Jpn. J. Protozool. Vol. 40, No. 2. (2007)

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Original

Gametic nuclear exchange during the conjugation of
Paramecium polycaryum
Xianyu YANG1* and Xinbai SHI2
1

School of Forestry and Biotechnology, Zhejiang Forestry University, Hangzhou - Lin’an
311300, China, 2Department of Biology, Harbin Normal University, Harbin 158025, China
Key words: Paramecium; Conjugation; Cross-fertilization; Protargol; Gametic nuclei

SUMMARY
The migratory gametic nuclear exchange is an
important event during the conjugation of ciliates.
So far, it is unclear whether this process occurs in
Paramecium polycaryum, and there are also someother contradictory descriptions of this species. To
clarify this issue, we studied the conjugation process in the Chinese strain of P. polycaryum bymeans of a modified protargol technique. The reciprocal gametic nuclear exchange and the formation of fertilized nuclei were observed in detail.
The results indicated the following: 1) P. polycaryum has cross-fertilization during its conjugation. 2) The transfer of migratory gametic nuclei
into the partner cell seems to be an active process.
3) The process of fertilized nuclear formation includes the attachment of a migratory gametic nucleus to a stationary one, membrane fusion of two*Corresponding author
Tel: +86-571-63744008 Fax: +86-571-63740809
E-mail: xianyu_yang@hotmail.com
Received: 6 June 2007; Accepted: 5 July 2007

kinds of gametic nuclei with separate nucleoplasm,
and the fusion of the nucleoplasm of two gametic
nuclei to form a typical fertilized nucleus.

INTRODUCTION
Paramecium polycaryum is a unicellular eukaryotic organism belonging to the Paramecium
genus that was originally reported by Woodruff
and Spencer (1923). The cells of this species are
70 to 110 µm in length and dorsal-ventrally flattened, with a slightly obliquely truncated anterior
end and a rounded posterior end. Each cell has asomatic nucleus (macronucleus), one to eight germinal nuclei (micronuclei), and two canal-fed contractile vacuoles, and it spirals to the left whenswimming. There is a centrally located mouth that
leads into the buccal cavity (oral apparatus, cytopharynx), which consists of several ciliary structures (Wichterman, 1986).
Concerning the sexual process (conjugation) of
Paramecium polycaryum, two reports have been
published so far, the first one by Diller on an In-

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Cross-fertilization in P. polycaryum

dian strain (1958), and the second one by Hayashi
and Takayanagi on a Japanese strain (1962). This
conjugation is induced by specific cell-cell interaction by mixing two complementary mating types
followed by formation of a conjugating pair(conjugant). The whole process of conjugation
includes a series of nuclear events, such as three
prezygotic nuclear divisions, formation of zygotic
nuclei (fertilized nuclei), three postzygotic nuclear
divisions, new macronuclear differentiation anddevelopment, and old macronuclear fragmentationand degeneration (Wichterman, 1986). The disintegration and regeneration of the ciliary structures on
the cytopharynx also happen during the conjugation. The first two prezygotic divisions are meiotic,
and all micronuclei take part in this process, while
only one of the meiotic products enters the paroral
region (the area surrounding the degenerated oral
apparatus) and survives. The surviving nucleus
then undergoes the third prezygotic division toform two gametic nuclei, one migratory gametic
nucleus (MiN) and one stationary gametic nucleus(StN) corresponding to the male and female gametic cells of higher organisms, respectively. Following the fusion of the MiN and StN, a fertilized
nucleus forms and undergoes three successive
postzygotic nuclear divisions. Four out of the eight
postzygotic division products differentiate into
new macronuclei (macronuclear anlagen), and theother four become micronuclei (Diller, 1958; Hayashi and Takayanagi, 1962). However, it is still
unclear whether true cross-fertilization (reciprocal
migratory gametic nuclear exchange) occurs in P.
polycaryum. Diller’s observation suggested selffertilization rather than cross-fertilization (1958),
while Hayashi and Takayanagi (1962) observed
gametic nuclei lying against a conjugant boundaryand mentioned cross-fertilization but did not provide any evidence of a reciprocal gametic nuclear
exchange.
Concerning the inheritance of the mating types,
Diller (1958) described caryonidal heredity, while

