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Conjugation: Mating in Tetrahymena thermophila
Conjugation is the sexual stage of the ciliate life cycle.
During this stage, two cells pair to exchange haploid gametic nuclei. The nuclear events of conjugation normally include meiosis,
gamete nucleus formation, fertilization, and nuclear
differentiation. Conjugation
includes the only - and very brief - haploid stage of the ciliate life cycle; it
follows meiosis and quickly ends at fertilization. This process is conserved among the
majority of
ciliates; however, the duration of each stage and other details can
vary among ciliate species. The following series shows the general order of
these steps, with details specific to Tetrahymena noted in the
accompanying text.
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Vegetatively growing T. thermophila cells contain one macronucleus
and one micronucleus. The number of macro-
and micronuclei found in other cilate species varies. Some contain 20 micronuclei and hundreds of macronuclei per cell.
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Ciliate cells propagate by asexual
division ("amitosis" in ciliates, left arrow), but under certain conditions cells will enter the
conjugation cycle (right arrow). T. thermophila cells can be induced to
conjugate by mixing cells belonging to one of seven different mating types.
At the start of conjugation in T. thermophila, micronuclei in paired cells undergo
meiosis, generating four haploid pronuclei. Three of these
pronuclei are destroyed, while the
remaining one divides to form two gametic nuclei: a "migratory"
pronucleus and a "stationary" pronucleus. Migratory pronuclei are
exchanged through a temporary junction of the two cells; these then fuse with a stationary pronucleus to form a zygotic nucleus in each cell.
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The zygotic nucleus divides twice to form four
identical nuclei, during which time the old macronucleus is
degraded. Two of the four zygotic clones develop into "anlagen"
(immature macronuclei), which undergo a wide array of
genome rearrangements, including chromosome breakage, programmed DNA
elimination, and telomere addition. In T. thermophila these
processes generate approximately 300 individual macronuclear
chromosomes. Each chromosome is then amplified to 45 copies,
completing development of the macronuclear genome. |
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One of the two remaining
zygotic clones is degraded; the other, the new micronucleus, divides mitotically during the
first asexual reproductive cycle. The daughter cells each receive
one micronucleus and one macronucleus in this division, yielding the
normal complement of nuclei found in vegetatively growing
T. thermophila cells. |
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