Conjugation and autogamy in ciliated protozoans

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Conjugation and autogamy in ciliated protozoans
Paul R. Earl pearl_at_dsi.uanl.mx Facultad de
Ciencias Biológicas Universidad Autónoma de Nuevo
León San Nicolás, NL 66450, Mexico
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A little bit of historySexual processes in
ciliates have been known since they were
discovered by the Dutch lens maker Antoni van
Leeuwenhoek over 300 years ago. The Danish
naturalist O. F. Müller (1786) was the second
observer of the sexual event conjugation. Others
like Balbiani (1858, 1861) followed, including
Hertwig (1889) and Maupas (1889). Giant steps in
the study of ciliate conjugation were taken by
Gary N. Calkins (1869-1943) with Paramecium
caudatum as in Calkins Cull (1907). Herbert S.
Jennings (1868-1947) in Paramecium had an
analogous role to that of Thomas H. Morgan
(1866-1945) in Drosophila. Autogamy was
discovered around 1934 in P. aurelia by William
F. Diller (1902-1984), and Tracy M. Sonneborn
(1904-1981) discovered mating types in 1938 in
Paramecium.
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Ehrenberg (1838) suggested immortality for the
ciliates, and Maupas (1889) found that they had
the periods of youth, maturity and senescence.
The rejuvenation of old clones follows a sexual
event. Without conjugation, autogamy or cytogamy,
the cell line will die out as found by the
remarkable French-Algerian protozoologist Maupas
(1842-1916).
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Sexual processesAutogamy is like conjugation,
except that the entire process occurs in single
cells. Ralph Wichterman (1904-1999) discovered
cytogamy. See his 1986 book on Paramecium, and
the book edited by Görtz (1988). Cytogamy is like
conjugation, except that there is no nuclear
exchange between the partners. It is independent
double autogamy.Ciliates have several features
of chromosome segregation that are unique among
eukaryotes, including their maintenance of 2
nuclei the germline micronucleus (MI), which
undergoes conventional mitosis and meiosis, and a
somatic macronucleus (MA) with a great number of
minichromosomes that divide by a poorly
understood amitotic process. Ciliates can
propogate without the MI !
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Each cell contains an MI with conventional
chromosomes and a transcriptionally much more
active polyploid MA that controls the cell. The
MA genome is com-posed exclusively of short
linear minichromosomes that typically contain
single coding regions. The MA DNA molecules are
derived from a copy of the MI genome during
conjugation through an extensive DNA
reorganization process that includes the
fragmentation of the MI chromosomes and the new
addition of telomeres to the ends of the formed
MA minichromosomes via reverse transcriptase
subunit of telomerase or TERT.
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P. aurelia does not really exist as a species any
longer since Sonneborn (1975) divided the aurela
complex into 14 syngens species. A species,
following Mayr (1957), is a sexually isolated
population. It cannot cross with any others. How
different species are morphometrically, or in
their DNA or RNA sequences does not involve the
species definition that is only concerned with
crossability. The sequences of base pairs from
several genes like the histone centromere gene
can be most helpful for phylogenetic analyses.
The polymerase chain reaction, its wonderous
results, blotting and other molecular techniques
are still in their protozoologic infancy,
although new insights are appearing at a fast
pace.
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Know your Darlington ! NeoDarwinism combines
Charles Darwin's theory of evolution by natural
selection and Gregor Mendel's theory of genetics
with random mutation as the source of variation.
Cyril D. Darlington (1903-1981) discovered
chromosomal crossover and outlined the stages of
meiosis in his 1937 textbook updated from his
1931 text. A reduction division MUST OCCUR in
meiosis. Reduction before fertilization is
gametic, whereas reduction after fertilization is
zygotic meiosis. Meiosis reduces the number of
sets of chromosomes by half, so that when gametic
nuclei recombine the life cycle diploidy of their
parents will be reestablished. In mitosis, 2
new chromosome sets segregate to 2 new cells. In
meiosis, homologous pairs of chromatids
(bivalents) segregate to become the gametes that
can fuse to become the synkaryon (fertilization
nucleus).
