Working with collaborator Matthew Arnegard, PhD, a postdoctoral fellow at the Fred Hutchinson Cancer Research Center in Seattle, WA, the scientists mapped the brain anatomy onto a phylogenetic tree (an evolutionary tree based on the similarity of DNA sequences), and they could see that there were two equally parsimonious ways to reconstruct the fishes' evolutionary history.
Either the complex brain was ancestral and the simpler brain evolved twice or the simpler brain was ancestral and the complex brain arose twice. To solve this riddle, they did what evolutionary biologists do, which is look at the "next outgroup member," the closest related fish that's not part of the Mormyridae family.
This fish has an area in the midbrain that is similar to a small, undifferentiated EL. This suggested the EL brain was probably the ancestral brain, and the more complex divided ELa/ELp evolved twice, once within the subfamily Mormyrinae and once within the subfamily Petrocephalinae.
Did fancy anatomy lead to rapid diversification?
If a communication system is to promote species diversity it must have both the capacity to create new signals (flexible stalk morphology) and the ability to distinguish those new signals from other signals (the broad distribution of knollenorgans and the complex brain).
"The only fish that have all three is a group of mormyrids we ended up calling Clade A for simplicity's sake," Carlson says.
To test the importance of these traits on signal divergence we analyzed the discharges of fish collected in two locales: the Ivindo River of Gabon, home to the largest known assemblage of the subfamily Mormyrinae; and Odzala National Park in the Republic of the Congo, home to the largest known assemblage of the subfamily Petrocephalinae.
"Statistical analysis showed us that the rate of signal divergence in Clade A was 10 times higher than among other fish within the Mormyridae," Carlson say
|Contact: Diana Lutz|
Washington University in St. Louis