Multiple ribosomal RNA gene loci in the genome of the homosporous fern Ceratopteris richardii

The genomes of homosporous ferns are largely uncharacterized, but they appe ar to differ from gymnosperms and angiosperms in key aspects, such as high chromosome numbers at the diploid level, and thus provide a unique perspect ive on plant genome structure and evolution. Using the model homosporous fe rn Ceratopteris richardii, loci encoding ribosomal RNA sequences (rDNA gene s) were detected using fluorescent in situ hybridization. At least two majo r rDNA loci were visible in all cases, and six or more weakly hybridizing s ignals were observed in most cytological preparations. These results are co nsistent with models of homosporous fern evolution via cycles of polyploidy followed by gene silencing. They are also consistent with other models of fern genome evolution. With the exception of the weakly hybridizing signals , these data are similar to analogous reports of one or two major rDNA loci in diploid angiosperms. These results suggest that the gross morphology of rDNA loci are similar between diploid homosporous ferns and angiosperms, b ut that important clues to rDNA gene and chromosome evolution in homosporou s ferns may reside in the analysis of their minor rDNA sequences.