Jpn. J. Protozool. Vol. 37, No. 2. (2004)

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Original
PCR primers for the amplification of the nuclear small subunit ribosomal
DNA sequences from polycystine radiolarians
Tomoko YUASA,1, * Osamu TAKAHASHI 2 and Shigeki MAYAMA3
1

Division of Mathematics and Natural Science Education, United Graduate School of Education,
Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan,2Department of Astronomy and Earth
Sciences, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan,3Department of Biology,
Tokyo Gakugei University, Koganei, Tokyo 184-8501, JapanSUMMARY

INTRODUCTION

Polycystinea (Radiolaria) is a class of planktonic
protists widely distributed in tropical, subtropical,
and even polar marine environments. Various
types of algae occur as intracellular symbionts inthe cells of the polycystines (e.g., Anderson, 1976)
which causes significant cross-contamination inpolycystine DNA extractions, complicating DNAbased molecular analyses. We designed potential
primers; Spu 191F and Spu 1731R, specifically forspumellarian polycystines, which are solitary andhave spongy or latticed siliceous shells. These
primers amplified 18S rDNA sequences directlyfrom the specimens containing symbiotic algae.
They will be used to accelerate phylogenetic
analyses of polycystine radiolarians, which havenot been established in culture and possess symbiotic algae.

Polycystinea (Radiolaria) is a class of planktonic protists widely distributed in tropical, subtropical, and even polar marine environments.
(Currently, many researchers use “Radiolaria” as aconventional name, which comprehensively indicates the Acantharea, the Polycystinea, and the
Phaeodarea (Table 1)). The first determination ofsmall-subunit ribosomal DNA (18S rDNA) sequences in the Polycystinea was reported by
Amaral Zettler et al. (1997). As templates for polymerase chain reaction (PCR) amplification, theyused the genomes of the swarmer cells of the colonial and skeletonless spumellarians that endogenously formed in individual vegetative cells. On theother hand, a molecular technique for the solitary
spumellarians with siliceous skeletons and shells
has not yet been established due to the difficulty ofculturing them.
Various types of algae occur as intracellularsymbionts in the cells of the polycystines;
dinoflagellates, prasinophytes, and prymnesiophytes (e.g., Anderson, 1976). In all polycystines,
algal symbionts are generally found in the rhizopodial network of the ectocytoplasm (Fig. 1)*Corresponding author

TEL: +81 42 329 7536
Fax: +81 42 329 7538
E-mail: r046003n@u-gakugei.ac.jp
Received: 28 June 2004; Accepted: 4 Nov 2004.

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PCR primers for polycystine radiolarians

Table 1 The present system of “Radiolaria” reviewed by Levine et al. (1980)

Kingdom PROTISTA Haeckel, 1866
Subkingdom PROTOZOA Goldfuss, 1818: emend. Owen, 1858
Phylum SARCOMASTIGOPHORA Honigberg and Balamuth, 1963
Subphylum SARCODINA Schmarda, 1871
Superclass ACTINOPODA Calkins, 1909
Class HELIOZOA Haeckel, 1866
Class ACANTHAREA Haeckel, 1881Class PHAEODAREA Haeckel, 1879
Class POLYCYSTINEA Ehrenberg, 1838 “Radiolaria”
Order Spumellarida Ehrenberg, 1875
Order Nassellarida Ehrenberg, 1875

Fig. 1. Schematic representation of the hostsymbiont configuration. Polycystine cell islargely divided into two cytoplasmic bodies:
endocytoplasm and ectocytoplasm, by porous
central capsular wall. The central capsule contains major cytoplasmic organelles: nucleus,
mitochondria, and other membranous organelles,
exclusive of digestive vacuoles (Anderson,1983). Symbiotic algae are generally found in theectocytoplasm.

(Anderson, 1976; Takahashi et al., 2003) andrarely have been observed within the central capsule (Anderson and Matsuoka, 1992). These causesignificant cross-contamination in polycystine
DNA extractions, complicating DNA-based molecular analyses. PCR amplification using polycystine-specific primers thus may overcome theselimitations. Amaral Zettler et al. (1997) used a
combination of spumellarian-specific primers inorder to obtain the 18S rDNA sequences; however,their primers are specific to the colonial spumellarians. In the present study, we designed primers
specifically for the spumellarians, which are solitary and have spongy or latticed siliceous shells.

MATERIALS AND METHODS
Polycystine samples were collected from northwest of Okinawa Island, Japan. The polycystines
included in the present study were Dictyocoryne
truncatum (Ehrenberg), Dictyocoryne profunda
Ehrenberg, and Spongaster tetras Ehrenberg(Family Spongodiscidae), whose sequences werealready reported by Takahashi et al. (2004), andDidymocyrtis tetrathalamus (Haeckel) (Family
Coccodiscidae). They all are classified into Order
Spumellarida and they are solitary and havespongy or latticed siliceous shells, about 200 µm
in diameter.
Single cells of each specimen were rinsed

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135

Fig. 2. Agarose gel electrophoresis of PCR products
from both a central capsule and ectocytoplasm of eachspumellarian species, amplified with an eukaryotespecific primers 90F/B. Templates of lanes 1, 2, 3, and4 are from the central capsules of Dictyocoryne truncatum, D. profunda, Spongaster tetras, and Didymocyrtis
tetrathalamus, respectively. Templates of lanes 5, 6, 7,
and 8 are from the ectocytoplasm of D. truncatum, D.
profunda, S. tetras, and D. tetrathalamus, respectively.
The all amplified fragments are 18S rDNAs. Molecularweight standards: ØX174RF Hae III digest in Lane 9.
λ-Hind III digest in Lane 10.

