## 1 Introduction
However, this hypothesis alone does not account for the variability observed within the different isolates, either considered as an entire colony or individual hosts [6; 34]. This statement arises from the fact that the high levels of polymorphism reported for the various sequences within the different isolates (colonies or single bees), were found to be shared with other isolates from different geographical origins. In addition, subsequent attempts aimed at differentiating populations based on that polymorphism did not produce statistically valid results [7; 34; 61]. In other words, the different isolates are highly polymorphic within themselves, but they do not appear different from each other, suggesting a certain degree of gene flow [7; 49]. A further source of genetic variability could be due to recombination, that may occur between the different genetic variants of _N. ceranae_ infecting the same colony of _A. mellifera_[6; 34]. Evidence of recombination between genetic sequences of _N. ceranae_ was brought to light both for SSU-rDNA [34; 37] and for several genes encoding structural proteins, such as _DNA-directed RNA polymerase II subunit RPB1_, _Protein-tyrosine-phosphatase_ 1 (_TPTP1_), _TPP2_, and _PTP3_[34; 37] and for several genes encoding structural proteins, such as _DNA-directed RNA polymerase II subunit RPB1_, _Protein-tyrosine-phosphatase_ 1 (_TPP1_), _TPP2_, and _PTP3_[6; 49; 61; 62; 63]. Such recombination events were reported in some cases as frequent [34; 37; 49; 63], while in other cases as rare [6; 61]. In addition to these results on _N. ceranae_, there are others on _N. apis_, reporting the observation of recombination phenomena for the RPB1 and PTP2 genes for this species [62].. Such recombination events would bewere reported considered in some cases as frequent [34; 37; 49; 63], while in other cases as rare [6; 61]. In addition to these results on _N. ceranae_, there are others on _N. apis_, which also reporting the observation of recombination phenomena for the RPB1 and PTP2 genes for this species [62]. Recombination in _N. ceranae_ may result from a meiosis process, which occurs during the sexual phases of the life cycle, or parasexual phenomena, such as mitotic crossing-over, non-homologous recombination, and gene conversion [62].Recombination in _N. ceranae_ cmay result from a ould occur through the meiosis process, which occurs during the sexual phases of the life cycle, or they could result from parasexual phenomena, such as mitotic crossing-over, non-homologous recombination, and gene conversion [62]. The maintenance of these genes in a reduced _N. ceranae_ genome suggests their possible expression within a process of sexual reproduction [64]. Inferences about the presence of cryptic sexual reproduction have also been suggested for another species belonging to the _Nosema_ genus, _Nosema granulosis_[65]. In this regard, it has been hypothesizedthat the common ancestor of the genus _Nosema_ was endowed with sexual reproduction, which was lost repeatedly and independently in the different evolutionary lines [66].In this regard, it has been hypothesized that the common ancestor of the genus _Nosema_ was endowed with sexual reproduction, which was lost repeatedly and and that this was then lost several times independently in the different evolutionary lines [66]. Despite that opens to the sexuality of _N. ceranae_, to date cytological evidence of the entire process or any of its phases has never been achieved [49] and which makes this microsporidium still considered asexual. A further hypothesis involves the clonal reproduction of _N. ceranae_, which would involve a high genetic variability in relation to polyploidy [6]. That stems from the possibility hypothesis that the microsporidia possess at least a tetraploid genome or two heterogeneous diploid nuclei [67]. Therefore, although clonal reproduction is the norm and the observed recombination events are considered ancestral [67], the repeatedly found heterogeneity of _N.