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EVOLUTION
The Building Blocks
of Evolution
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developmental biology and evolutionary biology. The researchers in RALF SOMMER’S
department at the MAX PLANCK INSTITUTE
FOR
DEVELOPMENTAL BIOLOGY
in Tübingen hope to identify the molecular foundations of evolution.
MANY PATHS LEAD
TO ONE GOAL
However, under normal conditions,
only three of these cells form the
actual vulva tissue – and they do so
in response to a signal generated by
the so-called anchor cell (AC). If
these cells are removed, the remaining three move into the center and
take on this task. However, if the
anchor cell is removed, then vulva
formation does not occur; the cells
remain simple skin cells. “All of this
takes place in one plane, allowing
the entire process to be observed
through the microscope without
having to continuously adjust the
focus,” says Sommer. And the researchers now also know which
signal molecules are involved. Nevertheless, there is an astounding
redundancy at the molecular level:
different signaling pathways with
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as it is not yet clear what findings might reveal a link between the fields of ecology,
Sommer began studying the development of this one-millimeter
long nematode, or rather that of its
egg-laying apparatus, while he was
still a post doc. Using laser microsurgery, the researchers can remove
one or more of a cell’s neighbors
during the developmental process
and then directly observe how the
remaining cell behaves under the
altered conditions. That is how they
know that, of the twelve cells that
form the ventral epidermis (the skin
in the abdominal region), six play a
special role: they are all capable of
participating in the formation of the
vulva – the technical term for the
egg-laying apparatus.
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hese are possibly the most interesting questions in developmental biology: How is it possible for a
single cell, the egg cell, to develop
into a complex organism? How does
a cell know what kind of tissue it is
associated with? What ensures that
certain organs appear only in certain
positions, such as the eyes always on
the head and not at the other end of
the body? What mechanisms play a
role in morphogenesis?
It is hoped that a small nematode
will help in finding the answers to
these questions. With its manageable 959 cells, Caenorhabditis elegans has become a favorite subject
of developmental biologists in recent decades – and there is a very
simple reason for this: the researchers can follow the fate of its every
cell under the microscope. Their cell
development is fixed in such a way
that, after the first cell divisions, it
is already possible to say which of
the two, four or eight cells will later
develop into the digestive tract or
the reproductive system.
Its genome sequence is likewise
known: in the 1990s, C. elegans was
the first multicellular organism to
have its genome completely sequenced. And a large number of mutants were analyzed to determine the
function of the individual genes. “If
there is one animal species whose
cellular development we understand
fairly well at the genetic and molecular level, it is this nematode,” says
Ralf Sommer, Director at the Max
Planck Institute for Developmental
Biology in Tübingen.
D EVELOPMENTAL B IOLOGY – J ÜRGEN B ERGER
T
Their research focuses on nematodes, and their approach is highly interdisciplinary,
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Scanning electron microscope
images of Caenorhabditis elegans (left)
and Pristionchus pacificus (right). The
insets show the respective egg-laying
structures at high magnification.
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EVOLUTION
EVOLUTION IS
CONSERVATIVE
The neo-Darwinists still assumed
that genes evolve quickly – in other
words, genes change through mutations, and the offspring retain the
D EVELOPMENTAL B IOLOGY
“We know that parasitism has occurred independently at least eight
different times in nematodes,” says
Sommer and, at the same time, points
out that the group of nematodes is
older than that of tetrapods, which
also includes humans. Over the
course of evolution, the nematodes
have evolved into more than a million species, making them the largest
phylum on Earth.
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mutations if they are associated with
certain advantages. This is known as
positive selection. However, due to
the high “innovation rate,” it would
not be expected that distantly related
organisms would still have the same
– that is, homologous – genes. “In
reality, though, what we have is primarily negative selection,” says
Sommer, “everything that works is
conserved, and what doesn’t work is
discarded.” In light of the broad diversification of anatomy and physiology that developed in the organisms over the course of their
evolution, this conservation is almost paradoxical. How can there be
diversification in spite of this conservation? How, then, the biologists
wonder, do biological diversity and
variability come about?
A Lego set may offer an appropriate analogy. The colorful but otherwise homogeneous-looking building
blocks can be put together in many
different ways due to their simple,
uniform fit, making it possible to
construct even quite complex objects. It works much the same in biology: In one organism, certain
genes can be expressed in very different regions of the body and used
MOLECULAR BIOLOGY
WITH A LOT OF FREEDOM
A comparative study of vulva formation between C. elegans and P.
pacificus uncovers surprising details:
The cell biographies of P. pacificus
have changed. Some cells are simply
obliterated by programmed cell
death, while others have lost the
ability to develop in a certain direction. In view of these variations, the
question is whether the underlying
molecular processes have changed,
too. And in fact they have: genes
were duplicated and integrated into
other genetic networks, and certain
signaling pathways now perform
opposing roles. While in C. elegans
they promote, for instance, the formation of the egg-laying apparatus,
in P. pacificus they inhibit it. “The
formation of morphological structures and the molecular processes
that are responsible for this are surprisingly strongly decoupled,” says
Ralf Sommer.
