R RN NA A i in nt te er rf fe er re en nc ce e i in n n ne em ma at to od de es s a an nd d t th he e c ch ha an nc ce e t th ha at t f fa av vo or re ed d S Sy yd dn ne ey y
B Br re en nn ne er r
Address: Institut Jacques Monod, CNRS - Universities of Paris 7 and 6, Tour 43, 2 place Jussieu, 75251 Paris cedex 05, France.
RNA interference (RNAi), the inactivation of gene
expression by double-stranded (ds) RNA, has become a
major method of gene inactivation in the past ten years. The
fact that the trigger for RNAi is composed of dsRNA was
discovered in the nematode worm Caenorhabditis elegans .
This gene-inactivation method is far from being applicable
to all nematodes, however, especially in the external
application mode used in C. elegans. A recent paper by
Shannon et al. in BMC Molecular Biology  describes its
successful use in two Panagrolaimus species that belong to a
different nematode suborder from C. elegans. This increases
the range of nematode species over which comparative
functional genomics is in principle possible, and reinforces
the accumulating evidence that susceptibility to RNAi is
widely distributed over nematode species.
I Is s R RN NA A i in nt te er rf fe er re en nc ce e a a u un ni iv ve er rs sa al l p ph he en no om me en no on n i in n
e eu uk ka ar ry yo ot te es s? ?
RNAi was first described in plants and has now been found
in a variety of unicellular and multicellular eukaryotes. The
mechanism of inhibition entails the cleavage of the dsRNA
trigger into smaller dsRNAs of 21-23 base pairs, called small
interfering (si)RNAs, which recognize the target mRNA and
lead to its destruction. Sensitivity to long endogenous
dsRNAs may be maintained by selection against the spread
of transposons or viruses or against the spurious expression
of other repetitive sequences, all of which are likely to be
transcribed in both directions to at least some degree, and
thus to produce dsRNA.
It is a particularly convenient feature of C. elegans that it is
sensitive to external dsRNAs provided very simply either by
soaking the worms in a RNA preparation  or by feeding
them with Escherichia coli bacteria expressing dsRNA from a
plasmid , as outlined in Figure 1. Sensitivity to external
dsRNAs is thought to be required for repression of viruses,
although this remains to be directly proven in C. elegans, for
which no natural viruses are known. It requires both a
mechanism for uptake of the dsRNAs into intestinal cells,
and the spread of the interference to other cells . In C.
elegans, this phenomenon has made possible systematic gene-
inactivation screens using a library of bacteria expressing
dsRNAs against most open reading frames. Similar gene
inactivation by feeding with bacteria expressing dsRNAs is
also possible in species of flatworms and cnidarians.
A Ab bs st tr ra ac ct t
The efficiency of RNA interference varies between different organisms, even among nematodes.
A recent report of successful RNA interference in the nematode Panagrolaimus superbus in
BMC Molecular Biology has implications for the comparative study of the functional genomics of
nematode species, and prompts reflections on the choice of Caenorhabditis elegans as a model
Journal of Biology 2008, 7 7: :34
Published: 13 November 2008
Journal of Biology 2008, 7 7: :34 (doi:10.1186/jbiol97)
The electronic version of this article is the complete one and can be
found online at http://jbiol.com/content/7/9/34
© 2008 BioMed Central Ltd
D Di iv ve er rs si it ty y i in n s se en ns si it ti iv vi it ty y t to o R RN NA Ai i a am mo on ng g n ne em ma at to od de es s
The efficiency of RNA interference is far from general,
however, even in nematodes (Figure 2). Until recently, the use
of bacteria expressing dsRNAs in nematodes was restricted to
C. elegans. Even within the Caenorhabditis genus, the second
most studied species, Caenorhabditis briggsae, was found to be
insensitive to external application of dsRNAs. Interestingly, C.
briggsae seems to be deficient in the uptake of dsRNAs in the
intestine, a deficiency that can be complemented by
expression of C. elegans sid-2 (systemic RNA interference
defective 2), a gene that was found in a genetic screen for
mutants defective in systemic RNAi . The sid-2 gene
encodes a putative transmembrane protein expressed in the
intestine and probably involved directly in the uptake of
dsRNAs from the intestinal lumen. Only one other tested
Caenorhabditis species (C. sp. 1 SB341) - and none of the close
relatives of C. elegans - was found to be sensitive to external
RNAi. However, so far all species of the genus tested have
been found to be sensitive to dsRNAs introduced by injection
into the gonad .
