MOLECULAR INSIGHTS INTO THE PHYLOGENY OF MORMYRIFORM FISHES AND THE EVOLUTION OF THEIR ELECTRIC ORGANS

In this report we generate a partial phylogeny of the mormyriform fish es using mitochondrial DNA sequences from twelve species of mormyrifor ms belonging to five genera. Electric organs and electric organ discha rges are also examined. We have sequenced and aligned 373 bases from t he mitochondrial 12S rRNA and 559 bases from the 16s rRNA from fourtee n species of the superorder Osteoglossomorpha. Two non-mormyriform gen era were used as outgroups. Three phylogenetic methods generated conco rdant partial phylogenies for these fish. Our analysis focuses on the genus Brienomyrus, which is a heterogeneous clade with at least eleven nominal species. Six morphs from Gabon had distinctive EODs but were morphologically 'cryptic' in that they all had the brachyistius-like b ody morphology. DNA analysis fully supports the EOD data that the six morphs represent distinct clades. The group from Gabon is monophyletic , while B. brachyistius from West Africa is a separate lineage. B. nig er, a second distinct lineage, is a sister group to the six species fr om Gabon. Petrocephalus is the sister group of all the genera of the s ubfamily Mormyrinae so far analyzed, thereby confirming previous osteo logical results. Gymnarchus niloticus is the sister group of the famil y Mormyridae, also confirming an earlier phylogenetic hypothesis based on morphology. The molecular data adds polarity to electric organ cha racteristics. Stalkless electrocytes appear to be primitive. Petroceph alus, with non-penetrating stalked electrocytes innervated on the post erior side, represents an ancestral state for the Mormyridae, while Ma rcusenius, Brienomyrus and Gnathonemus with penetrating-stalked electr ocytes, represent the apomorphic condition. Two species with doubly-pe netrating electrocytes innervated on the posterior side may represent a transitional stage. At least two species of Brienomyrus appear to ha ve reverted to non-penetrating stalked electrocytes, possibly through paedomorphosis.