Cytophotometric and autoradiographic evidence for functional apomixis in a gynogenetic fish, Poecilia formosa and its related, triploid unisexuals

Abstract

Amounts of DNA in individual Feulgen-stained nuclei from squash preparations of ovaries and testes from wide-caught and laboratory-reared stocks of Poecilia spp. were determined with an integrating microdensitometer. The DNA content of primary spermatocytes (4C) at zygotene, pachytene, or at metaphase I (3.3-3.4 pg) was approximately twice that found in secondary spermatocytes (2C) and four times that found for young spermatids (1C). Rarely, mature sperm were found with 2C DNA amounts. Nuclei from follicular epithelium and oogonia from both bisexual and diploid unisexual fish contained about 1.6-1.7 pg DNA; whereas, the DNA content of primary oocyte nuclei was about 3.5-3.7 pg DNA, indicating that just one cycle of chromosomal replication had occurred in these cells during the period of DNA synthesis before the visible onset of meiotic prophase. Similar results were obtained for triploid unisexuals whose 6C primary oocyte nuclei contained 5.0-5.1 pg DNA, which was twice the DNA content of 3C oogonia and follicular epithelial cells (2.4-2.5 pg DNA). Autoradiographic studies, designed to monitor the incorporation of 3H-thymidine by oogonia and primary oocytes in vivo and in vitro, also showed that there is no additional synthesis of DNA during the course of meiotic prophase in these unisexual fish. Therefore, we conclude that apomixis, not endoreduplication, is the cytological basis of reproduction in Poecilia formosa and its related, triploid biotypes.