DNA variation and ribosomal DNA constancy in two Crepis species and the interspecific hybrid exhibiting nucleolar organiser suppression

Summary The interspecific hybrid between Crepis capillaris and C. dioscorides exhibits suppression of the secondary constriction in the chromosome derived from C. dioscorides; each species normally has one chromosome pair with secondary constrictions. Such structures are known to be associated with the formation of nucleoli and are the probable sites of ribosomal-DNA (rDNA). The two species and the hybrid were studied with respect to DNA and rDNA variation to determine whether reduction of the highly redundant rDNA might occur as a consequence of nucleolar organiser suppression. The DNA content of C. dioscorides was three times greater than of C. capillaris . This is consistent with trends in the genus Crepis . Evolutionary advancement is correlated with a reduction of both chromosome number ( C. dioscorides , 2 n = 8; C. capillaris, 2n =6) and chromosome size. The chromosomes of the more advanced species, C. capillaris , are smaller than those of C. dioscorides . No alterations of the parental contributions of rDNA were observed in the hybrid, however; the two species and the hybrid each contained about 5000 rRNA genes. It was concluded that nucleolar organiser suppression must be at the transcription level.