twice in filtered seawater and the central capsule
was physically separated by a sterilized razor blade
from the ectocytoplasm, which contained endosymbiotic algae, as mentioned above. Each central
capsule and the ectocytoplasm was rinsed twice
again in distilled water, and then incubated for 30
minutes at 37°C in 0.2 µg/µl Proteinase K solution. This sample was used as a template for theamplification of 18S rRNA coding regions. Polymerase chain reaction (PCR) was accomplishedusing an eukaryotic-specific forward primer 90F(Hendriks et al., 1989): 5’-GAAACTGCGAATGGCTCATT-3’ and a reverse primer B (Medlin etal., 1988): 5’-CCTTCTGCAGGTTCACCTAC-3’.
The PCR amplification was performed using an
initial denaturation step of 95 °C for 3 minutes,
followed by 35 amplification cycles each consisting of 95 °C for 1 minute, 55 °C for 2 minutes, and
72 °C for 3 minutes. Each PCR product was purified by the GFX PCR DNA and Gel Band Purification Kit (Amersham Biosciences) and thencloned in the pGEM-T Easy Vector System(Promega) using E. coli JM109 Competent Cells(Promega). Five clones for each of the amplified
fragments from the inserts were sequenced using
flanking vector primers. The DDBJ accession
numbers for the 18S rDNA sequence data of D.
truncatum, D. profunda, S. tetras, and D. tetrathalamus are AB101540- AB101542, and AB193605,
respectively.

RESULTS AND DISCUSSION
The sequences of PCR products from ectocytoplasm of Dictyocoryne truncatum (Fig. 2, lane 5)
were branched in a clade of Class Haptophyta, andthose from Spongaster tetras and Didymocyrtis
tetrathalamus (Fig. 2, lanes 7, 8) were branched ina clade of Class Chlorophyta (data not shown).
These sequences of the amplified fragments probably originate from the symbionts in each spumellarian ectocytoplasm. On the other hand, Dictyocoryne profunda does not possess symbiotic algae(Takahashi et al., 2003); therefore, the PCR products from ectocytoplasm of D. profunda were notobtained (Fig. 2, lane 6). Finally, the sequences of
the amplified fragments were aligned with the corresponding region from the host spumellarians by
Clustal W ver. 1.81 (Thompson et al., 1994) (Fig.
3). Then, potential primer sequences were designated Spu 191F: 5’-GCGACTYACGAAGCCCTGTA-3’ and Spu 1731R: 5’-ACTTCGRGCCTCCCGTT-3’. The primers Spu 190F and Spu 1731R,
when paired with each other, using an initial denaturation step of 95 °C for 3 minutes, followed by
35 cycles of 95 °C for 1 minute, 57°C for 2 minutes, 72 °C for 3 minutes, amplified single fragments from the four spumellarian species (Fig. 4,
lanes 1-4). 18S rDNAs from each symbiotic algaein the ectocytoplasm of D. truncatum, S. tetras,

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Fig. 3. Comparison of designated primer positions by both the authors and Amaral Zettler et al. (1997).
The open squares by solid lines indicate designated primer regions by the authors, which are the forward
primer Spu 191F (positions 191-213) and the reverse primer Spu 1731R (positions 1885-2002) (primer
sequences see text). The open squares by broken lines indicate designated primer regions by Amaral Zettler et al. (1997), whose sequences are: the forward primer R906 (positions 952-969), 5’-TATTAGTATTTTRTCGTT-3’; the reverse primer R1451bio (positions 1612-1629), 5’-TATTGTAGCCCGTGCGCT-3’. Ammonia beccarii (Foraminifera) is added in this alignment as negative control.

Fig. 4. Agarose gel electrophoresis of PCR products from both a central capsule and ectocytoplasmof each spumellarian species amplified withspumellarian-specific primers 191F/1731R. Templates of lanes 1, 2, 3, and 4 are from the central
capsules of Dictyocoryne truncatum, D. profunda,
Spongaster tetras, and Didymocyrtis tetrathalamus, respectively. Templates of lanes 5, 6, 7, and 8
are from the ectocytoplasm of D. truncatum, D.
profunda, S. tetras, and D. tetrathalamus, respectively. The all amplified fragments are 18S rDNAs.
Molecular weight standards: ØX174RF Hae III
digest in Lane 9. λ-Hind III digest in Lane 10.

Jpn. J. Protozool. Vol. 37, No. 2. (2004)

and D. tetrathalamus could not be amplified bythese primers (Fig. 4, lanes 5, 7, and 8), and theectocytoplasm of D. profunda also could not be
amplified as expected (Fig. 4, lane 6).
These results given in the Figures 2, 3, and 4
show that the primers Spu 190F and Spu 1731R
are specific to the spumellarian polycystine sequences and amplified the 18S rDNA sequences
directly from the specimens containing symbiotic
algae. At present, there are very small numbers ofpolycystine species for molecular analyses. The
specific primers that were designated in the present
study will be used to accelerate phylogenetic
analyses and the in situ hybridization of polycystines, which have not been established in culture andpossess symbiotic algae.
ACKNOWLEDGMENT
We are grateful to two anonymous reviewers
for helpful comments and suggestions on the
manuscript. We would like to acknowledge Daisuke Honda of the Department of Biology, Facultyof Science and Engineering, Konan University for
his valuable technical advice and helpful discussions. We wish to thank Akihiro Takemura andYoshikatsu Nakano of the Tropical Biosphere Research Center of the University of the Ryukyus fortheir support and valuable advice. This study was
financially supported by Grants-in-Aid for Scientific Research (C) (Grant No.15540449) from the
Ministry of Education, Science, Sports, and Culture, Japan.
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