The premeiotic DNA replication of euchromatin and heterochromatin in Lilium longiflorum (Thunb.)

Abstract

The presynaptic stages of Lilium longiflorum have been analyzed in the light microscope. Seven substages can be defined on the basis of changes in morphology of the heterochromatin and the nucleoli. The duration of the presynaptic interval and the substages has been calculated from bud length measurements and cell stage gradients in the anthers. The premeiotic and somatic DNA replication has been investigated using3H-thymidine autoradiography. The premeiotic S phase lasts 50 hours whereas the somatic S phase in root tip nuclei is six times shorter. Moreover, premeiotic S phase nuclei house 2–3 times more heterochromatin than somatic nuclei. The premeiotic DNA replication can be divided into three periods: In early S phase the DNA of euchromatin is replicated; in mid S phase synthesis is arrested for a minimum of 9 hours; and in late S phase the DNA of heterochromatin is replicated. DNA synthesis in heterochromatin is correlated with a dispersal of these chromatin regions. Determinations of volume and staining characteristics of heterochromatin after the application of C and Q banding procedures indicate that the premeiotic heterochromatin comprises C bands, Q bands and interbands. The premeiotic DNA replication is discussed in relation to the observed chromatin dynamics, and a proposal is made for the significance of the replication sequence to the formation of the leptotene chromosome and the control of crossing-over.