Why focus on Biomphalaria
The biology of Biomphalaria
glabrata comprises many aspects that make this organism a logical
choice for a molluscan genome project. Below, such aspects are highlighted
from the standpoints of basic science and of infectious disease.
Obtaining the genome sequence of
the mollusc Biomphalaria glabrata:
A rationale from the standpoint
of basic science
The beginning of the 21st century will long be remembered
by historians as the age of genome biology. One of the challenges for biologists
during this exciting period is to determine a logical order with which
to procure the abundance of fascinating information lying within the genomes
of Earth’s various organisms. This challenge becomes particularly difficult
now that a first tier of genomes that encompass the most prominent model
organisms is in hand.
One of the major considerations in determining which eukaryotic organisms
make their way onto a planned list of genomes to be completed is to begin
to fill in some of the major phylogenetic gaps that exist in our current
sample. Thus far, only a small sample of genomes representing the Kingdom
Animalia is available: a nematode, an insect, and two mammals (humans and
mice). Other projects already underway will provide additional genomes
for a variety of vertebrates and medically significant parasitic animals,
including additional nematodes and the flatworm, Schistosoma mansoni.
The available sequences represent well the deuterostome lineage of animals,
and the ecdysozoan lineages, but leave relatively under-represented one
of the prominent groups of invertebrates, namely the more complex members
of the Lophotrochozoa. Below, the case is made for obtaining the genome
sequence for the freshwater snail Biomphalaria glabrata, a representative
of a prominent lophotrochozoan lineage, the Phylum Mollusca.
The Case for Obtaining a Molluscan Genome
One of the most prominent lophotrochozoan phyla,
and indeed one of the largest of all animal phyla, is the Mollusca. There
are an estimated 50,000 extant molluscan species, making it one of the
largest and most successful of all phyla. Furthermore, their success is
not transitory – molluscs have one of the best documented fossil records
of any animal group and have played a prominent role in animal life since
their origins in the Cambrian period over 550 million years ago. Of all
the animals on earth, perhaps none have adopted body plans and life styles
more distinctive than molluscs. Molluscs are remarkable for their possession
of a soft, mucus-covered body frequently protected by a shell and a file-like
radula for obtaining and processing food. They are often unusual in their
coloration and body organization, making them unique among animal life.
Molluscs occupy habitats ranging from the oceanic depths to the tops
of trees in tropical rain forests. As invertebrates go, molluscs like Architeuthis
are the largest and the octopus the most intelligent. Some molluscs like
squids have adapted to pelagic existence and are the equals of fish with
respect to their swimming speed and ability. Some of the longest lived
of all animals are molluscs: freshwater unionid bivalve species routinely
live for more than 100 years. The unionids also emphasize the point that
many molluscs are endangered and face extirpation. Molluscs also play a
prominent role in the lives of humans. No other invertebrate group is as
frequently exploited as a source of food or for products of commercial
significance. Molluscs also play an essential role in the life cycles of
many parasites, including some, like the digenetic trematodes, that are
widespread and significant pathogens of human beings and domestic animals.
Finally, there is an argument to be made purely from an aesthetic point
of view. Molluscs are often arrestingly beautiful – if you have ever watched
a cuttlefish or squid hover in the water, watching you watch them, you
know you are viewing one of the pinnacles of evolution’s accomplishments.
To our knowledge, presently there is no initiative underway anywhere in
the biological community to obtain the genomic sequence of a mollusc. The
prospects that a molluscan genome holds for the discovery of novel genes
and as yet unglimpsed biochemical or physiological capabilities are extraordinarily
Molluscs as Model Organisms
Although there is a wealth of information pertaining
to the paleontology and ecology of molluscs, the group as a whole is remarkably
understudied at the molecular level. This is true with respect to the application
of molecular methods to reveal phylogenetic relationships among molluscs,
and the unique developmental and physiological pathways undertaken to produce
their distinctive body plans. When molecular techniques have been applied
to molluscs, the results are often surprising, and provide general enlightenment
that extends well beyond the limited confines of molluscan specialists.
One example of how the study of molluscs has provided broad enlightenment
is in the field of neurobiology. Molluscs like the squid were used early
on to define the basic physiology of axonal conductance and the strong
tradition established in molluscan neurobiology is carried on today in
groundbreaking studies of molluscs like Aplysia and Lymnaea.
Studies of gastropods are providing fundamental insights into how neuronal
plasticity develops, and the underlying molecular and cellular basis of
learning and memory. Cephalopods, like the octopus, are prominent models
for the study of vision and capacity for problem solving in invertebrates.
