## 1 Introduction
In the literature, however, there are other cases of eukaryotes in which rDNA was found to violate this mechanism; polymorphic copies of rDNA in the same genome were found both in eukaryotic microorganisms, _Cryptosporidium parvum_[41], _Plasmodium berghei_[42], and _Prottotheca wickerhamii_[43], and in more complex eukaryotes, as grasshoppers [44] and flatworms [45]. The fact that rDNA variants are present in genome-wide does not necessarily mean that they are expressed in different ribosomes. However, in _N. ceranae_, the genetic variants of rDNA are transcribed and translated into functional proteins, suggesting changeable roles during the development life stages or under different environmental conditions [37].. Various subsequent studies have then demonstrated their actual existence, also and revealing revealed a high degree of polymorphism between them [6; 34; 37][6; 36; 38]. Additionally, in other microsporidia, such ase.g. _Encephalizoon cuniculi_, multiple copies of ribosomal genes are present, however, thesealbeit they are homogeneous within the same genome [38]. The point that ribosomal genes are repeated within the genome is considered normal in eukaryotes. However, the repetitions are usually homogeneous with each other, with donor donor donor minimal variations in their sequences [39]. This That occurs thanks to the phenomenon of concerted evolution, normally defined as the process by which the paralogous genes (genes repeated within the genome) present in a given species, show greater sequence similarity to each other, rather thancompared to ortholog genes (genes with different origins but with similar functions) are present in other related species [40]. Concerted evolution, therefore, provides for molecular mechanisms that prevent the independent evolution of repeated sequences, keeping them homogeneous within the genome. Ribosomal genes are among the best known examples one of the greatest examples of concerted evolution [39]. In the literature, however, there are other cases of eukaryotes in which the rDNA has beenwas found to escape violate this mechanism; polymorphic copies of rDNA in the same genome have beenwere found both in eukaryotic microorganisms, such as _Cryptosporidiumarumarum_[41], _Plasmodium berghei_[42], and _Prottoltea wickerhannii_[43], and in more complex eukaryotes, as eukaryotes more complex, such as grasshoppers [44] and flatworms [45]. The fact that rDNA variants are present in the genome-wide does not necessarily mean that they are expressed in different ribosomes. However, in _N. ceranae_, the genetic variants of rDNA are transcribed and translated into functional proteins, suggesting changeable roles during the development life stages or under differenttheir possible role during the different stages of development of microsporidium or in different environmental conditions [37]. On the other side, the _Hsp70_ gene codifies for one of the highest conserved proteins throughout the evolution [46]. That gene was found in a single copy in all organisms, including archaeobacteria, plants, fungi, and animals, as the coded protein is strictly related to species-specific functions [47; 48]. In the case of _Nosema_ spp., that protein protects the spores against overheating, therefore contributing to the microsporidium survival in hostile environments [49; 50]. The literature shows that the highly conserved single-copy gene _Hsp70_ represents a better alternative to rDNA genes in the molecular approach to species-specific identification, quantification, and genotyping [51; 52; 53; 54; 55; 56; 57; 58; 59; 60]. Several theories have been envisaged to explain the origin of the genetic variability observed in the _N. ceranae_ rDNA. One of the most frequently advocated is the possibility that single colonies of _A. mellifera_ become infected by multiple genetic variants of the microsporidium, thereafter multiplying asexually [7; 34; 61].