Gymnotiformes

Mormyroidea

Justification not yet generated — showing supporting facts

• The interval between the appearance of electroreception and the origin of myogenic electric organs in both Gymnotiformes and Mormyroidea is estimated to be between 16 and 19 million years or 22 and 26 million years, depending on the calibration method used.
• The mean ages of the Mormyroidea and the Gymnotiformes are very similar, with less than a 15% difference under both calibration strategies and largely overlapping credibility intervals.
• In the maximum likelihood tree of the Teleostei generated by the study, the Mormyroidea (African weakly electric fishes) and the Gymnotiformes (South American weakly electric fishes) are highlighted.
• African Notopteridae and Siluriformes are passively electroreceptive, whereas Gymnotiformes and Mormyroidea independently evolved specialized electric organs for producing weak electric discharges and high-frequency tuberous electroreceptors tuned to those signals.
• Maximum likelihood (ML) analyses consistently show that Gymnotiformes and Mormyroidea are each monophyletic groups to the exclusion of all other teleosts, with a bootstrap proportion of 100%.
• Reconstruction #2 for the origin of the Gymnotiformes and the Mormyroidea crown groups infers their ages to be 143.5 Mya and 124.8 Mya, respectively, which corresponds to the period before the complete separation of Africa and South America.
• In both Gymnotiformes and Mormyroidea, a similar duration of time elapsed between the initial origin of electroreception and the subsequent origin of electrogenesis, which was accompanied by the evolution of more complex electroreceptive systems.
• The study titled 'Comparable Ages for the Independent Origins of Electrogenesis in Teleosts' analyzed a single molecular dataset including representatives of South American Gymnotiformes and African Mormyroidea to infer the ages of their independent origins of electrogenesis.
• Mormyroidea and Gymnotiformes are used as comparative vertebrate systems for evolutionary neurobiology research.
• Mormyroidea are African weakly electric fishes, and Gymnotiformes are South American weakly electric fishes.
• Alves-Gomes estimated the age of the stem Mormyroidea to be 61–72 Mya and the age of the Gymnotiformes to be 79–117 Mya.
• The study re-examined the evolutionary ages of weakly electric teleost fish groups using complete mitochondrial genomes, an extensive taxonomic sampling of 27 species of Mormyroidea and Gymnotiformes, and a relaxed-clock Bayesian method for inferring phylogenetic relationships and divergence times.
• The researchers employed two distinct calibration strategies to estimate the divergence times of the Gymnotiformes and Mormyroidea on a phylogenetic tree, evaluating how absolute age estimates vary based on the calibration constraint methodology.
• Using reconstruction #2 with soft maximum age constraints, the estimated ages are 147.5 Mya (95% CI = 117.9–177.9 Mya) for Notopteroidei, 124.8 Mya (95% CI = 97.5–155.7 Mya) for crown-group Mormyroidea, 169.1 Mya (95% CI = 140.5–197.3 Mya) for Characiphysae, and 143.5 Mya (95% CI = 115.8–171.8 Mya) for crown-group Gymnotiformes.
• Tuberous electroreceptors in Mormyroidea and Gymnotiformes facilitate electrocommunication by detecting the electric-organ discharges (EODs) of neighboring fishes.
• Reconstruction #1 for the origin of the Gymnotiformes and the Mormyroidea crown groups favors a post-separation origin, dating them at 100.2 Mya and 93.7 Mya, respectively.
• The most recent common ancestor of the Mormyroidea and the Gymnotiformes is dated to 284.1 million years ago (with a credibility interval of 252.2–313.3 million years ago) under reconstruction #2.
• The ability to sense low-frequency, passive electric fields preceded the evolution of electrogenesis in Gymnotiformes and Mormyroidea.
• The Gymnotiformes (South American weakly electric fishes) and the Mormyroidea (African weakly electric fishes) independently evolved complex systems for object localization and communication via the generation and reception of weak electric fields, representing a case of convergent evolution.
• Inferences about the temporal context of evolutionary processes in Mormyroidea and Gymnotiformes are limited by the lack of a hypothesis for the timing of their origin and early diversification.
• Research on Mormyroidea and Gymnotiformes includes studies on reproductive character displacement, the role of communication in speciation, the effects of neural structure evolution on species radiation, and genetic mechanisms underlying evolutionary novelty.
• The Gymnotiformes and Mormyroidea lineages evolved electrogenesis independently but contemporaneously, occurring tens of millions of years after their most recent common ancestor.
• The Gymnotiformes and Mormyroidea lineages emerged around the time of the final fragmentation of western Gondwana.
• Figure S1 highlights the Mormyroidea (African weakly electric fishes) and the Gymnotiformes (South American weakly electric fishes) using light grey gradient boxes.
• The independent origins of the Mormyroidea and Gymnotiformes occurred at similar times, either slightly postdating or just predating the final separation of Africa and South America by continental drift.
• The most recent common ancestor of the Mormyroidea and Gymnotiformes was a non-electrogenic basal teleost that lived more than 85 million years before the origin of these electric fish lineages.
• Active electroreception, or electrolocation, occurs only in the teleost lineages Mormyroidea and Gymnotiformes, where the animal generates a weak electric-organ discharge (EOD) of less than 1 volt and detects distortions in the resulting field using high-frequency-tuned tuberous electroreceptors.
• The two clades of weakly electric fishes, Gymnotiformes and Mormyroidea, are nested within two distantly related groups of Teleostei: the Ostariophysi and the Osteoglossomorpha.
• The most recent common ancestor of the Mormyroidea and the Gymnotiformes is dated to 185.7 million years ago (with a credibility interval of 171.4–201.7 million years ago) under reconstruction #1.
• In Gymnotiformes and Mormyroidea, electric organs and tuberous electroreceptors facilitate electrical communication and active electrolocation, where objects are located and sensed via distortions in self-generated electric fields.
• The authors speculate that shared environmental conditions, such as climate during the early Late Cretaceous, may have contributed to the contemporaneous, independent origins of Gymnotiformes and Mormyroidea.
• Within teleost fishes, electroreception is restricted to two distantly related groups: the Siluriphysi (Gymnotiformes plus Siluriformes) and the Notopteroidei (Mormyroidea plus Notopteridae).