Another molluscan group that has provided novel biological insights is
the cone snails (Conus). A single species of cone snail can produce
over 100 different toxic peptides and studies of such conotoxins have provided
new insights into a diversity of mechanisms that can be used for post-translational
modification of peptides. The mechanisms used by molluscs to protect their
moist body surface from pathogens is a topic that has barely been approached
but should yield a wealth of new information regarding peptides and other
anti-microbial factors. Gastropods play an essential role in the transmission
of most species of digenetic trematodes, and studies of gastropod-digenean
interactions have done much to enhance our overall understanding of host-parasite
interactions. The evolution of sexuality in response to parasitism has
been effectively studied using a gastropod-digenean system. Because digeneans
typically castrate their molluscan hosts, they provide an excellent model
system to explore the physiological mechanisms exploited by parasites to
re-direct the energy metabolism of their hosts.
Biomphalaria glabrata as a Model Organism
One of the most commonly investigated of all molluscs
is the freshwater gastropod Biomphalaria glabrata. This distinction
is well-deserved because this snail serves as one of the most important
intermediate hosts for a widespread pathogen of humans, the digenetic trematode
mansoni. To a large extent, the geographic distribution of this snail
defines the distribution of S. mansoni in the Western Hemisphere. The snail
is widely distributed on several Caribbean islands, and in extensive areas
of South America, especially in Brazil. Biomphalaria glabrata also
hosts a variety of other digenetic trematodes and has been adopted as the
most commonly used model host to study the basic biology of digenean-snail
interactions. One of the advantages of working with B. glabrata
is that it is easily maintained in the laboratory.
Several studies underway with B. glabrata
highlight its contribution to fields like evolutionary biology, parasitology
or comparative immunobiology. As one example, B. glabrata has been
found to produce after exposure to digeneans a unique family of hemolymph
molecules termed FREPs (fibrinogen-related proteins). FREPs consist of
a unique juxtaposition of fibrinogen and immunoglobulin superfamily domains,
and have proven to be remarkably diverse in their composition. Biomphalaria
glabrata thus serves as a new model system to examine the nature and
diversity of non-self recognition molecules produced by invertebrates.
Other studies with B. glabrata have begun to reveal the phenomena
underlying adherence of invertebrate defense cells to foreign objects,
and the mechanisms used by such cells to kill helminth parasites. A cell
line derived from B. glabrata embryos has been used to support,
for the first time, the complete in vitro development of a digenetic trematode.
The genes responsible to resistance to digenean infection are actively
being sought using B. glabrata. The possible role of transposons
in altering susceptibility of snails to infection is also being actively
pursued using B. glabrata.
Examination of GenBank reveals that more sequence data by far are available
for B. glabrata (approximately 1400 nucleotide sequences) than for
any other mollusc. Collectively, molluscs have only about 8400 entries.
The more this database grows, the more B. glabrata will be used
as a model organism. The genome of B. glabrata is distributed on
18 pairs of small chromosomes and is estimated to be 9.31 x 108
base pairs in size (TR Gregory, University of Guelph, Ontario, Canada),
with a CG content of 46%.
The Rationale for Obtaining the Genome Sequence
for Biomphalaria glabrata
A concerted attempt to obtain the genomic
sequence for B. glabrata would be valuable for several reasons:
1. Genome sequencing
efforts would thus include a member of a large, as yet unrepresented lophotrochozoan
phylum, the Mollusca. The study of molluscan biology will be seriously
impaired if there is not an attempt to provide a genome sequence for a
2. B. glabrata
represents the most abundant class within the Mollusca, the Gastropoda,
a lineage that has been successful over geological time and that remains
3. B. glabrata
has already proven its worth as a valuable model for basic biological studies.
By providing a genome sequence, its value as a model would be further increased,
and the provision of the sequence would greatly assist ongoing studies
of B. glabrata biology. Furthermore, the application of novel techniques
to study proteomes and transcriptomes rely increasingly on the availability
of sequence data.
4. The genome itself
is likely to be interesting for several reasons. B. glabrata is
an exclusively tropical, aquatic, hermaphroditic organism, a combination
of attributes not associated with any animal thus far sequenced. The molluscan
genome will yield interesting insights regarding how a shell is formed,
how the moist molluscan body surface is protected from pathogens, and how
an asymmetrical body plan is produced.
5. There is an active
international community of scholars working with
B. glabrata to
both assist in procurement and analysis of the sequence data, and to use
it in the future.
Coen M. Adema and Eric S. Loker
Obtaining the genome sequence
of the mollusc Biomphalaria glabrata:
A rationale from the standpoint
of infectious disease
Freshwater snails of the genus Biomphalaria
are the intermediate hosts for Schistosoma mansoni, the most widespread
of the three species that cause schistosomiasis in humans. This progressive
and debilitating disease is one of the most intractable public health problems
in many parts of the developing world. By most estimates, up to 10% of
the world’s human population is infected with one of the three major schistosome
species. The decline in public health measures and sanitation, along with
construction of dams and new irrigation schemes, serve to spread the disease
into previously unaffected regions.