What does that mean? Nothing
less than that the molecular mechanisms of developmental processes
MPI
different molecules are involved, in
parallel, in the formation of a single
structure. The researchers are still
trying to determine why that is.
In the lab next door, under the
guidance of Christiane NüssleinVolhard, researchers study the fundamental principles of animal development, at first using the fruit
fly, and now the zebra fish, a vertebrate. Worm, fly, fish – all of these
organisms differ significantly in
their appearance, and one would
hardly assume that the same genes
and molecules were at work in them
– at least no more than, say, if one
were to compare the techniques of
manufacturing a shoe and a bicycle.
The great surprise of the past 10 or
20 years was that much of the basic
machinery is identical. The developmental processes and the functional
components on which they are
based were conserved over the
course of evolution.
the Genome Sequencing Center of
Washington University in St. Louis.
With 160 to 170 megabases (a megabase corresponds to one million letters in the genetic alphabet), the genome of P. pacificus is larger than
that of C. elegans (100 MB) and, with
29,000 genes, also has more genes
than its relative, which has just
19,000. The genome contains sequences that are highly similar to
genes that are typical for plant-parasitic nematodes.
BASED ON MATERIAL FROM THE
Ralf Sommer can follow the cell division processes of P. pacificus under the microscope.
In the microscope images on the right, the precursor cells are marked red and blue. Their division
(bottom image) is triggered by an induction signal from the cells of the genital system (green).
in different functional contexts. Other target molecules in the subsequent
signaling pathways then come under
the influence of this gene. In evolution, too, novel features in anatomy,
physiology and behavior occur as a
result of using existing components
in new combinations at different
times, in different places and to
varying degrees.
To obtain information on this type
of evolutionary transformation, we
must compare species that are closely related. “This cannot be studied
across distantly related phyla with
significant morphological differences,” says Ralf Sommer. The developmental biologist is convinced that
“The only way to make progress is to
compare organisms that exhibit key
differences in morphology and developmental biology, but that are
still so similar that the common origin of the structures to be examined
is beyond dispute.”
That is why, more than 10 years
ago, Sommer and his colleagues began to establish a further nematode
species as a satellite system for
comparison with C. elegans: Pristionchus pacificus. To the untrained
eye, the two nematodes look very
similar. In fact, however, their developmental lines diverged as far
back as some 280 to 450 million
years ago. Sommer prefers not to
narrow down the timeframe – the
molecular clock the researchers use
to determine the date is too imprecise. How different are the genomes
of two nematode species that have
been separate for so long?
The entire genome sequence of
Pristionchus pacificus has been
available since 2006. The sequencing
project was carried out on behalf of
the Max Planck Society and the US
National Institutes of Health (NIH) at
seem to have an enormous degree of
freedom. Apparently it can change
so drastically during evolution that
even homologous structures of closely related species are regulated by
entirely different molecular mechanisms. “This inevitably raises the
question of which selection mechanisms are responsible for these differences,” says the biologist. And in
order to answer this question, the researchers must analyze the nematodes’ ecology – how they live in the
wild – more precisely. This is because
every form of adaptation is a result
of the environmental conditions under which the animals live.
C. elegans prefers compost piles.
Although this is actually a manmade habitat and only came into existence at most 5,000 years ago, it is
easy to imagine that this small soil
nematode occurs naturally in a similar habitat. P. pacificus can certainly
be found in labs, where it is used as
a test animal, but until recently it
was hardly known where the worms
of this genus can be found in nature.
They were occasionally found on
beetles, “but the experts assumed
that the worm used the beetle merely
as a shuttle to get from one habitat
to another,” says Sommer. Yet, it was
the only clue the scientists had.
However, since beetles are not
exactly the developmental biologist’s
profession, Sommer found himself
a specialist: entomologist Matthias
Herrmann.
A NEMATODE WITH A
WEAKNESS FOR BEETLES
F IG .: C HRISTOPH S CHNEIDER ,
P HOTOS : MPI
FOR
D EVELOPMENTAL B IOLOGY (2)
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And he succeeded – initially right on
his own doorstep in Tübingen. Here,
as many as 70 percent of all dung
beetles are infested with nematodes
of the genus Pristionchus. “Our work
to date shows that representatives of
The lifecycle of the nematode P. pacificus begins with a first
cell division (top left). In the so-called bean stage (right),
all 558 cells of the embryo are present. Length growth occurs in
the juvenile stages (J1-J4). The mature worm has 959 cells.
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EVOLUTION
F IG .: C HRISTOPH S CHNEIDER , BASED ON MATERIAL FROM
THE MPI FOR D EVELOPMENTAL B IOLOGY – R AY H ONG
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Different Pristionchus species prefer different beetle species as their hosts. P. maupasi, for example, lives on cockchafers
(Melolontha melolontha), while P. uniformis prefers the potato beetle (Leptinotarsa decemlineata). The host is identified
through certain scents, including attractants that the beetles use for communicating with their conspecifics.
ADAPTING TO NEW
ENVIRONMENTS
also be possible, in the end, to understand macroevolution.
