, etc. The groups A, B, C, D and X, in this diagram could correspond to various bacterial or Eukaryotic phyla.

...
Wikipedia