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We tested for recombination within the DNA that codes for ribosomal RNA (rDNA) on the X and Y chromosomes of Drosophila melanogaster . The criterion was the appearance of intermediate bb alleles on X-Y recombinants from wild bb + and lethal bb 1 loci carried on parental chromosomes. Recombination within the rDNA occurs only when the rDNA of the X chromosome is inverted. We conclude that rDNAs of normal X and Y chromosomes have opposite orientation with respect to the centromere. The implications of this observation are discussed.

Si e notato che shocks di temperatura possono indurre una variazione di «puffing pattern» in ghiandole salivari di Drosophila. Tali «puffing» sono perfettamente reversibili e rappresentano zone di intensa sintesi di RNA. Si e notato che DNP e Na salicilato portano a simili variazioni di «puffing pattern».

THE multiple sequences of rDNA (DNA complementary to ribosomal RNA) of the Drosophila genome are localized at the bobbed locus, located in the X chromosome, position 66 and in the short arm of the Y chromosome1,2. Wild bobbed (bb+) is that locus which, without a partner, gives rise to a normal phenotype. That locus which in similar conditions is incapable of giving rise to a normal phenotype is called a bobbed mutation (bb) and contains fewer genes for rRNA. The number of genes for rRNA in different individuals can vary considerably. One mechanism for rDNA variation is unequal crossing over3. Another mechanism, described by Tartof4, becomes apparent when individual flies, carrying only one bobbed locus, are constructed and only if such a locus is on the X chromosome; that is, if one constructs Xbb+/O males (and also Xbb/O males) or Xbb+/XNO- females. Such individuals show a higher rDNA content than expected from the analysis of the same locus in Xbb+/Xbb+ females or in Xbb+/Ybb+ males. The increase of rDNA in this case is not inheritable4.

RNA/DNA hybridization experiments confirmed the existence of six genes for 23 s ribosomal RNA and six for 16 s ribosomal RNA for each Escherichia coli chromosome. The ability of nitrocellulose columns to separate pure from impure RNA/DNA hybrids led to the conclusion that most of the genes for ribosomal RNA are adjacent to each other.

GENE duplications are recognized as a powerful background for evolution1–3: thus, one of the genes maintains the function, the other accumulates mutations. Several cell functions, however, require multiple copies of the same gene4–7; and these are, generally, tightly clustered8–13. One might hence expect to find many mutations within blocks of duplicates; but in the cases tested, none were found9,12,14. Nevertheless, the genes of the cluster evolve15,16. I think that mechanisms like magnification are the basis for the “horizontal” evolution of tandemly duplicated genes16; a specific model has recently been proposed17. The mechanism I propose is as follows. Unequal or sister strand crossing over leads to the formation of partial deletions of the block or to blocks of increased redundancy18,19. Eventually there is selection against the latter20, while partially deleted blocks in the appropriate conditions can magnify: that is, can form a large number of new copies on those left in the block. This will lead to an increase of homogeneity which will be greater the lower the number of genes used as copies to rebuild the block. It seemed desirable to test whether deletions of genes for rRNA strong enough to be lethal were susceptible to magnification. This paper shows that this is feasible and strengthens a previous result by Atwood21.

In genetic combinations undergoing ribosomal DNA magnification, the frequency of crossing-over between X and Y chromosomes is increased at least 20-fold above that in similar combinations not undergoing magnification. Such recombination occurs at the ribosomal DNA level. The data are consistent with a model where extra copies of ribosomal DNA are formed which, after circularization, are integrated into the chromosome.