Ribosomal Rna Genes in Salivary Gland and Ovary of Rhynchosciara Angelae

Two facts make it important to determine whether the proportion of ribosomal R N A (rRNA) 1 genes in salivary glands of Rhynchoscza~a angelae varies throughout the end of the last (fourth) instar of larval development, namely: (a) In this instar, during the appearance of " D N A puffs," it is possible to observe many micronucleoli around the chromosomes (1). Perhaps the micronucleoli maybe ascribed to an extra synthesis of ribosomal D N A (rDNA), in analogy to what has been proposed in Hybosczara, to explain similar microbodies (2). (b) In s~tu hybridization experiments showed that the r R N A genes are located mainly at the heterochromatic ends of chromosomes X and C (3). As heterochromatic regions are less active in replication in polytene chromosomes (4-6), one would suspect an under-replication of the r R N A genes during polytenization, similar to what has been observed in Drosophila hyde~ (7). However, this was not observed for Rhynchosczara (18). To test both these hypotheses, we hybridized r R N A with salivary gland D N A from larvae of different ages at the end of larval life An alteration in the hybridization level during the polyteniza-don wouId indicate either an under-replication or amplification of the r R N A genes. In this paper, we verified that the proportion of r R N A genes does not change during a period of time corresponding to almost two cycles of D N A replication at the end of larval life. It was also observed that the number of r R N A genes in ovary 1Abbreviatwns used m thzs paper: rDNA, the DNA tissue of adult Rhynchoscia~a is higher than in salivary glands. The possible implications of these facts are discussed. Animals All experiments were performed with fourth in-star female larvae or with adult females, raised in the laboratory as previously described (8). The fourth in-star corresponds to nearly 65% of the total larval life and, for convenience, was divided into six periods, characterized by morphological and physiological events, as detailed elsewhere (9). Salivary gland DNA from three different larval ages was used in the hy-bridization experiments. The larvae used were' (a) 44-46 days old, corresponding to period III of the fourth luster; (b) 52-54 days old, when puffs are at maximum size and corresponding to period IV; (c) 55-56 days when most of the DNA puffs have contracted , corresponding …