Phylogenetic analysis of the South American electric fishes (order Gymnotiformes) and the evolution of their electrogenic system: a synthesis based on morphology, electrophysiology, and mitochondrial sequence data

The order Gymnotiformes (South American electric fishes) is a fascinatingassemblage of freshwater fishes that share the unusual ability to produceand sense electric fields used for electrolocation and socialcommunication. In the last few decades, the electrogenic and electrosensorysystems (EES) of these fish have served as an excellent model to studymotor and sensory physiology in vertebrates. In an attempt to the evolutionof characters associated with the EES in the group, we appliedmaximum-parsimony (MP), minimum-evolution (ME), and maximum-likelihood (ML)methods to analyze 302 aligned bases of the mitochondrial 12S rRNA and 416bases of the mitochondrial 16S rRNA of 19 gymnotiform genera representingall six recognized families. Six catfish genera (order Siluriformes) werealso sequenced and used as outgroups. The phylogenetic hypothesis resultantfrom molecular data analysis differs in some respects from previoushypotheses based on morphological studies. Our results were mostinformative within the family level, as we were unable to elucidate therelationships among deeper branches in this order with sufficientconfidence by using molecular data alone. The phylogenetic information ofboth mitochondrial DNA segments appears to be affected by functionalconstraints, and the resultant topologies were sensitive to differentweighting schemes and the algorithm used. Nonetheless, we found unanimoussupport for the following phylogenetic relationships: (1) the familySternopygidae is an unnatural group, and Sternopygus is the solerepresentative of a unique lineage within the order; (2) the familyHypopomidae is not monophyletic; and (3) the order Gymnotiformes iscomposed of at least six natural clades: Sternopygus, family Apteronotidae,a new clade consisting of the remaining sternopygids, families Hypopomidae+ Rhamphicthyidae, family Electrophoridae, and family Gymnotidae. Bycombining molecular, morphological, and physiological information, wepropose a new hypothesis for the phylogeny of this group and suggest a newfamily Eigenmanniidae n. (order Gymnotiformes).