Hayashi and Takayanagi (1962) described clonal
heredity. In Paramecium, there are three kinds ofmating type inheritance, i.e., “synclonal,”“caryonidal,” and “clonal” (Wichterman, 1986). A
synclone is a culture of the progeny of two cells
from one conjugating pair (exconjugant); when all
the cells in a synclone show the same mating type,
it is called synclonal heredity. When an exconjugant divides, micronuclei divide mitotically, whilemacronuclear anlagen (usually two or four depending on the number of postzygotic nuclear divisions
in different Paramecium species) segregate into
each of the daughter cells (Wichterman, 1986). A
cell line from one such daughter cell is designatedas a caryonide; when different mating types appear
among the caryonides from the same exconjugant,
it is called caryonidal heredity. In clonal heredity,
all the caryonides from one exconjugant show thesame mating types, and the caryonides from a partner exconjugant show another mating type, butthese two mating types are complementary. In
Diller’s experiment, exconjugants from the conjugating pairs were separately cultured, but conjugating pairs were observed in each culture of the exconjugant; therefore, caryonidal inheritance wasconcluded (1958). In contrast to Diller’s result,Hayashi and Takayanagi did not observe any conjugating pairs in the exconjugant culture except
when mixing the two exconjugant cultures fromone conjugating pair; they, therefore, concluded
that the outcome was clonal inheritance (1962).
To clarify this issue, we studied the entire conjugation process of P. polycaryum using a strain
collected from Harbin, a city located in northeast
China, by means of the protargol method and obtained more detailed information on gametic nuclear exchange and fertilized nuclear formation.
Here, we report the outcome and discuss the possible mechanisms of gametic nuclear exchange andthe process of fertilized nuclear formation in Paramecium.

Jpn. J. Protozool. Vol. 40, No. 2. (2007)

MATERIALS AND METHODS
Cell culture
Cells of P. polycaryum were isolated from water
samples collected from Majia Ditch, Harbin,
China. Collections were obtained from the ditch
under the bridge that was part of Hexing Road andcultivated according to the method described bySonneborn (1950) except that rice straw was usedinstead of lettuce powder. Conjugation was induced by mixing highly reactive cells of complementary mating types at room temperature (23-25°C). Conjugating pairs remained on the bottom of
the Petri-dish and were concentrated by micropipettes.
Staining and observation
After the induction of conjugation, cells were
fixed with saturated HgCl2 periodically and stainedusing a modified protargol method. This modification was performed by Shi and reported at the 4th
Symposium of Chinese Protozoological Society in
1987. The protocol used for staining consisted of
two parts, one for staining infraciliature, publishedby Shi and Frankel (1990), and the other for staining nuclei (Symposium of the Chinese Protozoological Society, 1987). The second part of thisprotocol is as follows: 1) Fix cells on a slide when
their nuclei have been scattered by the pressure
from the cover slip. 2) Perform step 7 to step 16 of
Shi and Frankel (1990), including secondary fixing
in a 1:1 (v/v) mixture of 95% ethyl alcohol andformalin (10 min); seal cells with 0.5% celloidin in
absolute ethyl alcohol, wash slides with distilledwater, and bleach with 0.5% KMnO4 (1-3 min)followed by 5% oxalic acid treatment (1-3 min). 3)
After bleaching and washing (step 16), add a stepof treatment with a 1% NaCl aqueous solution atroom temperature for one hour. 4) Continue fromstep 17 up to the completion of the procedure outlined by Shi and Frankel (1990), including immersion of slides in 1-2% protargol dissolved in dis-

