Cytology of meiosis in the triploid gyngenetic salamander Ambystoma tremblayi.

Female meiosis was analyzed in the triploid gynogenetic salamander Ambystoma tremblayi to determine the mechanism by which a stable chromosome number is maintained in this unisexual species. Gross details of the reproductive cycle and the cytology of meiosis were analyzed in 20 specimens and 320 oocytes involving all stages from early diplotene to the beginning of anaphase II. Ovulation apparently continues progressively involving a few oocytes at a time. Oocytes form the ovary contained chromosomes in diplotene, and diakinesis. The first metaphase was not observed since this stage occurs swiftly either immediately prior to or during ovulation. Oocytes in the most anterior region of the oviduct were in metaphase II, and those in the most posterior region were undergoing the beginning of anaphase II. Telophase II was not observed. Chromosome numbers obtained at all stages of prophase gave counts of approximately 42 bivalents, equivalent to the triploid somatic number known for this species. Similar numbers of dyads were obtained from metaphase II plates. This analysis supports earlier evidence suggesting that the triploid number of chromosomes in oocytes of A. tremblayi is doubled prior to meiosis, and the somatic number is restored by two normal meiotic divisions.