Outside the Caenorhabditis genus, the prospects seemed even
less bright. Despite extensive efforts, it has not been possible
to implement RNAi (by injection, soaking or feeding) in the
two laboratory ‘satellite’ model systems used for extensive
genetic screens and comparative studies to C. elegans - Oscheius
tipulae and Pristionchus pacificus. Morpholino oligonucleotides
had to be used instead, a poor and expensive alternative
The recent article by Shannon et al.  however encourages
the hope that other culturable nematode species may turn
out to be sensitive to RNAi. Panagrolaimus is a genus of free-
living nematodes more distantly related to Caenorhabditis
than are Oscheius and Pristionchus, and it encompasses many
different ecologies and reproductive modes. Burnell and
colleagues  show, using dsRNA-expressing bacteria, that
Panagrolaimus superbus and, to a lesser degree, Panagrolaimus
sp. PS1159 are sensitive to external RNAi. This study opens
the way for functional genomic analysis in these
Panagrolaimus species and suggests that other culturable
‘free-living’ nematodes may be sensitive to RNAi.
The situation is different for nematodes of medical or
economic interest. So far, only a few key species have been
tested, and we can now expect variation in RNAi efficiency
even between closely related animals. Several plant-patho-
genic nematodes are sensitive to external application (by
soaking in dsRNAs, in the presence of octopamine to induce
feeding behavior) and possibly also through expression of
dsRNAs from a transgenic plant . The insect-pathogenic
nematode Heterorhabditis bacteriophora is also sensitive to
dsRNAs administered by soaking . RNAi has been
unsuccessful or unreliable in nematodes that are vertebrate
pathogens, however, including various evolutionary groups
within nematodes (Figure 2) . In the genomes of these
nematodes, some of the genes encoding upstream compo-
nents of long dsRNA processing seem to be lacking or
In summary, the use of RNAi for functional genomics is so
far restricted to a few nematode species. The work of Burnell
and colleagues  suggests that the phylogenetic
distribution of the dsRNA effect on nematodes is capricious
and that researchers who find no effect in one species
should keep trying in a variety of other species. The complex
phylogenetic distribution also raises questions about the
evolutionary pressures, perhaps from pathogens, acting on
the mechanisms of response to various forms of internal or
external dsRNAs. It also makes retrospectively remarkable
the choice of C. elegans as the laboratory model system, 35
years before RNAi was discovered.
T Th he e l lu uc ck ky y c ch ho oi ic ce e o of f C C. . e el le eg ga an ns s
In the 1960s, Sydney Brenner, after his earlier work using
phage genetics, chose C. elegans to study development and
neurobiology of a multicellular organism. Brenner said:
“Thus we want a multicellular organism which has a short
life cycle, can be easily cultivated, and is small enough to be
handled in large numbers, like a micro-organism. It should
have relatively few cells, so that exhaustive studies of lineage
and patterns can be made, and should be amenable to
34.2 Journal of Biology 2008,Volume 7, Article 34Félixhttp://jbiol.com/content/7/9/34
Journal of Biology 2008, 7 7: :34
F Fi ig gu ur re e 1 1
Different modes of administration of dsRNAs for RNA interference.
External application of RNAs by soaking or feeding bacteria requires
the dsRNAs to cross the intestinal barrier and the siRNA signal to
spread systemically. In some species octopamine is required to induce
feeding behavior or electroporation may be used. Injection into the
body cavity only requires systemic spreading of the signal. Internal
application by injection into the syncytial female gonad ensures
transmission to the next generation, regardless of systemic spreading.
