Autoradiography with Meiotic Chromosomes of the Male Newt (triturus Viridescens) Using H3-thymidine.

HE very specialized meiotic events that occur in all sexually reproducing organisms have not been investigated in any great detail by means of high resolution tritium autoradiography. Most of the work so far has been done with animals because of technical difficulties in obtaining uptake of labelled nucleosides in the microsporocytes of plants (TAKATS 1962). LIMA-DE-FARIA (1959, 1961,1962) has studied the incorporation of tritiated thymidine by spermatocytes of the grasshopper and by oocytes of the Dipteran, Tipula. His studies demonstrated a marked asynchrony of deoxyribonucleic acid replication during the premeiotic DNA synthetic period. These observations were carried to the prophase of meiosis. MONESI (1962) studied the duration of the DNA synthetic period (S phase) prior to meiosis in the male mouse and concluded that there is a gradual lengthening of the S phase during successive spermatogonial divisions, culminating with the last premeiotic DNA replication lasting approximately 14 hours. TAYLOR (1958, 1959) utilized P32 and C14-labelled glycine and orotic acid in autoradiographic investigations of Lilium and Tulbaghia microsporocytes. He concluded that DNA was synthesized prior to leptonema. This paper reports an autoradiographic study of the meiotic chromosomes of the male newt labelled with H3-thymidine. It will be shown that aside from the main period of DNA synthesis in pre-prophase spermatocytes, a slow DNA synthesis occurs during a part of meiotic prophase.