(Credit: Jan-Peter Kasper/EPA/CORBIS)
Often, biologists talk about model systems: organisms that are particularly useful for research. One such organism is the axotol, Ambystoma mexicanum, a cool, but weird salamander:
Because of their large egg and embryo size, susceptibility to tissue grafting, and ability to regrow severed limbs and tails, “axolotls have a long history as primary amphibian models, especially in research areas involving embryonic development,” says Voss. He calls them a “re-emerging model organism” for scientists who study them with gene expression and other new tools. For example, cell and developmental biologist Elly Tanaka of the Center for Regenerative Therapies at the Dresden University of Technology in Germany says her lab was able to develop and breed transgenic axolotls, which “makes it easier to study the mysterious process of regeneration on a molecular level by driving gene expression in regenerating tissues.”
When a salamander regrows its severed tail, it must regenerate a portion of the spinal cord and the neurons inside. “How these particular vertebrates have kept this ability to regenerate while others have lost or blocked it fascinated me,” says Tanaka, who describes the axolotl as “an interesting and important organism for studies on the evolution of vertebrate traits.” Her lab has analyzed signaling pathways that control regeneration, such as “proteins that tell a regenerating cell whether it should form an upper arm or lower arm cells.” In a paper last year in Development, her group shed light on how the axolotl’s neural progenitor cells are activated to help regenerate a segment of spinal cord….
Among the stem cell scientists who use axolotls in their research is Andrew Johnson of the Institute of Genetics at the University of Nottingham in the U.K., who studies the production of primordial germ cells (PGCs) in the salamander’s embryos. “Axolotls are significant in that they share a mechanism that has been conserved during the evolution of mammals, in which PGCs are produced from pluripotent stem cells,” Johnson says. His group is investigating how such stem cells ignore signals that typically trigger somatic cells to differentiate.
But this is the habitat the axolotl is trying to survive in:
Leaning over the Traginera flatboat’s edge, Luis Zambrano surveys a canal floating with plastic bottles, Styrofoam cups, and a leafy carpet of invasive lilies. African tilapia fish ripple the brown water’s surface and a Chinese carp lurks underneath, but Zambrano sees no signs of his elusive goal: the axolotl salamander.
“We’ve spotted only a few in 6 months,” says Zambrano, a freshwater ecologist at the National Autonomous University of Mexico (UNAM) who is trying to count and preserve the feathery-gilled, 33-centimeter-long salamanders in their only natural habitat, the Xochimilco network of polluted canals and small lakes in and around Mexico City, the world’s third largest metropolitan area.
Five hundred years ago, axolotls–named for an Aztec god who transformed into a water animal to avoid being sacrificed–were common in the lakes around the Aztec capital. But as the wetlands receded, so did the axolotls, to the point that Zambrano now estimates a population density of only 100 per square kilometer of wetland, compared with estimates 10 times higher in 2004 and another six times higher than that in the 1980s. The species, Ambystoma mexicanum, is now classified as critically endangered by the International Union for Conservation of Nature.
Another threat faced by the axotol are introduced tilapia. While a small reintroduction/sanctuary program has started, it’s unclear if it will be successful. Even though there are many laboratory colonies, wild populations are critical for research:
Neurobiologist Alejandro Sanchez Alvarado, who studies the molecular basis of regeneration at the University of Utah School of Medicine in Salt Lake City, says losing the wild axolotls would be a tragedy. “Wild-type populations provide us with a window, a record of how biological traits evolve genetically,” he says. “Who is to say that unlocking the evolutionary mystery shrouding regenerative capacities in vertebrates will not come from studying wild axolotl gene pools?”