Chromosomal localization of 45S and 5S rDNA in 14 species and the implications for genome evolution of genus Epimedium

Studying the genome structure of Epimedium has been hindered by the large genomes and uniform karyotypes. Consequently our understanding of the genome organization and evolutionary changes of Epimedium is extremely limited. In the present study, the 45S and 5S rDNA loci of 14 Epimedium species were physically mapped by double-probe FISH for the first time. Results showed the following: (1) Chromosomes I and II of all 14 species examined, except for E. shuichengense, hosted one pair of 45S rDNA sites, respectively. Most of the 45S rDNA sites gave clear signals and were positioned in the distal regions of the short arms. (2) All species studied of section Diphyllon were found to have one pair of 5S rDNA sites localized in the interstitial regions of the long arm of chromosome IV, and the two species of section Epimedium, E. alpinum and E. pubigerum, had two pairs of 5S rDNA sites localized in the interstitial regions of the long arm of chromosomes IV and V, respectively. (3) In section Diphyllon, all species of small flower taxa, except E. shuichengense, had three pairs of 45S rDNA sites, clearly more than species of big flower taxa, except E. davidii, with two pairs of 45S rDNA sites. Based on the 45S and 5S rDNA distribution patterns and other chromosomal morphological characteristics, six pairs of chromosomes can be unambiguously identified in all 14 Epimedium species. The stable differentiation in 45S and 5S rDNA FISH patterns between the two sections suggests that chromosomal rearrangements and transpositional events played a role in the splitting of the two sections, and section Diphyllon may be more primitive than section Epimedium. In the same way, big flower taxa may be more primitive than small flower taxa in section Diphyllon.