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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.

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.

Last update: January 7, 2016
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