Nav: Home

How the African striped mouse got its stripes

November 02, 2016

A gene called Alx3 blocks the differentiation of pigment-producing cells in the skin of the African striped mouse, thus generating the mouse's characteristic light-colored stripes, a new study shows. The same gene also determines the stripe pattern of North American chipmunks, and the similar stripe patterns evolved independently in these two groups of animals, researchers report in this week's issue of Nature.

How animal color patterns develop and evolve is a longstanding question in developmental biology. HHMI Investigator Hopi Hoekstra of Harvard University studies the development and evolution of stripe patterns in wild mice because these stripes may play an important role in helping the mice escape predators. In addition, the wild mice are closely related to lab mice, for which a variety of genetic and genomic tools are available. "We are working on a non-model organism that is closely related to a model, so we can get the best of both worlds," Hoekstra explained.

To investigate how the African striped mouse got its stripes, Ricardo Mallarino, a postdoctoral fellow in Hoekstra's lab, first looked at how the stripes form during mouse embryogenesis. He found that the pigment producing cells in the skin, called melanocytes, failed to differentiate in regions that corresponded to the location of the light-colored stripe. The melanocytes that fail to differentiate cannot produce pigment, thus leading to light colored hair.

To figure out which genes were involved, Mallarino and colleagues used RNA sequencing to measure all the genes that were activated in the skin in the light colored stripe compared to those in the skin that grew darker hair. The team identified a gene called Alx3 that was highly expressed in the light colored stripe but not in the dark-haired areas. When they compared the expression of Alx3 in the African striped mouse and in the regular lab mouse, they found that all mice express Alx3 on their bellies, but the African striped mouse also expresses Alx3 in a striped pattern on its back. "It looks like what happened is that Alx3 has been co-opted from its role in making light-colored bellies, and now it's expressed where the stripe is going to form," explained Hoekstra.

The gene had not been implicated in pigmentation before, so the researchers weren't sure how the gene was affecting mouse hair color. To figure that out, the researchers conducted protein-DNA binding assays to identify the sections of DNA to which the ALX3 protein could bind. They discovered that ALX3, a transcription factor, binds to the promoter and represses MITF, a known regulator of melanocyte differentiation.

To further probe the function of the gene, the researchers over- and under-expressed it in lab mice and in cell culture, respectively. Using ultrasound-guided injections, the researchers introduced a lentivirus carrying the Alx3 gene into pregnant mice. The virus entered the developing embryos where it over-expressed Alx3 in the melanocytes. In these embryos, the melanocytes failed to differentiate, leading to a decrease in pigment synthesis. By contrast, in the cells where Alx3 was knocked down using small hairpin RNAs, the researchers found that melanin synthesis increased, leading to an increase in pigment production.

To see if this newly discovered mechanism controlling mouse hair color was a general phenomenon, the researchers turned to the North American chipmunk, which has similar stripes and which is evolutionarily separated from the African striped mouse by 70 million years. Analysis of skin biopsies from chipmunks showed a similar pattern of Alx3 expression. "The same mechanism has repeatedly evolved in two lineages for the same purpose," explained Mallarino.

Looking forward, the researchers would like to understand what controls where Alx3 is expressed, and whether variation in the timing and location of Alx3 expression can explain the evolution of novel color patterns in nature. "The next step is to figure out what controls Alx3 expression," said Mallarino. "Can you just tinker with where and when Alx3 is expressed to generate a diversity of striping patterns? We don't know the answer, but that is something we are excited about," added Hoekstra.
-end-


Howard Hughes Medical Institute

Related Evolution Articles:

Prebiotic evolution: Hairpins help each other out
The evolution of cells and organisms is thought to have been preceded by a phase in which informational molecules like DNA could be replicated selectively.
How to be a winner in the game of evolution
A new study by University of Arizona biologists helps explain why different groups of animals differ dramatically in their number of species, and how this is related to differences in their body forms and ways of life.
The galloping evolution in seahorses
A genome project, comprising six evolutionary biologists from Professor Axel Meyer's research team from Konstanz and researchers from China and Singapore, sequenced and analyzed the genome of the tiger tail seahorse.
Fast evolution affects everyone, everywhere
Rapid evolution of other species happens all around us all the time -- and many of the most extreme examples are associated with human influences.
Landscape evolution and hazards
Landscapes are formed by a combination of uplift and erosion.
New insight into enzyme evolution
How enzymes -- the biological proteins that act as catalysts and help complex reactions occur -- are 'tuned' to work at a particular temperature is described in new research from groups in New Zealand and the UK, including the University of Bristol.
The evolution of Dark-fly
On Nov. 11, 1954, Syuiti Mori turned out the lights on a small group of fruit flies.
A look into the evolution of the eye
A team of researchers, among them a zoologist from the University of Cologne, has succeeded in reconstructing a 160 million year old compound eye of a fossil crustacean found in southeastern France visible.
Is evolution more intelligent than we thought?
Evolution may be more intelligent than we thought, according to a University of Southampton professor.
The evolution of antievolution policies
Organized opposition to the teaching of evolution in public schoolsin the United States began in the 1920s, leading to the famous Scopes Monkey trial.

Related Evolution Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Setbacks
Failure can feel lonely and final. But can we learn from failure, even reframe it, to feel more like a temporary setback? This hour, TED speakers on changing a crushing defeat into a stepping stone. Guests include entrepreneur Leticia Gasca, psychology professor Alison Ledgerwood, astronomer Phil Plait, former professional athlete Charly Haversat, and UPS training manager Jon Bowers.
Now Playing: Science for the People

#524 The Human Network
What does a network of humans look like and how does it work? How does information spread? How do decisions and opinions spread? What gets distorted as it moves through the network and why? This week we dig into the ins and outs of human networks with Matthew Jackson, Professor of Economics at Stanford University and author of the book "The Human Network: How Your Social Position Determines Your Power, Beliefs, and Behaviours".