The organization of the ribosomal RNA genes of Chironomus tentans and some closely related species.

Southern gel analysis of total DNA from Chironomus tentans showed that the rRNA genes (rDNA) are homogeneous in structure. After cloning in Escherichia coli plasmid pBR313, the rDNA organisation was further studied by restriction fragment analysis and R-loop mapping. No heterogeneity could be detected by heteroduplex analysis of six different cloned rRNA cistrons. R-loop sizes of 1.69 and 3.63 kilobases (kb) were measured for the 18S and 28S rRNA coding sequences. The two spacers are 0.75 and 1.77 kb long. Southern gel analysis showed also a homogeneous rDNA structure for a Canadian population of C. tentans and C. pallidivittatus. The same technique indicated, however, that the rDNA of two other closely related species of C. thummi and C. melanotus is heterogeneous in structure. A possible correlation between this heterogeneity and the presence of heterochromatin in these species is discussed.