Matthias Herrmann examined
more than 4,000 beetles in various
countries in Western Europe and
found nematodes of the genus Pristionchus on more than half of them.
The European relatives of P. pacificus, P. maupasi and P. entomophagus, for example, live in cockchafers
(Melolontha melolontha) and dung
beetles (Geotrupes stercocarius); P.
uniformis prefers the potato beetle
(Leptinotarsa decemlineata). They all
have one thing in common: every
Pristionchus species has a preference
for a particular beetle species.
The search for further worm-beetle
associations also took Herrmann to
both America and Asia. The lab organism P. pacificus itself is the only
cosmopolitan species of its genus.
The researchers found it on various
scarab beetles, especially on the
oriental beetle (Exomala orientalis).
The question the scientists now raise
is: How do the blind nematodes find
their host beetles? Their ability to
This lifestyle we see with Pristionchus is known as necromeny. It requires a number of adaptations, as
the organism must survive in what is
actually, for it, a hostile habitat. It
must avoid the defense mechanisms
(such as toxins, enzymes or specific
defense cells) of its host. “Based on
the increase in genes in certain gene
groups, we can recognize, for instance, that the worm’s detoxification machinery has been powered
up,” explains Sommer. He hopes that
the necromenic lifestyle of the small
nematode will provide some insight
into the preliminary stages of parasitism. If they were to succeed in understanding the many small microevolutionary steps that occur when
adapting to a new environment (in
this case to the beetle), then it might
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to a source of food or to a sexual
partner. Such substances are particularly well suited as a recognition
feature, since they are species specific. Moreover, the worms recognize the scents that plants release
when attacked by herbivores, such
as the potato beetle. These scent
signals are actually intended to inform food-seeking predators about
the activity, occurrence and type of
pest – but in the end, they also tell
the worms that their host beetles
can be found on those plants.
P HOTO : MPI - M ATTHIAS H ERRMAN
THE SEARCH
INNOVATION
A bucket full of beetles (here June beetles) await examination
in the lab. At first glance, there is no way to tell whether these
dead beetles harbor nematodes of the genus Pristionchus. The
researchers will not know this until later in the decay process.
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follow a specific scent trail – what
scientists refer to as chemotaxis –
proves to be the key to greater understanding.
In a chemotaxis assay, Ray Hong,
a post doc in Sommer’s lab, offered
the nematodes various scents in a
petri dish and then measured how
long it took them to seek out the
relevant source. It turned out that
the creatures react specifically to
beetle attractants – in other words,
pheromones or sexual pheromones
that direct the beetles’ conspecifics
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Based on this assay, the researchers
in Tübingen were able to compile a
specific chemotaxis profile for each
species of Pristionchus. Comparing it
with C. elegans shows that the latter
reacts not only to entirely different
scents, but also to different concentrations and at a different speed. The
majority of the worms reached the
scent sources within an hour. With
P. pacificus, the researchers had to
exercise more patience: it took at
least two to three hours before the
bulk of the worms reached the source
of the scent, and in two cases, it even
took more than nine hours.
Nematodes do not have a complex olfactory organ, but merely a
P HOTO : MPI - M ATTHIAS H ERRMAN
this genus live as dauer larvae in the
beetles, but do not harm them in any
way. Rather, the worms wait for the
beetle’s death, whether due to natural causes or mycosis, and then feed
on the microorganisms that colonize
the rotting beetle during decay,” explains Herrmann. In the lab, the beetle specialist has to wait about a
week after killing the insects before
the mature worms appear.
P. pacificus prefers the oriental beetle (Exomala orientalis) as its host, while P. entomophagus seeks
out dung beetles (Geotrupes stercocarius). While P. pacificus responds particularly to long-chain fatty
acid esters, such as EDTA and myristate, P. entomophagus is attracted primarily by isopentylamines.
few olfactory neurons hidden in its
skin. The fact that P. pacificus does
not like any of the scents that C. elegans prefers suggests that the olfactory neurons differ significantly
between the species. “It is interesting that we find substantially fewer
genes for olfactory receptors in
the genome of P. pacificus than in
the genome of C. elegans,” says Ralf
Sommer. That is why the scientists
in the Department of Evolutionary
Biology are also studying the neuronal changes that took place when
the worms began specializing in
beetles as their hosts.
“We now have 22 different Pristionchus species in the lab, and more
than 80 P. pacificus strains that we
can use to study the effects of microevolution,” says Sommer. “In the
ideal case, just a few molecular
changes will result in a new phenotype. Ultimately, we are looking for
the phenotypically innovative steps.”
So what kind of novelty could be
stored in an organism and stabilized
through mutation to generate new
structures, new physiological functions or new behaviors? Some 150
years after Charles Darwin’s pioneering work ON THE ORIGIN OF SPECIES,
evolutionary biologists still have
some very tough nuts to crack.
CHRISTINA BECK
Entomologist Matthias Herrmann and his
dip net have traveled all over the world to
capture beetles. His main focus is on the
species that P. pacificus prefers as its host.
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