## 1 Introduction
class 2 integrons also harbor a typical 5'CS composed by the _intI2_ gene, (whose predicted integrase shared 46% identity with IntI1), the _attI2_, and Pc and PintI2 promoters (see below Sections 2.1.2 and 2.7), they carry a limited number of resistance gene cassettes [7]. The class 2 integrons normally include the _dfrA_, _aadA_, and _sat_ gene cassettes, which confer resistance to trimethoprim, streptomycin, and streptothricin, respectively, and _orfX_ of unknown function. Different from classes 1 and 2, the class 3 integrons are the rarest elements among the RIs/MIs. Despite class 3 integrons present a 5'CS backbone very similar to that found for class 1 integrons (see Sections 2.1.3 and 2.8), this other class codes an integrase 59% identical to IntI1, which are able to recognize the same gene cassettes, and harbors an _attI3_ longer than _attI1_ (131 bp versus 65 bp). Another integron category is known as chromosomal integrons (CIs), which have the same basic structure that defines an integron (_IntI_, _attI_, Pc, and PintI), but present several particularities relative to their mobile counterparts (MIs). Firstly, CIs are non-mobilizable platforms embedded in the chromosome of a large number of environmental Gram-negative bacteria [4]. These elements are greater in size relative to MIs and may comprise a significant portion of the genome. For example, the _Vibrio cholerae_ superintegon (IntVchA), which is the best-studied CI [8], harbors around 180 cassettes. Besides the remarkable difference in size between MIs (dozens of cassettes) and CIs (hundreds of cassettes), another important feature of CIs is that most of the gene cassettes found in these platforms codes for unknown proteins, which accounts for the host genomic plasticity (for revision see reference [4]). Furthermore, unlike the resistance gene cassettes carried on RIs/MIs, which are associated with unique and unrelated _attC_ sites, the recombination sites of CI cassettes are represented by almost identical repeats of approximately 120 bp that occur along the CI array flanking each gene. These repeats are species-specific in a way that they were named according to the organism where they were found, for example, the _V. cholerae_ CI harbors the _V. cholerae_ repeats (VCRs) as cassette recombination sites [8]. Additionally, the CIs have been pointed as the ancestors and the reservoirs of gene cassettes currently found in MIs [9]. In fact, a previous study demonstrated that the _qnrVC1_ gene cassette, firstly identified in a class 1 integron, was not associated with a typical _attC_, but with the recombination site that characterizes the _V. parahaemolyticus_ CI (_V. parahaemolyticus_ repeats--VPRs) [10]. Afterward, [PERSON] and [PERSON] [11] observed that several other _qnrVC_ alleles found in mobile platforms were always linked to CI-borne recombination sites from different _Vibrio_ species. This evidence strongly indicated that _qnrVC_ was an archeological trait of CIs in the current MIs, corroborating the previous hypothesis that chromosomal integrons are the source of cassettes present in drug-resistance integrons [9]. The main differences among RIs/MIs are featured in Table 1.  
The integrons are ubiquitous in nature and harbor an extraordinarily high diversity of gene cassettes. In fact, these platforms have been revealed as key elements of bacterial adaptation and genome evolution, playing a role that goes beyond that involved with the antibiotic resistance emergence [4; 7; 12; 13; 14]. Bacteria may take advantage of different mechanisms to bypass the antibiotic action. However, the emergence of antibiotic resistance is usually associated with biological burden leading to a fitness cost for the bacterial host [15]. The most prominent advantage of integrons for genome innovation and adaptive success