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RECENT
SPOTLIGHTS:
Why spiders have eight legs
 Every
school kid can tell you the difference between
an insect and a spider: insects have six legs
and spiders have eight legs. But why? Or in
other words: what are the genetic differences
between insects and spiders that make six legs
in the insect body plan, but eight legs in the
spider body plan? The Hox genes are known to
shape the body plan in insects. Thus, they are
likely candidates for shaping the spider body
plan as well. Indeed, we were able to show that
the Hox genes are involved in making a normal
spider. But we were surprised to find that they
were not doing this like we thought they would.
Legs in insects are mainly specified by the Hox
gene Antennapedia.
However, in spiders this gene is not even
expressed in its eight legs. And when we removed
the function of Antennapedia in our spider
model Parasteatoda
tepidariorum
we obtained a striking change of the spider body
plan: instead of eight legs the spiders now had
ten legs (see figure of a spider larva with
impaired Antennapedia
function). Even more surprising, when we removed
another Hox gene, Ultrabithorax, along with Antennapedia
then the resulting spider larvae even had
another pair of small legs, thus we obtained
spiders with (almost) twelve legs. Thus, instead
of promoting leg growth Antennapedia
represses legs in spiders and is responsible for
the spider body plan with "only" eight legs. It
is obvious that Antennapedia in insects and in
spiders works in different ways, and in our
future experiments we would like to address
these differences in more detail. This work has
been published in Proceedings of
the National Academy of Sciences.
Media coverage of this study: Universität
Jena | VBIO
| Ostthüringer
Zeitung | bionity
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Spider Distal-less
and the evolution of the mandible
 Most insects and
crustaceans have it: the mandible. The mandible
is an appendage on the head and serves as the
"jaw" useful for manipulating and chewing solid
food. Thus, the "invention" of the mandible
appendage was a very important step in arthropod
evolution, because it allowed new modes of
feeding using new kinds of food.
We know
how insects and crustaceans form their mandible:
they "shut down" a gene called Distal-less
(short: Dll)
in the body segment that will form the mandibles
(mandibular segment). But what about arthropods
that do not have a mandible? Spiders, for
example, do not have mandibles. Their segment
that corresponds to the mandibular segment is
the body segment that bears the first pair of
walking legs. We were able to show that shutting
the Dll
gene down in this segment of spiders does not
lead to the formation of mandibles. By contrast,
shutting down the Dll gene eliminated the whole
first walking leg segment and resulted in
spiders with only six legs (see figure: the
animal at the top has fully functional Dll,
whereas we removed Dll function from the first
walking leg segment in the animal at the
bottom). Thus, our results suggest that in
contrast to insects and crustaceans, spiders
were not able to evolve a mandible, because they
could not shut down their Dll gene
without losing a full body segment. We think
that this inability to evolve a mandible forced
the spiders to evolve a totally new way of
feeding: external digestion. This is a
remarkable example for how evolution always
finds a solution, even if some evolutionary
paths are blocked by developmental or genetic
constraints. This work has been published in PLoS Genetics.
Leg joints: a key innovation of the arthropods
 Primitive life
forms moved simply by contractions of their
body, similar to worms or slugs. The evolution
of legs was thus a huge leap forward
(literally!) to exploit new ecological niches
and explore new modes of life. Early legs were
simple outgrowths only capable of slow crawling,
but then a new invention introduced fast and
effective movements: the jointed leg. Actually,
this invention is so "ingenious" that it evolved
several times independently, for example in the
arthropods and in the vertebrates (yes, the
human leg is jointed too!).
The fact that leg joints evolved several times
in different animal groups sparked the idea that
leg joints could have evolved several times even
within a given animal group. Some scientists
therefore thought that leg joints evolved
several times in the arthropods. How leg joints
are made is well-known in the fruit fly Drosophila
melanogaster.
