Phylogeography and population genetic structure of the cardinal tetra (Paracheirodon axelrodi) in the Orinoco basin and Negro River (Amazon basin): evaluating connectivity and historical patterns of diversification

<p class="MsoNormal"><span class="Fuentedeprrafopredeter1"><span>The Neotropics contain one of the most diverse assemblages of freshwater fishes worldwide. Part of this diversity is shared between the Orinoco and Amazon basins. These basins have been separated for a long time due to the Vaupes Arch, rising between 10 - 11 Ma. T</span></span><span class="Fuentedeprrafopredeter1"><span>oday, there is only one permanent connection between the Orinoco and Negro </span></span><span class="Fuentedeprrafopredeter1"><span>(Amazon) </span></span><span class="Fuentedeprrafopredeter1"><span>basins, known as the Casiquiare Canal</span></span><span class="Fuentedeprrafopredeter1"><span>. </span></span><span class="Fuentedeprrafopredeter1"><span>However, alternative corridors allowing fish dispersion between both basins have been proposed. The cardinal tetra (<em>Paracheirodon axelrodi),</em> the most important fish in the ornamental world market, is distributed in both basins. Here we investigated </span></span><span class="Fuentedeprrafopredeter1"><em><span>P. axelrodi </span></em></span><span class="Fuentedeprrafopredeter1"><span>phylogeography, population structure, and potential routes of migration and connectivity between the two basins. A total of 468 bp of the mitochondrial gene (COI), 555 bp of the nuclear gene fragment (MYH6), and 8 microsatellite loci were analyzed. </span></span><span class="Fuentedeprrafopredeter1"><span>As a result, we found two major genetic clusters as the most likely scenario (K=2), but they were not discreetly distributed between basins. A gradient of genetic admixture was observed in Cucui and </span></span><span class="Fuentedeprrafopredeter1"><span>São</span></span><span class="Fuentedeprrafopredeter1"><span> Gabriel da Cachoeira, between the upper Negro River and the upper Orinoco. Samples from the middle-lower Negro River were highly structured. </span></span><span class="Fuentedeprrafopredeter1"><span>Cucui (Negro basin) was more similar to the Orinoco than to the rest of the Negro basin populations. </span></span><span class="Fuentedeprrafopredeter1"><span>However, substructure was also observed by the discriminant analysis, fixation indices and other hierarchichal structure analyses (K=3-6), showing three major geographic clusters: Orinoco, Cucui, and the remaining of the Negro basin. </span></span><span class="Fuentedeprrafopredeter1"><span>Unidirectional migration patterns were detected between basins: via Cucui toward Orinoco and via the remaining of the Negro basin toward Orinoco. Results from the Relaxed Random Walk analysis support a very recent origin of this species in the headwater Orinoco basin (Western Guiana Shield, at late Pleistocene) with a later rapid colonization of the remaining Orinoco basin and almost simultaneously the Negro River via Cucui, between 0.115 until about 0.001 Ma. Historical biogeography and population genetic patterns observed here for Cardinal tetra, seem to be better explained by river capture, physical, or ecological barriers than due to the geographic distance.</span></span></p>