ASSESSMENT OF GENE COPY NUMBER IN THE HOMOSPOROUS FERNS CERATOPTERIS-THALICTROIDES AND C-RICHARDII (PARKERIACEAE) BY RESTRICTION-FRAGMENT-LENGTH-POLYMORPHISMS

Homosporous ferns are generally considered polyploid due to high chrom osome numbers, but genetically diploid since the expression of isozyme s is generally controlled by a single locus. Gene silencing over evolu tionary time is one means by which this apparent contradiction can be explained. A prediction of this hypothesis is that silenced gene seque nces still reside in the genomes of homosporous ferns. We examined the genomes of Ceratopteris richardii and C. thalictroides for sequences which are similar to expressed gene sequences. Genomic DNA blots hybri dized with C. richardii cDNA clones showed that the majority of these clones detected multiple fragments, suggesting that most gene-like seq uences are duplicated in Ceratopteris. Hybridization signal intensity often varied between fragments of the same size between accessions, so metimes dramatically, which indicates that not all sequences are equiv alent, and may represent the products of silenced genes. Observed reci procal differences in intensity could be due to reciprocally silenced genes. In addition, an unusual segregation pattern for one locus follo wed by one probe may indicate homeologous chromosome pairing and segre gation.