Mice Are Here to Stay
Total Page:16
File Type:pdf, Size:1020Kb
Mice A re Here to Stay Wherever people live, there are mice. It would be difficult to find another animal that has adapted to the habitats created by humans as well as the house mouse has. It thus seemed obvious to Diethard Tautz at the Max Planck Institute for Evolutionary Biology in Plön that the species would make an ideal model system for investigating how evolution works. BIOLOGY & MEDICINE_Evolution Mice A re Here to Stay TEXT CORNELIA STOLZE he mice at the Max Planck mice themselves only within a range of can be a key factor in the emergence of Institute in Plön live in their 30 to 50 centimeters, they convey high- new species. For Tautz, the house very own house: they have ly complex messages. mouse is a model for the processes of 16 rooms where they can Diethard Tautz and his colleagues evolution: it would be difficult to find form their family clans and have discovered that house mice be- another animal species that lends itself T territories as they see fit. The experi- have naturally only when they live in so well to the study of the genetic ments Tautz and his colleagues carry a familiar environment and interact mechanisms of evolution. out to study such facets as the rodents’ with other members of their species. “Not only is this species extremely communication, behavior and partner- When animals living in the wild are adaptable, as demonstrated by its dis- ships sometimes take months. During captured, they lose their familiar envi- persal all over the globe, but we also this time, the mice are largely left to ronment: everything smells and tastes know its genome better than that of al- their own devices. Humans enter their different, and they are no longer able most any other species,” says Tautz. Af- realm only to clean up and provide to move about freely. And most impor- ter all, scientists have been breeding them with food and water. tantly, they lack contact with the fam- and studying mice in the laboratory for The mouse house in Plön meets its ily – a key element in the lives of these more than a hundred years, observing inhabitant’s requirements fully. After extremely social animals. their behavior and decoding their ge- all, house mice also live in large family nome. Today’s laboratory mice are de- clans in the wild and maintain exten- A MODEL FOR EVOLUTION scendants of the house mouse, but they sive physical contact and communica- are far less genetically diverse than tion with each other. The squeaking fa- The mice in the mouse house, in con- their wild relatives. miliar to humans is only a small excerpt trast, can indulge in their entire reper- The house mouse’s first ancestors of the rodents’ vocal and sound reper- toire of behaviors. This is the only way lived around 500,000 years ago in the toire. Most of their communication oc- the latest evolutionary development of region that is now Iran, where they curs in the ultrasound range, which is the rodents can be studied, as the re- split into subspecies. When humans not perceptible to the human ear. And sults of research carried out in the past settled there around 14,000 years ago while the sounds can be heard by the showed that differences in behavior and started farming, a new age began While the eyes of the house mouse (Mus musculus) tend to be underdeveloped, the rodent’s nose and ears play a very important role in the social life of its species: their vocalizations are so complex that the animals can even develop “dialects.” Such regional differences influence the mice’s choice of reproductive partner. Photo: MPI for Evolutionary Biology 1 | 18 MaxPlanckResearch 57 Mus musculus musculus Mus musculus domesticus America Mus musculus molossinus Original populations Mus musculus domesticus Mus musculus castaneus Mus spretus Mus spicilegus Mus macedonicus Mus musculus domesticus Colonization routes Contact zone between M. m. domesticus and M. m. musculus Additional contact zones for the mice: they couldn’t resist the tures of just under 5 degrees Celsius. To functional genes can form from parts corn fields and storerooms of the first research this diversity, there are mice of the genome within a short period of farmers. It’s hard to imagine a more re- from all over the world living at the time. These genome sections had pre- liable and convenient source of food. Max Planck Institute in Plön: in addi- viously baffled researchers, as they From then on, the house mouse tion to mice from Germany and France, don’t contain any information that conquered the world in the wake of there are conspecifics from the Faroe Is- can be translated into functional pro- humans. As part of this process, it fol- lands, Spain, Austria, Kazakhstan, Iran teins. For this reason, up to 90 percent lowed different dispersal routes: the and even Taiwan. of the mouse genome was classified as eastern European