During the 15 days of the second International Microgravity Laboratory mission (IML-2, STS-65), natural mating of male and female vertebrates occurred in microgravity and was videotaped for the first time. The Japanese Medaka fish Oryzias latipes mated successfully in space and young fish hatched during the flight (Figure 5-02). Back on the ground, the offspring produced healthy second-generation (F2) animals (Ijiri 1998).
Years before this video-based demonstration of successful fish mating in microgravity, several observations pointed to successful natural in-flight mating in other species. For example, the effect of spaceflight on fertilization, growth, and development of the fruit fly Drosophila melanogaster was observed during a 4-day Vostolc mission. It was likely that in-flight mating occurred because embryos were recovered after landing (Antipov et al. 1965). Later, a colony of nematodes Caenorhabditis elegans flew on board the European facility Bioraclc (see Figure 4-03) during the Spacelab IML-1 mission. The animals successfully reproduced twice in space and generated thousands of offspring (Nelson et al. 1995). Natural mating in microgravity by rats was tried during an 18.5-day spaceflight on board the Russian Cosmos-1129 biosatellite flight. Post flight investigations on the flight dams revealed that ovulation, copulation, and fertilization had occurred in microgravity, but due to some unclear events, females were prevented from delivery of young. This experiment was never repeated so that until today, no rat or other mammal has undergone birth in space. It is worthwhile to note, however, that in this early experiment, the ground-control females, too, failed to deliver pups (Ronca 2003b).
2 The pituitary is an endocrine gland located at the base of the brain, which secretes important hormones including growth hormone and sexual hormones.
3 The period of time after giving birth.
4 Each month an egg develops inside the ovary in a fluid filled pocket called a follicle.
In 1998, the tropical freshwater snail Biomphalaria glabrata flew on board Space Shuttle STS-89 and STS-90 missions. Videotaping of Biomphalaria in orbit revealed that the snails were easily dislodged from the aquarium wall, while on Earth they spent most of their time attached to the walls. Once separated from the wall they floated through the water, which gave them the chance to contact another snail in orbit. As these snails are hermaphrodites, mating pairs were often seen floating attached to one another. Therefore, after landing of the spacecraft, embryos of all developmental stages were present (Marxen et al. 2001).
In frog Xenopus laevis, the first successful fertilization in space was done during a ballistic rocket flight in 1988, using a fully automated hardware. The experiment was successfully repeated on another sounding rocket flight in 1989, and then on board the manned IML-1 and IML-2 Spacelab missions (Ubbels 1995), and the Japanese Spacelab-J mission, where eggs were fertilized in vitro by manipulations of the crew (Souza et al. 1995). Successful automatic fertilizations in microgravity were also obtained in sea urchins (Marthy et al. 1994) on board a sounding rocket and during Space Shuttle flights (Schatten et al. 1999a).
Another strategy for natural in-flight fertilization is possible in those animal species that require no in-flight mating. In these species, mating and insemination is performed on the ground before launch. Inseminated females store the sperm in a compartment of the body called spermatotheca and use the sperm cells at the moment of egg deposition. The advantage of this approach is that the time of fertilization, and therefore the age of embryos can precisely be determined by the experimenter.
This type of fertilization was successfully performed in two urodele species, the salamander Pleurodeles waltl (experiments "Fertile" on the Russian space station Mir in 1996 and 1998; Aimar et al. 2000) and the newt Cynops pyrrhogaster (experiment "Astronewt" on board IML-2 in 1994 with a repetition in 1995; see Izumi-Kurotani and Kiyomoto 2003 for review). The female newts keep spermatozoa in their cloacae ready to fertilize eggs after hormonal stimulation of ovulation. Thereafter, egg laying occurs within 24 to 48 hours. Presence of spermatozoa in the perivitelline space and of spermatic spots on the surface of the eggs in microgravity can be considered as a proof that the development of embryos is not based on parthenogenesis5. During the two "Fertile" projects on board Mir, about 56% of eggs were successfully fertilized, as calculated from the number of living embryos at the two-cell stage, or at later stages from the number of eggs in the spawning. By comparison, the ground experiments revealed a ratio of 51%, suggesting that
5 Parthenogenesis is a form of reproduction in which the egg develops into a new individual without fertilization by sperm.
occurrence of egg fertilization was not affected by microgravity (Aimar et al. 2000).
Crickets use a fertilization strategy similar to that of salamanders. After insemination by a male, the female cricket keeps the sperm in its spermatotheca. Fertilization occurs during the process of egg laying, which is activated by offering a suitable substrate to the female. By means of this technique, we were able to obtain successful in-flight fertilization in the house crickets Acheta domesticus during the Italian Soyuz Taxi flight "Enei'de" to the ISS in 2005. After the flight, embryos were recovered, suggesting that eggs could develop for eight days in microgravity.
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