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How the zebrafish gets its stripes: Uncovering how beautiful color patterns can develop in animals

Max-Planck-Gesellschaft

The zebrafish, a small fresh water fish, owes its name to a striking pattern of blue stripes alternating with golden stripes. Three major pigment cell types, black cells, reflective silvery cells, and yellow cells emerge during growth in the skin of the tiny juvenile fish and arrange as a multilayered mosaic to compose the characteristic colour pattern. While it was known that all three cell types have to interact to form proper stripes, the embryonic origin of the pigment cells that develop the stripes of the adult fish has remained a mystery up to now. Scientists of the Max Planck Institute for Developmental Biology in Tübingen have now discovered how these cells arise and behave to form the ‘zebra’ pattern. Their work may help to understand the development and evolution of the great diversity of striking patterns in the animal world.

New research by Nüsslein-Volhard’s laboratory published in Science shows that the yellow cells undergo dramatic changes in cell shape to tint the stripe pattern of zebrafish. “We were surprised to observe such cell behaviours, as these were totally unexpected from what we knew about colour pattern formation,” says Prateek Mahalwar, first author of the study. The study builds on a previous work from the laboratory, which was published in June this year in Nature Cell Biology (NCB), tracing the cell behaviour of silvery and black cells. Both studies describe diligent experiments to uncover the cellular events during stripe pattern formation. Individual juvenile fish carrying fluorescently labelled pigment cell precursors were imaged every day for up to three weeks to chart out the cellular behaviours. This enabled the scientists to trace the multiplication, migration and spreading of individual cells and their progeny over the entire patterning process of stripe formation in the living and growing animal. “We had to develop a very gentle procedure to be able to observe individual fish repeatedly over long periods of time. So we used a state of the art microscope which allowed us to reduce the adverse effects of fluorescence illumination to a minimum,” says Ajeet Singh, first author of the earlier NCB study.

Surprisingly, the analysis revealed that the three cell types reach the skin by completely different routes: A pluripotent cell population situated at the dorsal side of the embryo gives rise to larval yellow cells, which cover the skin of the embryo. These cells begin to multiply at the onset of metamorphosis when the fish is about two to three weeks old. However, the black and silvery cells come from a small set of stem cells associated with nerve nodes located close to the spinal cord in each segment. The black cells reach the skin migrating along the segmental nerves to appear in the stripe region, whereas the silvery cells pass through the longitudinal cleft that separates the musculature and then multiply and spread in the skin.

A striking observation is that both the silvery and yellow cells are able to switch cell shape and colour, depending on their location. The yellow cells compact to closely cover the dense silvery cells forming the light stripe, colouring it golden, and acquire a loose stellate shape over the black cells of the stripes. The silvery cells thinly spread over the stripe region, giving it a blue tint. They switch shape again at a distance into the dense form to aggregate, forming a new light stripe. These cell behaviours create a series of alternating light and dark stripes. The precise superposition of the dense form of silvery and yellow cells in the light stripe, and the loose silvery and yellow cells superimposed over the black cells in the stripe cause the striking contrast between the golden and blue coloration of the pattern.

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Bumphead parrot fish declare their arrival with a crunch

BY SUSAN MILIUS

The sound of the world’s largest parrot fish swimming toward him, says Douglas McCauley, is not some watery swish, swish. It’s crunch, crunch. “You can hear a school of them before you see it,” he says.

Bumphead parrot fish (Bolbometopon muricatum) grow to “about the size of a junior high school kid” as McCauley puts it.  And feeding is a noisy business because they eat — and loudly digest — what’s essentially rock.

The fish gouge out hunks of reef and snap thumb-sized coral branches. But what McCauley finds even more impressive are the noises of the parrot fish’s down-deep throat teeth, which can grow wider than half dollars, milling the coral chunks.

Crushing coral uncovers what the fish really want: fleshy polyps and other tiny organisms hiding inside. Bumpheads excrete the broken-up coral as gravel and a plume of white sand that “just hovers,” McCauley says, “as if you had opened a carton of milk underwater.”

So prodigious a grinder of coral is the bumphead that more than four tons of coral sediment land on the reef in a year from the excretions of a single parrot fish. Calculating that number required 130 days underwater from McCauley, now at the University of California, Santa Barbara, and a series of helpers carrying syringes the size of turkey basters for collecting excreted coral. On two atolls in the Northern Line Islands in the Pacific, McCauley eased as close as he could to one bumphead at a time and recorded what it ate and pooped as long as he could keep it in sight. On his best day, that was 5.3 hours. If he took his eyes off a fish for just 30 seconds, he could lose it. “I kept a PowerBar in my sleeve,” he says.

His hard-earned data reveal a dilemma for people who love coral reefs. Bumpheads can help corals (for example, by mowing down smothering algae) but in crowds over decades they may reduce the diversity of coral species, McCauley and his colleagues suggest July 26 in Conservation Biology. And the charismatic fish themselves rank as vulnerable to extinction, a conservation conundrum with a terrestrial parallel. The big, loud, wonderful, alarming bumpheads, McCauley says, are “underwater elephants.” 

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