Synthesis of satellite DNA in Rhynchosciara hollaenderi

During the latter part of the fourth instar development of Rhynchosciara larvae, DNA synthesis occurs in both germ cells and somatic cells, even though these cells do not undergo mitosis. CsCl density gradient analysis has revealed the synthesis of d(AT) satellite DNAs in the testis and in somatic tissues such as salivary gland, fat body, and gastric cecum. In these studies it has been shown that there is a tissue-specific variation in the relative proportions of synthesis of d(AT) satellite and main-band DNA in the testis during the fourth instar. The initiation of synthesis of the d(AT) satellite in the testis corresponds with the appearance and formation of the highly characteristic mitochondria which develop during the maturation of the spermatocytes. This satellite DNA has been shown to be circular and has a density of 1.681 gm/cm³ in CsCl. Synthesis of a similar circular DNA cannot be detected in somatic tissues, although these tissues do synthesize a d(AT) satellite of similar density. We conclude from these studies that the specific synthesis of a unique circular DNA may be an essential part of the morphogenetic development of the highly specialized mitochondria formed during spermatocyte maturation.