Nucleic acid changes in the whole body and several organs of the silkworm, Bombyx mori, during metamorphosis

Abstract

The levels of DNA and RNA have been determined in the whole body and in several organs of the silkworm, Bombyx mori, at frequent intervals from the mature larva until adult emergence. Total DNA decreased steadily from the pharate pupa to emergence, falling to one-third of the initial level. Total RNA decreased rapidly in the pharate pupa, then increased during adult development, and again decreased before emergence. The distribution of the nucleic acids changed greatly. In the pharate pupal stage, both nucleic acids disappeared rapidly from the silk gland, and simultaneously increased in the fat body. During pharate adult development the fat body lost both DNA and RNA, and the ovary and the integument including some imaginal organs gained them. Before emergence, there was considerable loss of both nucleic acids from the ovary and of RNA from the integument. In the testis, the levels of DNA and RNA showed little change. The ratio of RNA to DNA reached maxima in the pharate pupa in the fat body and integument, and in the pharate adult agin in these two organs and also in the ovary. This is interpreted as reflecting active synthesis of protein. The incorporation of injected 3H-uridine into RNA of various organs almost paralleled their respective rates of increase of RNA. On the other hand, 3H-thymidine labelled most actively the DNA of the integument in the pharate pupa although net increase of DNA was not occurring, whereas it contributed little label to other tissues whose DNA contents were increasing rapidly. This is interpreted as indicating the transport of DNA (perhaps in degraded form) from the silk gland to the fat body for later use in development of the adult organs.