A Shock from the Past: The Evolution of Electric Organs in Weakly Electric Fish

Aditya Uppuluri

Title: A Shock from the Past: The Evolution of Electric Organs in Weakly Electric Fish

Article Title: Genomic Basis for the Convergent Evolution of Electric Organs

Source: Science

Summary: A new study published in Science finds that similar transcriptional factors may explain the evolution of electric organs in different types of fish.

Researchers from Michigan State University, University of Wisconsin, Harvard Medical School, and other institutions find that similar expression of certain transcription factors explains the evolution of electric organs in certain types of fish.

One of the most interesting adaptations seen in nature is the electric organ. The electric organ is an organ that allows fish to produce an electric field that ranges in strength. Electric organs may be used for defense, communication, and perception. Current information suggests that the electric organ evolved in fish six different times. Electric organs are also known as electrocytes. Electrocytes are essentially groups of modified muscle cells. Unlike the muscle cells of other organisms, electricity generating muscle cells do not have a mechanical component; they do not exert a physical force. Electrocytes work by using ion channels to create voltage differences between the inside and outside of the specialized muscle cells. Anatomically, the arrangement of these specialized muscle cells resembles the stacking of batteries in a flashlight. The shape of the electrocyte varies with the species of fish.

The researchers looked at parts of the genomes of five species of electric fish: Electrophorous electricus, Sternopygus macrurus, Eigenmannia virescens, Brienomyrus brachyistius, and Malapterurus electricus. For experimental purposes, the researchers constructed mRNA sequences from the cells of various electric organs and other internal organs such as the kidneys, brain, and heart. Once these sequences were acquired, scientists compared the genes in these sequences to see if any were regulated in a different way. The scientists looked closely at genes that were highly up-regulated or highly down-regulated in the electric organs but not in skeletal and cardiac muscles. The researchers looked at genes responsible for muscle cell differentiation, the presence of structural proteins, voltage-dependent ion channels, and deactivation of the excitation-contraction function of muscles.

When the researchers compared these genes, they found trends in some genes across the electricity generating fish that were studies. Some genes, such as those that deposited collagen in the specialized muscle cell were highly up-regulated the electric fish species even though the species were not closely related. On the other hand, genes associated with the excitation-contraction pathway were highly down regulated in many species of electric fish.

After analyzing the sequences of these five species of electric fish, the researchers concluded that there were common regulator pathways in many types of electric fish. Scientists as far back as Charles Darwin suggested that there must be some sort of convergent evolution that explains the presence of the electric organ in different lineages. This study supports the theory that common networks of gene regulation and expression were selected for during the evolution of the electric organ in fish.

Labels: Electric Organs, Convergent Evolution, Electric Fish, Electrocytes

Citation:

Gallant J. R., L. L. Traeger, J. D. Volkening, H. Moffett, P. Chen, C. D. Novina, G. N. Phillips Jr., R. Anand, G. B. Wells, M. Pinch, R. Guth, G. A Unguez, J. S. Albert, H. H. Zakon, M. P. Samanta, and M. R. Sussman. 2014. Genomic basis for the convergent evolution of electric organs. Science 344:1522-1525.