Mormyroids

Justification not yet generated — showing supporting facts

• The small eye sizes of many mormyroids and gymnotiforms may be a direct result of their active electrosensory systems superseding the importance of vision.
• Gymnotiforms and mormyroids both possess high-frequency electroreceptors, which are derived from similar lateral line receptor precursors and underlie active electrolocation and electrocommunication.
• The authors refer to gymnotiforms and mormyroids as a textbook example of convergent evolution to highlight their independent origins from phylogenetically unrelated and phenotypically dissimilar teleost ancestors.
• The estimated time interval between the origins of teleost electroreception and the origins of electrogenesis is 22.7 million years for mormyroids and 25.6 million years for gymnotiforms under reconstruction #2.
• Gymnotiforms and mormyroids evolved novel myogenic electric organs (EOmyo) that are developmentally derived from skeletal muscle progenitor cells known as myoblasts.
• H.W. Lissmann proposed that the convergence upon an anguilliform body form with ribbon fin propulsion, seen in all gymnotiforms and some mormyroids, may have evolved to minimize body axis bending during active electrolocation.
• Similar patterns of amino acid substitution occurred in regions of the sodium channel α-subunit in both gymnotiforms and mormyroids, which are thought to contribute to electric signal variation.
• Gymnotiforms and mormyroids originated independently from non-electrogenic ancestors at approximately the same period in Earth's history.
• Zakon et al. and Arnegard et al. demonstrated that in both gymnotiforms and mormyroids, the same sodium channel α-subunit paralog (gene duplicate) was co-opted from skeletal muscle for exclusive expression in myogenic electric organs (EOmyo).
• Both mormyroids and gymnotiforms have diversified to a similar degree, with each group comprising approximately 200 named species.
• The estimated time interval between the origins of teleost electroreception and the origins of electrogenesis is 16.6 million years for mormyroids and 18.7 million years for gymnotiforms under reconstruction #1.
• Reduced mouth sizes and restricted gill openings in mormyroids and gymnotiforms likely evolved to reduce interfering electrical emissions from electrically active epithelial tissues.
• Mormyroids and gymnotiforms, with the exception of Electrophorus electricus, are classified as "weakly" electric fishes because the external potentials they produce are typically imperceptible to humans without amplification.
• Whole genome duplication (WGD) occurring just prior to the radiation of teleosts contributed to the origin of novel electrogenic systems in mormyroids and gymnotiforms.
• Myogenic electric organs in mormyroids and gymnotiforms are similar in many ways, but also exhibit significant differences between lineages and great diversity in structure and function among species.
• The elongate snouts or trunks of some mormyroids and gymnotiforms may have arisen secondarily as an adaptation to the specific types and habits of prey organisms found in the ecological niches exploited by active electrosensory predators.
• The two groups of weakly electric fishes, mormyroids and gymnotiforms, originated at nearly the same time in evolutionary history, with similar time intervals passing between the whole genome duplication (WGD) at the base of the teleost radiation and the origins of novel myogenic electric organs in each lineage.

Africa

The origin of the Gymnotiformes is linked to Africa through the geological history of continental drift, as their evolutionary timeline is compared to the separation of Africa and South America [1], [2]. While Gymnotiformes are specifically South American, their evolutionary history is geographically contextualized by the historical connection between Africa and South America [3]. — 3 supporting facts