Variation and inheritance of nuclear ribosomal DNA clusters in Streptocarpus (Gesneriaceae) and their biological and phylogenetic implications

Chromosomal fluorescent in situ hybridization ( FISH) and PCR fragment intensity analysis were used to determine the number and sizes of 45S nuclear ribosomal DNA ( nrDNA) arrays and their inheritance in two Streptocarpus species, Streptocarpus rexii and Streptocarpus dunnii. A strong correlation between 45S nrDNA signals and PCR band intensity was observed. They revealed two loci in both species, with variation in nrDNA copy number within arrays: S. rexii had relatively homogeneous loci, while one individual of S. dunnii was heterozygous at the main locus, possessing a large and a small nrDNA array, and homozygous at the second locus, which was very small. This small nrDNA array was not PCR detectable and explains the non- Mendelian results in some backcrosses. Our results indicated that nrDNA sequences in Streptocarpus were highly homogenized and that the inheritance of nrDNA was additive without any detectable recombination. PCR segregation of 45S nrDNA markers in backcross plants supported the hypothesis that the two loci detected by FISH were unlinked. The minute locus in S. dunnii may indicate a mechanism by which the number of nrDNA sites can change in plants. The backcross results demonstrated the dynamics of nrDNA inheritance and the swiftness of the replacement of parental types within two generations when hybridization is involved in plant evolution.