> homologous recombination events between
> rRNA gene units of N. apis and N. ceranae
> infecting the same cell are not impossible...

Citation please?  Maybe a link to a pdf of the full text of the paper?

Just to put that quote into better context, it seems that the situation is
far more complex than one might first think, and that apparent
"recombination" does not necessarily imply any sort of hybridization.

"Cross-hybridization of rRNA sequences from organisms as taxonomically
diverse as plants and animals was observed, suggesting very high selective
pressure to preserve a specific nucleotide sequence. This conservation
appeared to account for the uniformity of sequence between the different
copies of the genes within each organism."

and

"First, individuals from the same species had different numbers of rRNA
genes. Second, the number of rRNA genes retained by most individuals
appeared to be in excess of the number needed for survival. The variation in
number of rDNA units found among individuals was hypothesized to occur by
homologous recombination between rDNA units located at different positions
within the loci on two chromosomes. Such 'unequal crossover' events would
generate one recombinant chromosome with more rDNA units and another
chromosome with fewer units."

and

"These studies suggest that unequal crossover is the major driving force in
the evolution of the rRNA genes with sister chromatid exchange occurring
more often than exchange between homologs."

All quotes above from:
http://www.genetics.org/content/175/2/477.full
"Finely Orchestrated Movements: Evolution of the Ribosomal RNA Genes"
Eickbush and Eickbush
Genetics February 2007 vol. 175 no. 2 477-485
doi: 10.1534/genetics.107.071399

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