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Macronuclear regeneration. Reciprical
fertilization in the synkaryon produced a
genetically new MA. Most commonly the old MA is
degenerated and resorped. However when the new MA
fails to develop, fragments of the old MA are
able to grow and resume control of the
exconjugant cell. Macronuclear regeneration was
discovered by Sonneborn in Paramecium in
1940.The MA genome has only a small of the MI
genome (510) thus, the chromatin elimination
involves as much as 95 of the DNA. Two types of
DNA processing form the MA chromosomes 1)
deletion of DNA sequences that are ISEs and
retention of the MA genomic sequences--the MDSs
and 2) fragmentation of chromosomes 5' and 3' of
genes coupled with the new addition of
telomeres.As we move along, we find new
directions leading still farther from textbook
cytology.
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General conjugation with emphasis on
TetrahymenaIn the crescent soon after pairing,
the 10 diploid chromosomes of Tetrahymena are
reduced to 5 haploid MIs in meiosis and are later
exchanged between the conjugants. In each cell of
the pair, the migratory MI fuses with the
stationary MI to form a new zygote which is the
synkaryon. The fertilization nucleus proceeds
through 2 mitotic divisions, usually, resulting
in 4 daughter nuclei. Two of the nuclei can serve
as the germ line MI for the daughter cells
following the next round of cell division. One of
the nuclei will segregate to the new MA, and one
to the new MI. The old MA will be degraded unless
needed to salvage a malfunctioning new MA. One of
2 MI in the 1st exconjugant is eliminated so that
the 2nd excongugant regains 1 MI 1 MA.
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Soon after mitosis, chromosomes within the
nucleus are destined to differentiate, undergoing
multiple rounds of replication to form polytene
chromosomes. The
chromosomes are radically fragmented to
minichromosomes, telomeres are added to sequences
of the MDSs, and the DNA of the remaining non-MAs
and the old MA fragments are degraded.
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The illustration above fits classic meiosis.
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The stages of the crescent after Sugai
Hiwatashi (1974)
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First prize goes to the people of the Yao
laboratoryThe best interpretation of
conjugation is by Marcella D. Cervantes, Xiaohui
Xi, Danielle Vermaak, Meng-Chao Yao and Harmit S.
Malik (J Biol Cell 17 485-497, 2006), mainly
because they have used the most recent technology
which is that of immunofluorescence with
centromeric antibody comple-mented with DAPI
fluorescent dye specific to DNA as in their 3
figures given here.
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Remember that the somatic MA is highly polyploid
and consists of amplified, highly rearranged
segments of the MI genome. The MA is
transcriptionally active and is responsible for
much of the gene expression in ciliates, whereas
the MI is active at special times like
meiosis.The completion of meiosis sets up a
cascade of events that leads up to the formation
and diversification of the germline MI MA. One
of 4 meiotic MIs is selected as a gamete and
undergoes 1 round of mitosis. The paired cells
exchange MIs, and then the 2 gametic nuclei, one
from each parent cell, fuse forming the diploid
zygotic nucleus--the synkaryon or zygote.These
MIs are the stationary and the migratory nuclei
and usually identical. Nevertheless, different
allopolyploids might well be encountered.
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Tetrahymena thermophila has 10 diploid
chromosomes. They are represented by the
brilliant green staining of the appropriate
antigen, centromeric histone.
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Stage III of the crescent and the distribution of
Histone 3. Credit to Cervantes et al, 2006
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Destruction of the old MA. Credit to Cervantes et
al, 2006
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Let's review conjugation again !When
conjugation is induced, the germ MI enters
meiosis and forms a synkaryon after reciprocal
exchange of gametic nuclei between the mates. The
synkaryon divides 3 times--usually--and
differentiates into new macro- and micronuclei.
The newly developed macronucleus (MA anlagen)
begins to function, the old MA degenerates and
the cells separate to be exconjugants. Eliminate
extras like 2 not 1 MIs in excongugants to fit
the condition. That's it !FPD first prezygotic
division, TPD third prezygotic division, PZD post
zygotic division.
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Exconjugant of Paramecium with 4 MA anlagen. The
Mis are lost among MA fragments. Basic fuchsin,
fast green.
An older exconjugant of Paramecium with 2 new
MAs.