Intestine is in red, gonad in green.
into body cavity
Soaking in dsRNAs
genetic analysis.” (Proposal to the Medical Research
Council, October 1963). He later added: “Since a nervous
system is essentially a cellular network, we had to be able to
observe junctions between cells and their processes and this
could only be achieved with the electron microscope, which
has the necessary resolution.” (Nobel Lecture, 2002).
After trying several other exotic organisms and isolating
“nematodes from nature to find the best one” , Brenner
chose C. elegans for several reasons: its easy culture in large
numbers on two-dimensional surfaces of agar plates with E.
coli and in defined liquid media; its fast generation time (3.5
days); its mode of reproduction with self-fertile hermaphro-
dites and facultative males for crosses; its transparency in
light microscopy and good contrast in electron microscopy;
the constancy of its neuronal composition (which was
known at least for other nematodes such as Ascaris, though
at the time not for C. elegans) .
Several common free-living nematode species could have
met most of these criteria, for example in the Oscheius,
Rhabditis or Pristionchus genera. Within the Caenorhabditis
genus, another obvious candidate was C. briggsae. Brenner
indeed first intended to work on C. briggsae (Proposal to the
Medical Research Council, October 1963) which was the
species that Ellsworth Dougherty, his nematode contact in
Berkeley, was beginning to culture axenically. However, the
Dougherty C. briggsae strain turned out to grow less well
than a C. elegans strain isolated in Bristol (N2). So Brenner
finally chose the latter (J. Hodgkin, personal communi-
cation). Brenner was surely a visionary when he turned to
studies of development and behavior of C. elegans, but there
http://jbiol.com/content/7/9/34Journal of Biology 2008,Volume 7, Article 34Félix 34.3
Journal of Biology 2008, 7 7: :34
F Fi ig gu ur re e 2 2
Phylogenetic relationships and RNAi susceptibility of nematode species. Phylogenetic relationships are redrawn from [32,33]. A indicates transition
to animal parasitism. P indicates transition to plant parasitism. Susceptibility of each species to internal and external modes of RNAi administration is
indicated on the right. A + sign indicates that RNAi has been successful, a - sign that it has not. A +/- sign indicates poor efficiency. ND, not
determined. Colors of species name indicate the status of genome sequencing: green, published; orange, ongoing or planned; red, not planned.
*I. Nuez and MAF, unpublished data.
Caenorhabditis sp. 1 SB341
Panagrolaimus sp. PS1159
RNAi by injection
+ (gonad only)
- (rescued by Ce-sid-2)
+ (w/ octopamine)
+ (w/ octopamine)
+ (w/ octopamine)
(rescued by Ce-sid-2)*
(but use of morpholinos)
(but use of morpholinos)
are several future developments he could not possibly have
foreseen when he chose C. elegans over C. briggsae and other
nematode genera. The first of these is that C. elegans is so far
the only hermaphroditic free-living species in which external
application of dsRNAs inactivates gene expression.
Panagrolaimus superbus is a male-female species and P. sp.
PS1159 is parthenogenetic. Furthermore, the N2 strain that
Brenner chose as a reference is much more sensitive to RNAi
in the germ line than other C. elegans isolates (such as the
CB4856 strain commonly used for single nucleotide poly-
morphism (SNP)-based mutant mapping; ). Second,
easy transgenesis of C. elegans is possible because of its
syncytial germ line: any form of injected DNA recombines
and forms an additional chromosome that is frequently
passed onto later generations . Transgenesis by injection
has so far proved impossible in Oscheius tipulae, Pristionchus
pacificus and Panagrolaimus species and is much more
difficult in C. briggsae (the efficiency of establishing lines is
lower and there is more silencing and mosaicism ).
Transgenesis in parasitic nematodes has so far been
restricted to transient expression [13,14]. Third, C. elegans
can be frozen, as was successfully achieved by John Sulston
in 1969 . Freezing Pristionchus pacificus , some
Rhabditis species or other Caenorhabditis species such as C.
sp. 3 has proved much more difficult. And finally, the
efficiency of chemical mutagenesis by ethane methyl
sulfonate is a balance between toxicity and mutagenic effect;
mutagenesis is less efficient in Oscheius tipulae, for example,
than in C. elegans .