115

tilled water at 50°C for 30 min, development of thecells in 0.1% hydroquinone in a 5% Na2SO3 aqueous solution, fixing in 5% Na2S2O3·5H2O, dehydration in ethanol series, clearance in xylene, andmounting of the cells. This method can be used tostain spindles, fibril systems, nuclear membranes,
nucleoli, chromatin, and chromosomes. Preparations were observed and photographed under a
Nikon microscope.
Measurement of the immature period and mating-type inheritance
To investigate the mating-type inheritance inthis experiment, exconjugants from the same conjugating pairs were separately grown, and the caryonides of some exconjugants were also separately
grown. After ciliate conjugation, caryonides go
through a period during which they have no mating ability (activity), known as the immature period, which is measured by the number of cell divisions. Here, the daily isolation method (Takagi and
Yoshida, 1980) was used to count the cell divisions; according to this method, one cell is kept ina fresh culture medium; then, the number of cell
divisions is calculated the following day. Again,
only one cell is transferred to a fresh culture medium, while the other cells are gathered and supplied with enough medium to keep them growing(mass culture), as in the case of the daily isolated
cells. This operation is repeated daily until theappearance of mating activity, which was tested bymixing the caryonide mass culture with the original cell cultures that had been used to induce theconjugation.

RESULTS
Mating-type inheritance in P. polycaryum
Forty-three clones of P. polycaryum were established from the water samples of Majia Ditch, andgrouped into two complementary mating types. To

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Cross-fertilization in P. polycaryum

know the inheritance of mating types, the exconjugants of ten and caryonides of five conjugating
pairs were separately grown, and their mating activity was periodically checked. All caryonides
were mature by 45 fissions. No conjugating pairs
appeared in the exconjugant cultures. Four caryonides from one exconjugant showed the same mating type, while two caryonide groups from two
exconjugants of one conjugating pair showed different but complementary mating types in allcases. These results indicated that the inheritanceof mating types in the Chinese strain of P. polycaryum was clonal rather than caryonidal, a finding which is in agreement with the conclusion of
Hayashi and Takayanagi (1962).
Gametic nuclear exchange in P. polycaryum
In Paramecium, the micronuclear division is
mitotic (the macronuclear division is amitotic), as
in other ciliates, but the process is strikingly different from that in metazoan cells in two respects(Wichterman, 1986). One is that the nuclear envelope remains intact during all the stages of division, and the other is that centrioles are absent inciliates. The modified protargol method developed
by Shi is an excellent staining technique that yields
high resolution for both fibrillar systems (Shi andFrankel, 1990) and nuclei-related structures, suchas spindles, nuclear membranes, nucleoli, chromatin, and chromosomes of ciliates (Shi et al., 1997;
Watanabe et al., 1996). To discern the contradictory reports on the issue of the reciprocal migratory gametic nuclear exchange in P. polycaryum(Diller, 1958; Hayashi and Takayanagi, 1962), we
studied the entire conjugation process using a cellstrain collected from Harbin by means of the protargol method. A typical pattern of three prezygotic
nuclear divisions and three postzygotic nuclear
divisions was observed in our study, in agreementwith previous reports (Diller, 1958; Hayashi and
Takayanagi, 1962).
The strain of P. polycaryum used in this experi-

Fig. 1. Meiotic divisions of micronuclei during the conjugation of P. polycaryum. Double arrowheads indicate conjugant boundaries of each conjugating pair in all panels. A. A conjugating pair at the early stage. No obvious morphological changes in either micronuclei (small arrowheads) or macronuclei (large arrowheads). The oral apparatus begandisintegrating (solid arrows). B. Late meiotic prophase - crescent stage. Small arrowheads: two crescent micronuclei;
solid thick arrow: disintegrated oral apparatus. C. Second prezygotic nuclear division. Small arrowheads: nuclei atdifferent stages (metaphase, anaphase, and telophase); solid thick arrow: faint conical structure of the former oral
apparatus; thin small arrow: spindle of the telophase nucleus. Scale bars: 20 µm.

