Cell Division, Mitotic Recombination and Onset of Meiosis by Yeast Cells during Space Flight (YEAST)

Cell and molecular biology

Saccharomyces cerevisiae (yeast)
Cell culture chambers: 32

Delayed synchronous

Biorack US2 Experiment Hardware

The principal objectives of the flight experiment were to determine the effects of the space flight environment on cell yield, survival, and ability to undergo meiosis and to monitor mitotic chromosome segregation and recombination in the space flight environment. Two types of yeast cultures were flown: mitotic cell cultures in which yeast cell populations are established by budding and mitosis, and meiotic cell cultures in which yeast cells undergo meiosis and acospore formation. The mitotic cell cultures are microbial analogs of human somatic mitotic cell division, while the meiotic cell cultures are microbial analogs of human meiosis and gamete formation.

Two yeast diploid hybrids were prepared. STS42-1 is a Rec+, Rad+, Spo+ strain capable of mitosis or meiosis at both 22 °C or 36 °C. STS42-2 is a temperature- conditional strain that exhibits Rec+, Rad+, Spo+ at 22 °C, but Rec-, Rad-, and Spo- at 36 °C and grows mitotically at either temperature. Two cultures of each strain were incubated inflight under four conditions: in microgravity at 22 °C and 36 °C, and in a 1 G centrifuge at 22 °C and 36 °C. The ground cultures were incubated under similar conditions: static at 22 °C and 36 °C, in a 1 G centrifuge at 22 °C and 36 °C.

There was no marked enhancement or reduction in total cell yield due to microgravity conditions. The incubation temperature appears to be the principle factor in the total cell yield: cell densities of cultures incubated at 22 °C were greater by a factor of 2 or less than the densities of those incubated at 36 °C. The average survival of the STS42-1 flight culture cells ranged from 51% to 75%, with the 22° cultures being the highest. The average survival rate of the STS42-2 flight cell cultures ranged from 24% to 72% with the 22 °C cultures being the highest. The most striking discovery was the higher-than-expected recovery of Rec- intergenic mitotic recombinants from the 36 °C flight cultures. Rec- is recombinate deficient at 36 °C in ground controls. STS42-2 also preserved its Rec- phenotype during flight at 36 °C with respect to resistant segregants due to gene conversion, events that result in mitotic segregants and failure to initiate meiosis. One hypothesis for this behavior is a difference in the nature of the lesion that initiates mitotic recombination in flight as opposed to ground.

Bruschi, C.: Microgravitational Effects on Chromosome Behavior (7-IML-1). First International Microgravity Laboratory Experiment Descriptions, T.Y. Miller, ed., NASA TM-4353, 1992, pp. 33–38.

Esposito, M.S. and C.V Bruschi: Diploid Yeast Cells Yield Homozygous Spontaneous Mutations. Current Genetics, vol. 23(5–6), May–Jun 1993, pp. 430–434.†

Bruschi, C.V. and M.S. Esposito: Cell Division, Mitotic Recombination and Onset of Meiosis by Diploid Yeast Cells during Space Flight. In: BIORACK on Spacelab IML-1, C. Mattock, ed. Noordwijk, The Netherlands: European Space Agency SP-1162, 1995, pp. 83–93.