Evolutionary dynamics of rDNA clusters on chromosomes of moths 1 and butterflies ( Lepidoptera ) 2 3

31 We examined chromosomal distribution of major ribosomal DNAs (rDNAs), clustered in the nucleolar 32 organizer regions (NORs), in 18 species of moths and butterflies using fluorescence in situ 33 hybridization (FISH) with a codling moth (Cydia pomonella) 18S rDNA probe. Most species showed 34 one or two rDNA clusters in their haploid karyotype but exceptions with four to eleven clusters also 35 occurred. Our results in a compilation with previous data revealed dynamic evolution of rDNA 36 distribution in Lepidoptera except Noctuoidea, which showed a highly uniform rDNA pattern. In 37 karyotypes with one NOR, interstitial location of rDNA prevailed, whereas two-NOR karyotypes 38 showed mostly terminally located rDNA clusters. A possible origin of the single interstitial NOR by 39 fusion between two NOR-chromosomes with terminal rDNA clusters lacks support in available data. In 40 some species, spreading of rDNA to new, mostly terminal chromosome regions was found. The 41 multiplication of rDNA clusters without alteration of chromosome numbers rules out chromosome 42 fissions as a major mechanism of rDNA expansion. Based on rDNA dynamics in Lepidoptera and 43 considering the role of ordered nuclear architecture in karyotype evolution, we propose ectopic 44 recombination, i.e. homologous recombination between repetitive sequences of non-homologous 45 chromosomes, as a primary motive force in rDNA repatterning. 46