Credit: Carl D. Hopkins
In what seems like a freshwater version of playing hard to get, a peculiar species of electric fish in Gabon's Ivindo River Basin may provide a rare snapshot of the evolutionary divergence of one species into two.
Researchers from Cornell University found that genetically identical fish are sending out two different electric signals, and certain male members of the species ignore some signals emitted by females, responding only to pulses similar to their own. The strict selectivity for specific signals observed in these electric fish may eventually result in different mating groups, leading researchers to surmise that the fish could be on the verge of speciation.
"Evolution is a historical, inferential science—you can't really see it happening before your eyes," said Matt Arnegard, a neurobiology and behavior postdoc at Cornell and lead author of the study, which appears in the June issue of The Journal of Experimental Biology. "We think maybe this is an example where we're really close to seeing it happen before our eyes."
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Arnegard and his mentor, professor Carl Hopkins, caught male fish and studied their responses to electrodes emitting both types of signals that could come from prospective mates. Some males responded to both type I and type II female signals, while others ignored type I female signals but vigorously attacked the electrode that emitted the type II signal. The researchers found that the electric signal receptors of all the male fish generated different responses to each signal, indicating that they could distinguish them.
Surprisingly, when Arnegard compared the DNA sequences between the selective and non-selective fish, he found they were mostly the same.
"We came across this thing that really violates that pattern in all other electric fish that we know of," Arnegard said. "One hypothesis is they're so close to when they form two species that the genomes are largely identical."
According to Peter Dijkstra, a graduate researcher in behavioral biology at the University of Groningen in the Netherlands, it would be difficult to determine if these electric fish are actually on the verge of speciation. Instead, he said, the distinct types could be simply due to different reproductive tactics or may be the result of two previously separate species that merged into one but still exhibit features of both ancestral species.
Arnegard hopes to solve that mystery. He is heading back to Africa to more closely examine the breeding conditions and courtship behaviors of the fish, and he also plans to be involved in sequencing a gene that functions in producing the electric signals. Different DNA sequences between the two types would provide more evidence that the species is diverging.
"There's going to be a lot more surprises in store," Arnegard said. "This is going to be a revealing system."