The most thoroughly studied snail host for schistosomes
is Biomphalaria glabrata. It is the one most closely associated
with schistosomiasis in the Western hemisphere, but has also been reported
in Egypt, having been introduced there accidentally sometime during the
last few years. Of the schistosome snail hosts, it is the easiest one to
maintain in the laboratory, and the volume of scientific literature on
this species exceeds that of all others combined.
Since this snail species is linked to such an important
human health problem, studies on its genetics have largely been focused
on how this may affect its relationship with S. mansoni. These studies
began in the mid-1950’s, when it was found that susceptibility to infection
S. mansoni is a heritable trait. Since then, we have found out
a great deal about the genetics of parasite/snail compatibility. It is
hoped that by identifying genes and their products that interfere with
parasite survival in the snail, we may develop better methods of controlling
transmission of schistosomiasis to humans. Information on the nature of
those genes involved in the host parasite relationship however is still
rudimentary. Efforts to identify genetic loci using a marker driven positional
mapping approach is now under way, but this method is technically challenging
because little in the way of either genetic/physical or RFLP maps exist
for the B. glabrata genome.
Much of the problem centers around the genome size
of B. glabrata, which is estimated to be 9.31 x 108 base
pairs in size (TR Gregory, University of Guelph, Ontario, Canada). The
chromosomes (haploid number = 18) are small, relatively monomorphic, and
have been organized into groups according to size and shape. To better
understand the molecular make-up of B. glabrata, various gene libraries
(cDNA, genomic, cosmid, BAC) have been constructed, and several laboratories
are actively engaged in gene identification efforts.
It is clear that the entire phylum Mollusca is under-represented
in proportion to its numbers and importance. A recent search of the public
databases revealed the following information (table) on the nucleotide
sequences of molluscs that have been deposited.For B. glabrata several
genes have been sequenced and characterized, and there currently (November
2001) are approximately 1400 ESTs deposited in GenBank.
|Mollusca (all species)
| Biomphalaria sp
| B. glabrata
| Aplysia sp.
| Oncomelania sp.
| Lymnaea sp.
| Bulinus sp.
Compared to the genomic studies of invertebrate vectors
of other parasitic diseases (most notably several mosquito species), molecular
biology studies of these molluscs are considerably less advanced.
The genomes of several invertebrates have now been sequenced and the molecular
make-up of organisms such as Drosophila and Caenorrhabditis elegans
are forthcoming. In the field of tropical medicine sequencing of the malaria
parasite and its vector host Anopheles gambiae are in progress,
and several other genome projects are underway, including that of S.
mansoni. We realize that mapping the genome of B. glabrata would
be one of the larger sequencing efforts to be undertaken in the infectious
disease arena. The molecular information already gathered on this species
however places it in the forefront of molecular studies for any member
of the phylum Mollusca. In the animal kingdom, members of this phylum are
second only to those of the Arthropoda (insects, spiders, crustaceans)
in their numbers and diversity. Information on the genome of B. glabrata
would also have relevance for molluscan species that serve as hosts for
a number of other trematode, and some nematode, infectious agents.
Besides schistosomiasis, diseases such as fascioliasis, clonorchiasis,
and paragonimiasis represent only a few of the snail transmitted diseases
with worldwide medical and economic impact.
Since schistosomes alternate between a vertebrate
and invertebrate host, we believe that ongoing sequencing efforts for S.
mansoni will produce some significant gaps in our knowledge without
having comparable sequence information for its snail host. The degree of
parasite differentiation in the snail is much greater than it is in the
mammalian host, and gene expression by the parasite in snail tissue is
likely more varied.
As evidenced by the success of the human genome project,
and the sequencing efforts of several other complex genomes, the technology
certainly exists for sequencing this species as well. Such an effort will
likely be the most efficient way (in labor and material cost) to advance
our molecular knowledge in such a complex system.
We propose that, for a meaningful start to a genome
project for this organism, our collective efforts could be organized into
2 major phases. The approach proposed is flexible and may be adjusted on
consultation with other investigators.
Phase 1 – Presequencing approaches
Using a multigroup effort, the labor for this could
be partitioned in the following way
- Initiate "gene
discovery" (EST) projects by individual investigator labs
BAC libraries with inserts at least 120Kb with 5- to 6-fold coverage
cDNA libraries from specific regions or tissues of interest for EST-based
- Map new or
already known genes to the BACs so that contigs can be identified
- Map BACs to
Phase 2 - Sequencing approaches
- Employ whole genome
shotgun sequencing, or shotgun sequencing of relevant chromosomes or BACs
In summary, schistosomiasis research has been supported by NIH-NIAID
for roughly 50 years. As a consequence great strides have been made in
understanding the biology of the parasite and, most notably, deciphering
immune components of the disease process in the mammalian host. We feel
the time is appropriate now to sequence the genome of B. glabrata,
thus giving more relevance to the information coming from the sequencing
efforts for S. mansoni, and spearheading the NIH-supported sequencing
movement into an entirely new phylum of medically and economically important
Fred Lewis and Matty Knight