Japanese flounder. (Credit: Songlin Chen, Yellow Sea Fisheries Research Institute, CAFS, Key Laboratory for Sustainable Development of Marine Fisheries, Ministry of Agriculture, Quingdao, China.)


New research by an international team featuring a Texas A&M University-affiliated biologist may net new profit for flounder producers and progress for the broader aquaculture industry, in addition to potential insight into vertebrate body shapes.

Recent findings concerning newly discovered intermediate fish fossils showing partial asymmetry, in combination with the genome of the Japanese flounder (Paralichthys olivaceus) recently sequenced by a team led by Texas A&M Institute for Advanced Study (TIAS) Faculty Fellow Dr. Manfred Schartl, is shedding new light on the current understanding of flatfish evolution -- in particular, how their asymmetric body is formed during metamorphosis.

Schartl, professor and head of the Department of Physiological Chemistry at the University of Würzburg, Germany, and a visiting professor in the Texas A&M Department of Biology, is a co-principal investigator for the Chinese Ministry of Agriculture-based team that worked jointly with researchers across the United States, Norway, Japan, Portugal and Germany to analyze the Japanese flounder genome and related information. By identifying the genetic underpinning of this unique developmental process, the team hopes to answer lingering questions about the evolutionary origin of asymmetry and also provide insight into the mechanisms that control body shape in vertebrates.

Specifically, the team compared the Japanese flounder's genomic analysis with that of a distantly related flatfish they previously had sequenced. By cross-referencing the two genomes with the genes exhibited during metamorphosis, they detected a central role for a key developmental trigger responsible for regulating the flounder's trademark craniofacial asymmetry: retinoic acid, which not only establishes the asymmetry in skin pigment, but also interacts with another factor, thyroid hormone, to guide the process of eye migration.

The team's findings are published online today (Dec. 5) in Nature Genetics.

"Experiments by manipulating these pathways during metamorphosis demonstrated a critical involvement of retinoic acid in establishing the flounder's asymmetric pigmentation and, via molecular cross-talk with thyroid hormones, in modulating the process of eye migration," said Schartl, a world leader in cellular and molecular biology of Xiphophorus model systems including platyfish and swordtails and a senior author on the team's paper -- the bulk of which was written during his TIAS-related time at Texas A&M last winter. "These signaling molecules and the regulatory networks they activate are well known from other vertebrates and also govern the heavy changes in flatfish development."

Beyond narrowing down the catalyst for eye migration, Schartl said the team was surprised also to find an unexpected cause for pigmentation irregularities. Remarkably, the same visual pigments that capture light in the eye are expressed in the skin of the flounder larvae, where they sense differences in illumination and respond by generating retinoic acid gradients that serve as the underpinning for asymmetry generation.

"Flounders are highly priced food fish," Schartl said. "Failures in metamorphosis are a frequent problem in flounder aquaculture, accounting for many millions of dollars of losses in production. Understanding how these unique creatures develop may help the industry, in addition to illuminating a longstanding evolutionary puzzle."

Schartl notes that although flatfishes have the most extreme asymmetric body morphology of vertebrates as adults, they start out their life fully symmetrical like any other fish. However, during a spectacular metamorphosis, the symmetric larvae are transformed into asymmetric juveniles living on the seabed, at which point both eyes end up on the same side of the head and the downward-facing side of the fish loses its skin pigment. At the same time, they switch from living and feeding in open water to doing so on the seabed -- a transformation that demands radical changes in both physiology and behavior.

"The origin of flatfish asymmetry is one of the most extraordinary anatomical specializations among vertebrates, and its evolutionary origin has intrigued scientists for over a century," Schartl said. "Charles Darwin was at a loss to explain the 'remarkable peculiarity' of flatfish anatomy. He remained puzzled -- how did these asymmetric fish evolve from a symmetric ancestor?"

As a 2015-16 TIAS Faculty Fellow, Schartl continues to collaborate with Texas A&M biologist Dr. Gil Rosenthal, an expert in swordtail fish evolution and mating behavior, along with faculty, research scientists and graduate students in the Texas A&M Health Science Center and Texas A&M College of Veterinary Medicine and Biomedical Sciences. A member of the National Academy of Sciences of Germany and a recipient of the Heisenberg Award from the German Research Foundation, Schartl is globally renowned for his career work explaining the molecular-genetic basis of cancer formation using non-mammalian models and for translating basic evolutionary research into discoveries with clear and direct impacts on human health. He has elucidated key mechanisms in swordfish whereby genetic variation generates variation in physical traits and also addressed important questions surrounding the development of melanoma in interspecific hybrids while taking a lead role in sequencing the first genome in the genus.

The team's paper, The genome and transcriptome of Japanese flounder provide insights into flatfish asymmetry, can be viewed online along with related figures and captions.

To learn more about Schartl's research, visit http://www.uni-wuerzburg.de/?id=42039.

To learn more about the Texas A&M Institute for Advanced Study, go to http://tias.tamu.edu.

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Contact: Shana K. Hutchins, (979) 862-1237 or shutchins@science.tamu.edu or Dr. Manfred Schartl, 0931-31-84148 or phch1@biozentrum.uni-wuerzburg.de

Hutchins Shana

  • Symmetric larva, just before metamorphosis (above), compared to larva at early metamorphosis (below), the stage at which one eye started to move to the flounder's other side. (Credit: Songlin Chen, Yellow Sea Fisheries Research Institute, CAFS, Key Laboratory for Sustainable Development of Marine Fisheries, Ministry of Agriculture, Quingdao, China.)

  • Dr. Manfred Schartl

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