Jpn. J. Protozool. Vol. 40, No. 2. (2007)

ment had 1-6 micronuclei, all of which enteredmeiosis through a long prophase; on the other
hand, the ciliary structures on the cytopharynx
disintegrated gradually and disappeared mostly
before the first meiotic division (Fig. 1A, B). Finally, a conical structure remained as the trace ofthe former oral apparatus (thick arrow in Fig. 1C);
its surrounding area is the so-called paroral region.
After meiosis, only one of the meiotic productsthat entered the paroral region survived and underwent the third prezygotic division, while the re-117

maining ones degenerated (Fig. 2A, A’). During
this division, the nucleus destined to be the migratory gametic nucleus (MiN) kept its location in theparoral regions (right cell in Fig. 2B, B’), but thenucleus destined to be the stationary gametic nucleus (StN) was far from the paroral region (blackarrow in Fig. 2C). After this division, one MiN andone StN formed in each mate of the conjugating
pair (left cells in Fig. 2B, B’, C, C’). MiN was stillin the paroral region and seemed to have attachedto the conjugant boundary (left cell in Fig. 2C, C’),Fig. 2. One of the meiotic products or the migratory gametic nuclei (MiN) located in the paroral regions during the
conjugation of P. polycaryum. The framed portions in A, B, and C are magnified in A’, B’, and C’, respectively.
Double arrowheads: conjugant boundaries in all panels. A. A conjugant after meiosis. Thick arrow: one meiotic
product entered the paroral regions; thin arrows: meiotic products outside the paroral region. A’. One meiotic nucleus entered the paroral regions (arrows). B. A conjugating pair around the stage of the third prezygotic nuclear
division. Right cell: telophase of the third prezygotic division. Hollow arrow: nucleus destined to be the MiN localizing in the paroral region; thin arrow: spindle. Left cell: after the third prezygotic nuclear division. Black arrow: around-end spindle-shaped stationary gametic nucleus (StN). B’. Right cell: MiN lying against the conjugant boundary. Left cell: two parts of the StN (arrow). White arrowhead: chromatin; black arrowhead: clear area surrounding
the chromatin. C. The same conjugating pair as in B but with a different focus. Right cell: a StN out of focus (solid
arrow). Left cell: a round-end spindle-shaped MiN (hollow arrow). C’. A migratory gametic nucleus (hollow arrow). White arrowhead: chromatin part; black arrowhead: clear area surrounding the chromatin. Scale bars: 20µm
in A, B, and C and 10µm in A’, B’, and C’.

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Cross-fertilization in P. polycaryum

while StN remained slightly far from the boundary(right cell in Fig. 2B, B’). Although both MiN and
StN located differently in the cells, both of themshowed similar morphological changes: a roundend spindle-shaped nucleoplasm (chromatin) surrounded by a thin layer of a clear area (left cells in
Fig. 2B, B’, C, C’). When the MiN was crossing
the conjugant boundary, the chromatin part became
thinner and longer (Fig. 3A, B), while the clear
area at the anterior part of the MiN became larger
and sharper, containing some faint filaments (thinarrows in Fig. 3B).
Zygotic nuclear formation in P. polycaryum
Following the reciprocal MiN exchange, theprocess of nuclear fusion between MiN and StN(zygotic nuclear formation) initiated. At the beginning, the MiN attached to the StN when both ofthem extended, showing a bipolar-spindle shape,
and were arranged side by side along their longitudinal axes (Fig. 4A, A’); at that time, the membrane border between two gametic nuclei was observed (small black arrow in Fig. 4A’). The membrane border then disappeared, while two parts of
the nucleoplasm still remained separated, showing
a heterogenous appearance (Fig. 4B, B’). Finally,
two parts of the nucleoplasm fused completely, anda typical zygotic nucleus formed (Fig. 4C, C’).

