The timing and role of DNA synthesis in insect gland development

Abstract

Metamorphosis of the non-dividing labial gland cells of Manduca sexta involves their transformation from silk to salivary function. In an investigation of the role of DNA synthesis in this process, the timing of DNA synthesis was studied in vivo and in vitro by incorporation of [ 3H]-thymidine and quantitative estimates of DNA content were made at selected periods of development. While DNA synthesis both precedes and occurs during this process of transformation, the most prominent period of synthesis appears to be coupled to gland growth, but occurs too late to trigger adult differentiation. Blood levels of [ 3H]-thymidine were assayed to determine the efficacy of label in vivo. Following single injections, approximately 60% of the radioactivity remains in the blood during the first 4 hr, but is reduced by 21 hr. The rate of [ 3H]-thymidine catabolism varies among individual insects so that between 2 and 17% [ 3H]-thymidine is actually available after 4 hr in vivo. Continuous perfusion of [ 3H]-thymidine into the blood was used to investigate incorporation into gland cells, in addition to using single injection studies.