A radioautographic study of the time course of male meiosis in the newt Triturus vulgaris

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Abstract
In male Triturus vulgaris at 16 °C the pre-meiotic synthesis of DNA (S-phase) takes 9-10 days. The S-phase starts 1-2 days after completion of the ultimate spermatogonial mitosis, and extends into leptotene. Heterochromatic regions, most of which lie close to the centromeres, start and finish replicating about 1 day later than other parts of the chromosomes. Synapsis starts 6 days after the end of pre-meiotic S and is completed 8 days later. Pachytene lasts for 4 or 5 days, diplotene for 1 or 2 days, and first metaphase occurs 20 or 21 days after the end of pre-meiotic S. Label distribution at first metaphase in spermatocytes stemming from spermatogonia which incorporated [3H]thymidine during the penultimate intermitotic S shows that random segregation and assortment of labelled and unlabelled chromatids took place at the ultimate spermatogonial mitosis.