In the history of science, a model organism may remain
successful because of unexpected turns of chance, and C.
elegans might not have become so popular had some of the
features mentioned above been lacking. A further
retrospective bias is introduced by the fact that C. elegans has
been studied more than other species. Methods have been
optimized for C. elegans N2: for example, culture conditions
have to be changed for optimal culture of other C. elegans or
C. briggsae strains (higher agar concentration because of
worm burrowing ); immunostaining has been
optimized for C. elegans; and so on. On the other hand,
other discoveries might have been made if another species
had been chosen. The recent finding of Caenorhabditis sp. 9,
a male-female species that can hybridize with the
hermaphroditic C. briggsae (MAF, unpublished data) may
make C. briggsae a popular species for some evolutionary
studies. In contrast, a close relative of C. elegans that can
hybridize with it is still missing.
In addition to these methodological advantages, it is starting
to become clear that the choice of strain and species has also
influenced the biological results. Taking the example of
vulva development, the effect of gonad ablation is clear cut
in C. elegans: vulval tissue does not form at all, because of the
lack of induction by the gonadal anchor cell. Yet in
Panagrolaimus sp. PS1159, the outcome of the same ablation
is much less clear: some vulval tissue forms in a variable
manner depending on the individual . At a smaller
evolutionary scale, Wnt pathway mutations were not found
in the original screens for vulva mutants on the N2 strain, but
have a stronger effect in the genetic background of other C.
elegans wild isolates (J. Milloz, I. Nuez and MAF, unpublished
data). Brenner’s choice is therefore also important for the
textbook picture that emerges from studying a model
organism. Even given the overall universality of biological
mechanisms and molecular pathways, the rapid evolution of
partial redundancy between processes and molecular
pathways means that key results of experimental
manipulations (such as cell ablation and gene inactivation)
are highly dependent on the species and strain chosen as the
N Ne em ma at to od de e g ge en no om me es s
C. elegans was the first multicellular organism to have its
genome fully sequenced . With the increase in
sequencing capabilities, genome sequencing of various
nematode species is under way [21,22], regardless of the
possibilities for functional studies. The successful use of
RNAi in Panagrolaimus superbus makes it a ‘superb’ candidate
for genome sequencing.
In the Caenorhabditis genus, the genome sequences of five
species are now available, with the best assembly being that
for C. briggsae . Molecular divergence is high, which
makes these genomes useful for the annotation of
conserved noncoding regions of the C. elegans genome.
Most other nematode genome and expressed sequence tag
sequences (completed or planned) are for animal- or plant-
parasitic nematodes, because of their medical or economic
importance. Annotation of sequences from these parasites
can make use of the good annotation of the C. elegans
genome. The draft assembly of the genome of Brugia malayi
(the cause of filariasis) suggests conservation of large-scale,
but not small-scale, synteny and of many operons .
Functional characterization of gene function may be
possible for plant parasites using RNAi interference, but
direct characterization of gene function in the vertebrate
parasites will be difficult because of the present lack of gene-
inactivation techniques. Alternative methods for delivery of
dsRNAs or siRNAs will be needed.
Besides their use for the development of nematicides, these
nematode genomes will be of great interest for genome
evolution studies. The nematodes form an abundant and
highly diverse group of animals. Perhaps because of the
34.4 Journal of Biology 2008,Volume 7, Article 34 Félixhttp://jbiol.com/content/7/9/34
Journal of Biology 2008, 7 7: :34
short generation time of many of them, genome evolution is
rapid in nematodes. Large sequencing projects will provide
tools to study genome evolution at different evolutionary
scales: from intraspecific evolution to large-scale divergence
A Ac ck kn no ow wl le ed dg ge em me en nt ts s
I thank past and present members of my lab and other colleagues who
shared their experience with various difficult nematode strains. I
acknowledge funding by the Centre National de la Recherche Scientifique
(CNRS) and grants from the Association pour la Recherche sur le
Cancer and the Agence Nationale de la Recherche.
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