Conversion from mitosis to meiosis: morphology and expression of proliferating cell nuclear antigen (PCNA) and Dmc1 during newt spermatogenesis.

The conversion from mitosis to meiosis is a phenomenon specific to the cellular progenitors of gametes; however, the mechanism or mechanisms responsible for this conversion are poorly understood. To this end, some morphological and molecular changes that occur during the initiation of meiosis in newt spermatogenesis are reported in the present paper. In situ morphologic studies revealed that spermatogonial stages comprise two phases: early mitotic generations (G1-G4) and late mitotic generations (G5-G8). Morphologic conversion from secondary spermatogonia to primary spermatocytes occurred during the intermediate stage of premeiotic DNA replication. The expression of proliferating cell nuclear antigen (PCNA), a DNA polymerase-delta auxiliary protein, in spermatogonia was weak in G1, highest during DNA synthesis (S), decreased in G2 and was not detectable in dividing cells. Complementary DNA for newt homologs of DMC1 (disrupted meiotic cDNA), which is an Escherichia coli RecA-like protein specifically active during meiosis, were isolated. The newt Dmc1 mRNA was first expressed significantly during the preleptotene stage and this continued into the spermatid stage. These observations present a basis for investigating the mechanism(s) controlling the conversion of newt spermatogonial cells from mitosis to meiosis.