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The life cycle of Brachionus plicatilis can be closed by two modes of reproduction.
During female parthenogenesis the amictic females produce amictic (diploid, 2n
chromosomes) eggs which develop and hatch into amictic females. Under specific environmental
conditions the females switch to a more complicated sexual
reproduction resulting in mictic and amictic females.
Although both are not distinguishable morphologically, the mictic females produce haploid
(n chromosomes) eggs. Larvae hatching out of these unfertilized mictic eggs develop into haploid males.
These males are about one quarter of the size of the female; they have no
digestive tract and no bladder but have an over-proportionated single testis which is filled
with sperm. Mictic eggs which will hatch into males are significantly smaller
in size, while the mictic fertilized eggs are larger and have a thick, faintly granulated outer
layer.
These are the resting eggs that will only develop and hatch into amictic females
after exposure to specific environmental conditions. These can be the result of
changes in environmental conditions eventually creating alternations in temperature
or salinity or changing food conditions. It should be emphasized that the rotifer
density of the population also plays an important role in the determination of the
mode of reproduction. Although the mechanism is not completely understood it is
generally accepted that the production of resting eggs is a survival of the population
through unfavourable environmental conditions as drought or cold. To increase the population density of rotifers, it is
necessary to keep them in the asexual reproduction phase by excluding the
factors which induce sexual reproduction of rotifers in mass culture (Dhert, 1996).
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