The Mitochondrial Phylogeny of the South American Electric Fish (Gymnotiformes) and an Alternative Hypothesis for the Otophysan Historical Biogeography
In order to address the mitochondrial phylogeny of the order Gymnotiformes within the Otophysi, two gonorynchiform genera were used to root the tree, and 15 genera from each remaining otophysan order (Cypriniformes, Characiformes, Siluriformes and Gymnotiformes) were sampled. The final data matrix consisted of two fragments (701 aligned nucleotides) of the 12S and 16S mitochondrial ribosomal RNA genes. Both molecules appear to be under functional/structural constraints, that can limit their efficiency as molecular marker for speciation events within a short time window. Nevertheless, 12S and 16S rRNA appear to retain enough phylogenetic information to determine branching order within the Otophysi, which is congruent with morphological data. Cypriniformes is the sister group of (Characiformes + Siluriformes + Gymnotiformes), and Characiformes is the sister group of the Siluriphysi (Siluriformes + Gymnotiformes). Adopting this hypothesis as the correct alternative, the fossil records of each order were reviewed in conjunction with their present distribution and current theories about the fragmentation of Pangea. The following scenario has been proposed: the ancestral Otophysi differentiated in freshwater habitats in Eurasia, from a lineage that invaded continental waters during the Early Cretaceous. Cypriniformes differentiated in Asia, colonized Africa but never reached South America. Cypriniform representatives arrived in North America by the Beringian land bridge and/or using a less obvious path through the Europe–Greenland–Labrador connection. The final separation of South America and Africa, between 80 and 125 million years ago, represented a vicariant event for already differentiated characiform clades, and sister-group relationships are found between African and South American genera. Characiforms reached North America only after the uplift of the Panamanian Isthmus, and not using the same route as the cypriniforms, probably for reasons associated with the current absence of characiforms in high latitudes. Siluriformes and Gymnotiformes differentiated within the isolating South America. It is proposed that catfish were able to disperse through shallow brackish/marine water by the Late Cretaceous and attained a cosmopolitan distribution by the Early Tertiary. Gymnotiforms never left South America, which may be related to functional constraints placed upon the efficiency of the Electrogenic and Electrosensory Systems (EES) by brackish (low resistivity) water.