The electric pulses emitted by electric fish can be quite variable in their duration: and as it turns out the reason can be quite “shocking.” Jason Gallant, assistant professor in Michigan State University’s Department of Integrative Biology in the College of Natural Science, has received a three-year, $680,000 National Science Foundation, or NSF, grant to continue work on a discovery that this variation may be due to unusual changes in a common protein called a potassium channel.
Gallant and members of his lab are broadly interested in the origin and diversity of unusual phenotypic and behavioral traits involved in animal communication signals.
“In this project, we will explore how genetic changes contribute to the evolution of new animal signaling behaviors, specifically in weakly electric fishes from Africa known as mormyrids,” Gallant said. “These fishes use electricity not only to navigate through their environments but also to communicate with one another. Evidence suggests that when these fish develop new electric signals, these signals somehow facilitate speciation.”
To generate electric pulses, the fish need both a sodium channel and a potassium channel. In earlier research, Gallant and his collaborators at the University of Texas, Austin noticed mormyrids have two copies of a potassium channel that most vertebrates, including humans, have only one of.
“What’s interesting about this phenomenon is that the potassium channel in the electric organ accumulates mutations, which then change the function of the potassium channel,” Gallant said. “Normally, the potassium and sodium channels in every other vertebrate are highly constrained to function very particularly. If you get one wrong amino acid, you get problems like seizures or the inability to move.”
However, electric fish use these redundant potassium channels to make electric signals, an ability unique only to a handful of fish species. Evolution has acted on these channels by altering specific amino acids to operate much more rapidly than they normally work in the nervous system or muscles of humans. The end result is the ability to produce extremely brief pulses of electricity. This discovery is helping advance the understanding of the physics of ion channels work.
“When we look at the electric fish, we see a really strange substitution that actually teaches us about the physical properties of the ion channels themselves,” Gallant said. “As a result, we are actually able to understand better how the charge on these particular amino acids contributes to the overall speed of the channel. This can be really important for developing drugs and for understanding how particular mutations contribute to diseases called channelopathies—diseases such as certain types of seizures or paralysis that affect ion channels, which can be very serious for motor and nervous system function.”
After discovering how electric fish accomplish these brief pulses, Gallant and his lab are turning to understand how some electric signals become very long. Despite the strong selection on mormyrids for brevity early in their evolution, electric pulse duration varies widely across the more than 200 species of mormyrids: long duration EOD pulses have evolved at least three times from short-duration EOD pulse ancestors. Gallant’s hypothesis is that these long-duration EODs are the result of additional mutations in the same potassium channelgene. This is the hypothesis that will be pursued under this new NSF grant.
“In this project, we will test this hypothesis by capturing some of these species in Gabon with very long electric pulses. We’ll then bring them back to the lab here in Michigan, and sequence their potassium channel genes, and also evaluate how they physically function. The last step will be to examine which factors contribute to the evolution of long-duration EODs in the first place: for that, we will be conducting high-tech behavioral studies to ask whether these pulses allow fishes to find food differently or attract mates more effectively,” Gallant said.
This multi-year investigation will provide a unique opportunity to understand the connection between genotypes (a particular set of genes carried by an individual) and phenotypes (the individual’s physical characteristics). This is possible because a change in a single ion channel gene actually leads to a direct change in the behavior of the animal: in this case the behavior can be measured with just two electrodes, making the enterprise much simpler.
“A counterexample might help explain the value of this study based on the research we’ve already done,” Gallant said. “Say that you were studying birds and asserted that a gene mutation affected bird song. You would have to know every step along the nervous system circuit and how every muscle along the way contributes to some change in that bird’s song. Measuring a change in the ion channel is comparatively simpler. The story we are hoping to tell with this project is that we will identify mutations that cause alterations in the genome that lead to consequential changes in the phenotype of the animal.
“Additionally, while we’re in the field, we’re going to explore the factors that influence this evolution, and there are numerous possibilities,” Gallant added. “One is that having a longer signal may allow a fish to catch bigger prey. Another might be that longer signals are sexier to males or females, so selection based on those channels include mutations that make those discharges longer. We’ll be sorting all of this out in the field.”