If other arthropods have evolved their leg
joints by themselves, then there should be
almost no similarities between the joint-making
mechanisms in these arthropods and the fly Drosophila.
We have therefore studied the making of leg
joints in spiders and have found that
joint-making in the spider and the fly uses much
the same genes (see some examples in the figure)
and much the same mechanisms. Thus, we propose
that leg joints were invented only once in the
arthropods and are therefore one of the key
innovations of this animal group. This work has
been published in Developmental
Biology.
The legs of velvet worms: precursors of
arthropod legs?
 Velvet
worms are peculiar animals: neither fully worms
nor fully arthropods they are something
in-between and might be called a "missing link"
or "living fossil". Similar to arthropods they
have legs, but these are not jointed. As
explained above, the origin of a joint-making
genetic mechanism was among the key events in
arthropod evolution and thus velvet worms appear
to be at an evolutionary stage before the origin
of true arthropods.
Surprisingly, we were able to show that certain
parts of the joint-making mechanism of
arthropods are already present in velvet worms
(see figure showing the expression of the gene extradenticle
in a velvet worm embryo). Thus, parts of the
joint-making mechanism are active in the velvet
worm legs, even though these legs do not have
joints. Thus, we propose that these mechanisms
first evolved in velvet worms for a different
function and were then re-used and brought to
perfection in the arthropods. This is an
interesting example for how complex genetic
mechanisms can evolve part by part by having
different functions at different stages of their
evolution. This work has been published in Evolution &
Development.
Wnt genes: Ancient
landscapes of an old gene family
 Where´s
the head, and where´s the tail?
Where´s left, right, and where do I place
the organs? For a developing embryo these are
very important questions. The so-called Wnt
genes are helping the embryo making the right
decisions regarding all of these questions. Wnt
genes are therefore known from all animals- from
humans and mice to flies, worms and sea
anemones. Given their importance for embryonic
development and their function in so many
processes it is hardly surprising to find that
there can be over 10 different Wnt genes in a
species (the figure shows the evolutionary tree
of these Wnt genes). Surprisingly, most of these
genes are expressed in very similar patterns,
suggesting that they are more or less all the
same. Together with our collaborators we have
studied the Wnt genes from spiders, beetles,
millipedes and segmented worms and found that
they are not all the same, but rather they
appear to combine their action to achieve their
many functions. Thus, if a species has e.g. 10
different Wnt genes these genes cannot only
serve 10 different functions, but by combining
their functions in all possible combinations
they could theoretically serve 10 to the power
of 10 functions. This is what we have termed
"Wnt landscapes" and each combination of Wnt
genes in these landscapes has its own special
role in development. This work has been
published in BMC Evolutionary
Biology.
Segmentation in diplopod myriapods: derived
peculiarity or precursor of the insect pair-rule
system?
 Usually
a body segment of an arthropod can bear only a
single pair of legs. Diplopods are different:
most of their trunk segments bear two pairs of
legs- hence the name "diplopods". This so-called
"diplosegmentation" was long regarded as a
peculiarity of this arthropod group with no
relevance for the question of the origin of
segmentation. This changed when studies of
segmentation in the fruit fly Drosophila
melanogaster
revealed that the segments were genetically
produced in pairs ("pair-rule system").
Diplosegmentation and the pair-rule system could
thus trace from a common ancestral segmentation
mechanism. We have studied segmentation gene
expression in the pill-millipede (the figure
shows a pill-millipede embryo stained for the
genes engrailed
and pairberry
and a summary of segmental gene expression in
the pill millipede, the flour beetle and the
fruit fly). Our results indicate that
segmentation in the millipedes is derived and
shows conserved mechanisms on the ventral side,
but millipede-specific mechanisms on the dorsal
side. These millipede-specific mechanism lead to
diplosegmentation, and thus our results imply
that diplosegmentation is not related to the
insect pair-rule system, but has newly evolved
in the millipedes. This work has been published
in EvoDevo.
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