Discovery and phylogenetic analysis of a riverine species flock of African electric fishes (Mormyridae: Teleostei)

The evolution of species-specific mate recognition signals is of particular interest within speciose monophyletic groups with restricted distributions (known as "species flocks"). However, the explosive nature of speciation in these clades makes difficult the reconstruction of their phylogenetic history. Here we describe a species flock of riverine mormyrid fishes from west-central Africa in which electric signals may play a role in the reproductive isolation of sympatric species. In our recent field collections, totaling more than 1400 specimens from many localities, we recognize 38 forms that are distinct in their morphologies and electric organ discharge (EOD) characteristics. Of these 38, only four clearly correspond to described species. Here we treat these forms as operational taxonomic units (OTUs) in a phylogenetic analysis of cytochrome b sequence data from a sample of 86 specimens. We examined support in the molecular data for the monophyly of these 38 OTUs considered together, the monophyly of each phenotypically delimited OTU considered individually, and for relationships among OTUs congruent with those inferred from the distribution of morphological and EOD character states. Trees obtained by both maximum-parsimony and maximum-likelihood analyses, rooted with sequence data from outgroup taxa, provide evidence for the monophyly of these 38 OTUs with respect to other mormyrid fishes. The small genetic distances between many distinct forms suggest their recent divergence. However, in many instances the cytochrome b tree topology fails to support the monophyly of individual OTUs and close relationships between OTUs that are similar in morphology and EOD characteristics. In other cases, individuals from distinct OTUs share identical or nearly identical haplotypes. Close examination of these cases suggests that unnatural OTU definition is not the sole cause of this pattern, and we infer an incongruence between the mitochondrial gene tree and the organismal phylogeny caused by incomplete mitochondrial lineage sorting and/ or introgression across forms. The apparently rapid diversification in this clade of riverine electric fishes and the problems associated with recovering a meaningful species-level phylogeny from mitochondrial data parallel findings in other species flocks. Selection on EOD waveforms as mate recognition signals may be involved in the radiation of these fishes. This is the first description of a freshwater fish species flock from a riverine, as opposed to a lacustrine, environment.

Basal teleosts and the question of elopomorph monophyly. Morphological and molecular approaches

The methodology used by previous authors to resolve the relationships of the elopomorphan taxa is criticized. The morphological characters that have been proposed to support the monophyly of the Elopomorpha are reviewed and it is shown that most of them are weak. A new hypothesis of relationships is proposed on the basis of nucleotidic sequences of ribosomal RNA 18S, 16S and 12S. In order to really test all the possible relationships, the monophyly of the Elopomorpha was not considered a priori. The tree was rooted on Amia calva and Lepisosteus osseus and the ingroup taxa sampling was subsequently increased. The obtained topology shows that the Elopomorpha is a non-monophyletic taxon. Elopiforms, anguilliforms, albuliforms and notacanthiforms are considered here as four monophyletic, incertae sedis taxa among basal teleosts.

Molecular systematics of the african electric fishes (Mormyroidea: teleostei) and a model for the evolution of their electric organs

We present a new molecular phylogeny for 41 species of African mormyroid electric fishes derived from the 12S, 16S and cytochrome b genes and the nuclear RAG2 gene. From this, we reconstruct the evolution of the complex electric organs of these fishes. Phylogenetic results are generally concordant with earlier preliminary molecular studies of a smaller group of species and with the osteology-based classification of Taverne, which divides the group into the Gymnarchidae and the Mormyridae, with the latter including the subfamilies Petrocephalinae (Petrocephalus) and Mormyrinae (all remaining taxa). However, we find that several genera previously recognized by Taverne are non-monophyletic. Within the Mormyrinae, the genus Myomyrus is the sister group to all the remaining taxa. Other well-supported clades within this group are recovered. A reconstruction of electrocyte evolution on the basis of our best-supported topology suggests that electrocytes with penetrating stalks evolved once early in the history of the mormyrids followed by multiple paedomorphic reversals to electrocytes with non-penetrating stalks.

Phylogenetic relationships of mormyrid electric fishes (Mormyridae; Teleostei) inferred from cytochrome b sequences

The Mormyridae are African osteoglossomorph freshwater fishes of great interest because of their electric organs. They have become an important model in studies of electrophysiology and behavior but their phylogenetic relationships are poorly known. Phylogenetic relationships among mormyrids were determined by comparing cytochrome b sequences (588 bp) of 27 species belonging to 15 genera. Results showed that the Petrocephalus species (subfamily Petrocephalinae) are the sister-group of all other mormyrids (subfamily Mormyrinae). The monophyly of the Mormyrinae was supported, as well as three original intra-Mormyrinae clades. Three genera, Marcusenius, Pollimyrus, and Brienomyrus, were found to be polyphyletic with high support. Some of these polyphylies are tentatively explained. The results confirmed that the lateral ethmoid bone was lost several times within the Mormyrinae. These findings emphasize the necessity of systematic studies and taxonomic revision of the Mormyridae. The tree obtained from the mitochondrial data showed a single rise of each electrocyte type except for electrocyte with penetrating stalk ("Pa"). Constraining the single occurrence of electrocyte type Pa did not require an excessive number of extra steps (1.86%).

Molecular phylogenetics and the evolution of antarctic notothenioid fishes

The monophyly of the antarctic fish suborder Notothenioidei and the monophyly of its earliest family the Bovichtidae have been investigated with 12S and 16S mitochondrial DNA sequences. New data from Cottoperca, Pseudaphritis, Harpagifer and several outgroups, in addition to available sequences, show that the bovichtids are paraphyletic. Pseudaphritis is the sister group of all the non-bovichtid notothenioids. The same results are found from two independent genetic markers, the nuclear 28S rDNA and the 12S and 16S mitochondrial rDNA. This reliably refutes a previous hypothesis that placed Pseudaphritis as the sister group of all the remaining notothenioids (including Cottoperca and Bovichtus). Bootstrap analyses show that the Notothenioidei are monophyletic (although members of the suborder Trachinoidei have not been surveyed). Subsequent data from hemoglobin composition confirm the present relationships. After discussions between members of the European Science Foundation (ESF) network during its last two meetings, we point out here some fundamental aspects of comparative biology to improve understanding between the physiologist community and phylogeneticists. The most important points are differences in how the concept of homology is used and differences in the consideration of adaptation. When adaptation is evoked or questioned, endless speculations and untestable scenarios are often developed. We strongly advocate the use of phylogenetic trees for testing hypotheses of adaptation (through multiple character mapping). Such a "research program" in comparative biology has the power to improve knowledge because it can potentially lead to new experiments for testing adaptive hypotheses.