DISCUSSION

Fig. 3. Reciprocal gametic nuclear exchange in P. polycaryum. Framed portion of A magnified in B. A. A
conjugating pair at the stage of MiN exchange. Thickarrow: a crossing MiN; thin arrow: a stationary gametic
nucleus. An elongated macronucleus is observed in eachconjugant. Double arrowhead: conjugant boundary;
MA: old macronuclei. B. A crossing migratory gametic
nucleus. White arrowhead: thinner and longer chromatinpart; black arrowhead: enlarged and sharp clear area at
the anterior part of the MiN containing some faint filaments (thin arrows). Scale bars: 20µm in A and 10µm
in B.

In this report, we found a “clonal” mating-type
determination in the Chinese strain of P. polycaryum, which is in agreement with the results
reported by Hayashi and Takayanagi on a Japanesestrain (1962) and contradictory to Diller’s report(1958). All caryonides obtained in this study
showed an immature period of mating activity(mating activity appeared by 45 fissions after conjugation), while in P. tetraurelia, one of the wellstudied species and known to have a “clonal” mating type inheritance, its progenies only haveautogamy immature period rather than immature
period of mating activity (Wichtermen, 1986).
These results, in conjunction with our observationof cross-fertilization might suggest that P. polycaryum of Chinese strain represents a different
mating type determination style of Paramecium
genus. Our results might also suggest that certain
differences exist between the Chinese and Japanese strains and the Indian strain and that thesedifferent strains belong to different sibling groups,

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Fig. 4. Process of fertilized nuclear formation in P. polycaryum. A-C. Conjugants at different stages of fertilized
nuclear formation. Framed portions in A, B, and C are magnified in A’, B’, and C’, respectively. MA: old macronuclei assumed a twisted rope-like shape. Double arrowheads: conjugant boundaries. A. The MiN (solid arrows) attached to the StN (hollow arrows). A’. Bipolar spindle-shaped MiN (solid arrow) and StN (hollow arrow). White
arrowhead: chromatin part of the MiN; black arrowhead: clear area at the anterior part of the MiN; thin small arrow:boundary between two gametic nuclei. B. Two parts of the nucleoplasm from two MiN and StN. B’. Fused nuclear
membrane of the MiN and StN (arrow) and their separate nucleoplasm (white arrowheads). Black arrowhead: clear
area at the anterior part of the MiN. C and C’. Typical fertilized nuclei (arrows). Scale bars: 20 µm in A, B, and C
and 4 µm in A’, B’, and C’.

Fig. 5. Schematic representation of gametic nuclear exchange and fertilized nuclear formation of P. polycaryum
based on the protargol staining images. Only gametic nuclei or fertilized nuclei are shown; the dotted line indicatesthe conjugating boundary. A. Telophase of the third prezygotic division. B. Soon after the third prezygotic division,
the MiN locates in the paroral regions lying against the conjugating boundary, and the StN locates farther from theboundary. Both pronuclei show a round-end spindle shape and are covered by a thin clear area. C. Stage of gametic
nuclear exchange showing enlarged and sharp clear areas at the anterior parts of the MiN. D. The MiN attaches to theside of the StN, and both of them show a bipolar-end spindle morphology. The membrane border between the MiN
and the StN is observed. E. The membrane borders between MiN and StN disappear; two parts of nucleoplasm show
heterogenous appearance. F. Formation of typical fertilized nuclei.

