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.G. Gallardo et al. J. Exp. Mar. Biol. Ecol. 252 2000 97 –107
1. Introduction
The monogonont rotifer Brachionus plicatilis is capable of reproducing both asexually and sexually. In asexual or amictic reproduction, an amictic female rotifer produces
clones of herself via ameiotic parthenogenesis, whereas in sexual or mictic reproduction, females produce mictic daughters whose meiotically produced eggs, if not fertilized, will
become haploid males, or if fertilized will become resting eggs cysts. Resting eggs undergo dormancy thereby ensuring survival of the species during adverse environmen-
tal conditions. Initiation of mixis reproduction therefore is a critical phase of the life cycle that has important ecological and evolutionary consequences for the population
Pourriot and Snell, 1983; Snell and Boyer, 1988.
The occurrence of sexual or mictic reproduction depends on external and internal factors. Several external factors including population density, temperature, salinity, and
food have already been investigated, whereas only few internal factors have been studied Hagiwara and Hirayama, 1993. These internal factors include genetic variation among
clones Hino and Hirano, 1976; Snell and Hoff, 1985, culture history Hino and Hirano, 1985, 1988, cumulative generation Hino and Hirano, 1977, and aging Snell and
Childress, 1987. Hagiwara et al. 1994 indicated that a water soluble substance extracted from rotifer biomass increased sexual reproduction but this substance has not
been characterized.
Our initial survey of several vertebrate and invertebrate hormones for effects on B .
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plicatilis, found that porcine growth hormone GH, 0.0025 IU ml and gamma-amino
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butyric acid GABA, 50 mg ml increased population growth, whereas juvenile
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hormone JH, 0.5 mg ml enhanced mictic female production and serotonin 5-
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hydroxytryptamine, 5-HT; 5 mg ml enhanced both population growth and mictic
female production in batch cultures Gallardo et al., 1997. However, since these experiments were in batch cultures, the life history of individual rotifers was not known.
We therefore conducted individual culture experiments to determine the intrinsic rate of natural increase r, net reproduction rate Ro, and mixis reproduction of hormone-
treated rotifers. Subsequent experiments with individually cultured rotifers exposed to GH and GABA showed that GH enhances reproduction when culture conditions are
optimum, whereas GABA enhances reproduction when rotifers are stressed Gallardo et al., 1999. Following this same protocol, we conducted individual culture experiments
with JH and 5-HT to confirm their effects on mixis induction. Individual culture reduces the effect of population density on mictic female production Hino and Hirano, 1976,
1977; Snell and Boyer, 1988; Carmona et al., 1994, thus, we could investigate the effect of hormones on mictic female production of isolated females. Further, individual culture
experiments were conducted to determine if JH or 5-HT were still effective in a stressful environment, as was the case of GABA which enhanced population growth through
amictic reproduction even at low food or high free ammonia Gallardo et al., 1999. Snell and Boyer 1988 reported that low food level and high free ammonia con-
centration suppress mictic female production.
Identification of endogenous hormones by dot blot immunoassay and high-per- formance liquid chromatography HPLC of rotifer samples showed the presence of
GABA and 5-HT in rotifers Gallardo et al., 2000. These findings and results of the
W .G. Gallardo et al. J. Exp. Mar. Biol. Ecol. 252 2000 97 –107
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present study will provide insights into the regulation of the rotifer life cycle by hormonal signals and provide means for manipulating mictic reproduction of B
. plicatilis cultures.
2. Materials and methods