house mouse (Mus junk DNA. However, the Max Planck musculus musculus) populated north- FROM VEGETARIAN working group discovered that these ern Asia and eastern Europe and adapt- TO CARNIVORE originally functionless DNA sequenc- ed to the continental climate there. es could give rise to new genes. “They The western European house mouse When necessary, the house mouse even really are complete reinventions – some- (Mus musculus domesticus) reached adapted its originally vegetarian eating thing that previously would have been western Europe, with its Atlantic cli- habits. The descendants of the former considered impossible,” explains the mate, on Phoenician merchant ships sea-faring mice, for instance, feed on evolutionary biologist. crossing the Mediterranean. Then, insects, worms and larvae. On the is- By genetically analyzing several with the first European seafarers, they land of Helgoland, they even became mouse species, Tautz and his colleagues made it as far as America, Australia, Tai- accustomed to eating dead sea birds. discovered that only a small percent- wan and even remote archipelagoes They are so flexible that, as long as they age of the DNA is translated into pro- such as the Faroe Islands. have a good food supply, mice don’t teins. Nevertheless, almost every DNA With the first European whalers, the even need direct access to water. section is transcribed into an RNA mol- mouse also penetrated almost as far as A few years ago, Diethard Tautz dis- ecule and can thus be a candidate for a Antarctica, populating regions where it covered a possible key to the evolution- new gene. Sections of DNA that are quickly adapted to average tempera- ary success of the house mouse: new, read but not translated into proteins Graphic: TRENDS in Genetics, vol.19, no.1, January 2003 58 MaxPlanckResearch 1 | 18 BIOLOGY & MEDICINE_Evolution Left page Origin and range of the house mouse: Two million years ago, their closest relatives M. spretus (yellow), M. spicilegus (gray) and M. macedonicus (green) separated; today their ranges overlap in different regions of Europe. Half a million years ago, subspecies of the house mouse then emerged in the area that is now India and Iran. The rodents gradually spread across the globe in the wake of the first farmers: the eastern European house mouse (M. m. musculus) migrated to northern Asia and eastern Europe, the southeastern Asian house mouse (M. m. castaneus) to eastern Asia, and the western European house mouse (M. m. domesticus) to western Europe. From there, the western European house mouse spread to all the other continents by ship. The different subspecies encountered each other in different regions of the world and cross-bred with each other (purple). One example of this is a narrow hybrid zone in central Europe (red line). A new hybrid subspecies, M. m. molossinus, emerged in Japan as a result of the contact between the eastern and Asian house mice. Right Diethard Tautz at the demonstration enclosure in the visitors room at the Max Planck Institute for Evolutionary Biology. The mice live here in a highly diverse environ- ment under nearly natural conditions. The actual experiment rooms are fitted with the same structural elements. are thus called precursor genes or pro- togenes. Whether they become genes or not depends on the environment: if an RNA molecule assumes an import- ant function, the coding section of the DNA remains active and is retained. If the RNA molecule fails to find a useful task, the section becomes non-coding DNA again. But new genes also arise through a second mechanism: when a gene for a protein arises from an originally non-coding section due to a change in the DNA reading frame. A reading frame comprises three consecutive let- ters of the genetic alphabet. Each of these triplets stands for an amino acid into which the genetic code is trans- lated. If this reading frame shifts, new triplets arise and the sequence of let- ters can be translated into amino ac- ids. The scientists in Plön identified several genes that were overwritten due to such a change in the reading Graphic: TRENDS in Genetics, vol.19, no.1, January 2003 Photo: Tom Pingel frame. One example is the Hoxa9 gene 1 | 18 MaxPlanckResearch 59 BIOLOGIE & MEDIZIN_xxxxxxx Diethard Tautz and his colleague – a gene that controls embryonic de- nosaurs became extinct, and in the most Christine Pfeifle have completed all the velopment. In rodents and primates, recent lineage leading to mice. preparations. The inhabitants will now this gene uses an additional, alterna- In addition, whenever the house be left undisturbed for months; their behavior is recorded by cameras. tive reading frame of this kind. mouse encountered a new habitat over The rooms are equipped with wood The researchers suspect that genes the course of evolution, new popula- shavings for burrowing and digging, frequently form anew.