Evolutionary dynamics of rDNA clusters on chromosomes of moths and butterflies (Lepidoptera)

We examined chromosomal distribution of major ribosomal DNAs (rDNAs), clustered in the nucleolar organizer regions (NORs), in 18 species of moths and butterflies using fluorescence in situ hybridization with a codling moth (Cydia pomonella) 18S rDNA probe. Most species showed one or two rDNA clusters in their haploid karyotype but exceptions with 4-11 clusters also occurred. Our results in a compilation with previous data revealed dynamic evolution of rDNA distribution in Lepidoptera except Noctuoidea, which showed a highly uniform rDNA pattern. In karyotypes with one NOR, interstitial location of rDNA prevailed, whereas two-NOR karyotypes showed mostly terminally located rDNA clusters. A possible origin of the single interstitial NOR by fusion between two NOR-chromosomes with terminal rDNA clusters lacks support in available data. In some species, spreading of rDNA to new, mostly terminal chromosome regions was found. The multiplication of rDNA clusters without alteration of chromosome numbers rules out chromosome fissions as a major mechanism of rDNA expansion. Based on rDNA dynamics in Lepidoptera and considering the role of ordered nuclear architecture in karyotype evolution, we propose ectopic recombination, i.e., homologous recombination between repetitive sequences of non-homologous chromosomes, as a primary motive force in rDNA repatterning.