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Cross-fertilization in P. polycaryum

such as different syngens in P. caudatum and P.
bursaria (Wichterman, 1986).
The gametic nuclear exchange was here studied
using the modified protargol method. As reported
above, this modified protargol method developed
by Shi for staining the nucleus can stain not only
chromatin but also the nuclear membrane, and theimage obtained by this method gives higher resolution than the traditional nuclear staining methodsof Feulgen or DAPI. Therefore, this method isextremely useful when the nuclear behavior of
ciliates, especially, nuclear divisions, is studied.
Our observations indicated the existence of reciprocal gametic nuclear exchange and true crossfertilization in the Chinese strain of this species(Fig. 3, Fig. 4). More detailed nuclear behavior
during the processes of nuclear exchange and fertilized nuclear formation is summarized in Fig. 5.
After the third prezygotic division, the shape ofboth the MiN and the StN changed into round-end
spindles, and each of them consisted of a chromatin part and the clear area surrounding the chromatin (Fig. 2, B’, C, C’, Fig. 5B). These round-end
spindle-shaped gametic nuclei probably correspond to the description of biconvex lens-shaped
migratory gametic nuclei, and the clear area surrounding the chromatin corresponds to the structure of intranuclear microtubules lining the gametic nuclear membrane in P. caudatum(Nakajima et al., 2001). During the MiN transfer,
we observed that the clear area at the anterior partof the MiN became enlarged and sharper (Fig. 3A,
A’, Fig. 5C), containing some faintly stained filaments (thin arrows in Fig. 3B’); this structure
might correspond to the tip of the crossing MiN of
P. caudatum, in which many microtubules arealigned in the direction of its movement (Nakajima
et al., 2001). Other morphological studies on P.
caudatum (Shi, 1998) and P. dubosqui (Watanabe
et al., 1996) using the protargol method also indicated the sharp and pointy anterior part of thecrossing migratory gametic nuclei. All these obser-

vations might suggest that gametic nuclear transfer
does not take place through amoeboid movement
but, rather, as an active style.
Concerning the gametic nuclear exchange inciliates, there are mainly two opinions. One focuses on the amoeboid movement of the MiN(André and Vivier, 1962; Inaba et al., 1966), andthe other emphasizes that extranuclear microtubule
structures, either the microtubule meshwork on theback of MiN in Tetrahymena (Orias et al., 1983) orthe extranuclear bundles of microtubules in P. aurelia (Jurand, 1976), play an essential role in gametic nuclear transfer. Recent studies by Nakajima
et al. (2001) indicated that both cytoplasmic(extranuclear) microtubules and intranuclear
microtubules are necessary for gametic nuclear
transfer in P. caudatum, and they suggested an
active process of MiN migration. The pointy andsharp anterior part of the crossing MiN shown by
protargol staining (Shi, 1998; Watanabe et al.,
1996; current study) provides support for the ideaof an active process of MiN transfer.
After the third prezygotic nuclear division, both
the MiN and the StN assumed a round-end shape,
and the chromatin part was surrounded by a clear
layer (Fig. 2B, B’, C, C’, Fig. 5B) consisting of
microtubules (intranuclear microtubules) (Nakajima et al., 2001). These intranuclear microtubulesof the MiN may function in the active migration of
the MiN itself (Nakajima et al., 2001), while thefunction of the similar structure in the SP is not yet
understood. After reciprocal gametic nuclear exchange, the MiN first attached to the side of the
StN when both assumed the bipolar-spindle shape(Fig. 4A, A’, Fig. 5D); then, the nuclear membranes of two gametic nuclei fused while theirrespective nucleoplasm remained separated (Fig.
4B, B’, Fig 5E). Finally, the nucleoplasm from two
gametic nuclei fused completely, and a typicalfertilized nucleus formed as a result (Fig. 4C, C’,
Fig. 5F). The question of how the MiN recognizes
the StN and fuses with it then has to be answered.

Jpn. J. Protozool. Vol. 40, No. 2. (2007)

Both morphological and structural changes occur
in both kinds of gametic nuclei before and aftergametic nuclear transfer. All these changes suggest
some certainly active preparatory processes forgametic nuclear transfer and/or MiN - StN recognition.

ACKNOWLEDGEMENTS
The authors thank all the members of Prof.
Shi’s lab for their kind support. This research waspartially supported by a grant from the National
Natural Science Foundation of China (No.
30670297).

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