Tuesday, March 18, 2014

[Ichthyology / Evolution • 2012] Comparable Ages for the Independent Origins of Electrogenesis in African and South American Weakly Electric Fishes

Figure 2. Morphological convergences between African and South American electric fishes.
Mormyroid African electric fishes (left column) are facing gymnotiform South American electric fishes (right column) with similar aspects of morphology (such as elongate bodies, extended tube-like snouts, reduced eyes, and/or small mouth sizes). Anterior portion of body shown above small image of whole body (except for Petrocephalus sullivani); electric organ discharge waveform shown for every species (each trace 5 ms in total duration with head-positivity plotted upwards).
 (A) Mormyrops zanclirostris, 175 mm standard length (SL), Ivindo River, Gabon,
(B) Sternarchorhynchus oxyrhynchus, 220 mm total length (TL), Rio Negro, Brazil;
(C) Mormyrus proboscirostris, 232 mm standard length, Ubundu, Congo River, D.R. Congo;
(D) Rhamphichthys sp., 305 mm TL, Rio Negro, Brazil;
(E) Mormyrops anguilloides, 195 mm SL, Yangambi, Congo River, D.R. Congo;
(F) Gymnotus sp., 195 mm TL, Rio Negro, Brazil; and (G) Petrocephalus sullivani, Ogooué River, Gabon; (H) Eigenmannia sp., Apure River, Venezuela.
Species A–D feed on benthic invertebrates, species E, F are piscivorous, and G, H feed on pelagic invertebrates.


One of the most remarkable examples of convergent evolution among vertebrates is illustrated by the independent origins of an active electric sense in South American and African weakly electric fishes, the Gymnotiformes and Mormyroidea, respectively. These groups independently evolved similar complex systems for object localization and communication via the generation and reception of weak electric fields. While good estimates of divergence times are critical to understanding the temporal context for the evolution and diversification of these two groups, their respective ages have been difficult to estimate due to the absence of an informative fossil record, use of strict molecular clock models in previous studies, and/or incomplete taxonomic sampling. Here, we examine the timing of the origins of the Gymnotiformes and the Mormyroidea using complete mitogenome sequences and a parametric Bayesian method for divergence time reconstruction. Under two different fossil-based calibration methods, we estimated similar ages for the independent origins of the Mormyroidea and Gymnotiformes. Our absolute estimates for the origins of these groups either slightly postdate, or just predate, the final separation of Africa and South America by continental drift. The most recent common ancestor of the Mormyroidea and Gymnotiformes was found to be a non-electrogenic basal teleost living more than 85 millions years earlier. For both electric fish lineages, we also estimated similar intervals (16–19 or 22–26 million years, depending on calibration method) between the appearance of electroreception and the origin of myogenic electric organs, providing rough upper estimates for the time periods during which these complex electric organs evolved de novo from skeletal muscle precursors. The fact that the Gymnotiformes and Mormyroidea are of similar age enhances the comparative value of the weakly electric fish system for investigating pathways to evolutionary novelty, as well as the influences of key innovations in communication on the process of species radiation.

Sébastien Lavoué, Masaki Miya, Matthew E. Arnegard, John P. Sullivan, Carl D. Hopkins and Mutsumi Nishida. 2013. Comparable Ages for the Independent Origins of Electrogenesis in African and South American Weakly Electric Fishes. PLoS ONE. 7(5): e36287. doi: dx.doi.org/10.1371/journal.pone.0036287