Figure 4. Inhibition of TOR signaling prevents meiotic onset in Oikopleura dioica.

At standard culture densities (top row), the O. dioica germline is characterized by a coenocystic cytoplasm consisting of a homogenous population of proliferative mitotic nuclei prior to day 3 of development. Following meiotic onset, (late day 3) the germline coenocystic cytoplasm consists of equal numbers of smaller meiotic nuclei (MN, arrowheads) arrested in prophase I and larger endocycling nurse nuclei (NN). Germline nuclei of growth arrested animals (Dense condition) retain a homogeneous undifferentiated size and do not commence meiosis as evident from the persistence of the proliferative mitotic mark H3pS28 (arrows) similar to Day3 animals before meiotic onset under standard conditions. There was a decrease in IdU incorporation in germline nuclei over time (D3–D18; day 3 to day 18). Similarly, germ line nuclei of O. dioica cultured at standard density, do not commence meiosis when treated with the TOR inhibitor CCI-779 (7.5 μM) for 24 h, 48 h or 72 h and continue to proliferate mitotically. Scale bars = 20 μm.