Molecular Phylogeny And Divergence Time Estimation Of The Bagrid Catfishes (Actinopterygii:Siluriformes)

Bagridae (Actinopterygii:Siluriformes) is a large and morphologically diverse family of catfishes with21genera and more than220species, which are distributed in Asia and Africa. This group of fishes are generally not as well studies as some other catfish groups. The alpha level taxonomy and the phylogenetic relationships within Bagridae remains controversial. Now molecular phylogenetics provide new tools for solving biological systematic classification. In the present study, molecular phylogeny and divergence time analyses were conducted in the bagrid catfishes based on the complete mitochondrial genome sequences. The main results are as follows:1. we determined the complete mitogenome sequences for these bagrid catfishes, Bagrichthys obscurus, Bagrus orientalis, Bagrus bajad, Bagrus docmak, Hemibagrus macropterus, Hemibagrus spilopterus, Mystus bocourti, Mystus mysticetus, Mystus rhegma, Mystus singaringan, Pseudomystus siamensis, and Tachysurus sinensis, using LA-PCR technique. These mitogenomes were16521-16796bp in length. The structural organization and arrangement of these mitochondrial genes were similar to the typical vertebrate mitochondrial genomes, consisting of two rRNAs,22tRNAs,13protein-coding genes, and a non-coding control region. There was more A and T in the overall base composition of the heavy strand (AT=57.8%). The extent differs of lengths in the bagrid species mitochondrial genomes, as the protein-coding genes have similar lengths but the lengths of the non-coding regions differ. The non-coding regions of M. bocourti is shortest in lengths (876bp), while the non-coding regions of B. docmak is longest in lengths (1151bp).2. Totally54mitogenome sequences were used in this study, including20bagrid species (belonging to9genera),29other related catfishes,5representative species from Characiformes, Cypriniformes and Gonorynchiformes were used as outgroups. Phylogenetic trees were reconstructed using Bayesian inference and maximum likelihood methods. The results show that:(1) the representatives of bagrid species form a monophyletic group;(2) The three genera Pelteobagrus, Pseudobagrus and Leiocassis from East Asia form a monophyletic group;(3) Bagrichthys and Pseudomystus from Southeast Asia form a sister-group relationship;(4) The three genera Bagrus, Hemibagrus and Mystus form a monophyletic group, and the species of Bagrus from Africa form a monophyletic group;(5) Tachysurus simnsis is not a species in Bagridae but a species in Ariidae.3. For a comprehensive analyses of the bagrid systematics based on more representative species, we used all available cytb gene sequence data (50bagrid catfishes belonging to15genera) retrieved from GenBank in a separate phylogenetic analysis. Phylogenetic trees were reconstructed using Bayesian inference and maximum likelihood methods. The results show that bagrid catfishes were divided into6monophyletic groups, genera of Pelteobagrus, Leiocassis and Pseudobagrus from East Asia form a monophyletic group; genera of Pseudomystus, Hyalobagrus, Nanobagrus, Leiocassis and Bagrichthys from Southeast Asia form a monophyletic group; genera of Bagrus, Hemibagrus and Mystus form a monophyletic group; the three genera of Rita, Batasio and Horabagrus form a monophyletic group, respectively. 4. Divergence times of bagrids were estimated using Bayesian relaxed molecular clock method. The origin of the Siluriformes was estimated here as about114.7Mya in the Cretaceous period. Of particular interest for bagrid groups, the origin of which, is estimated as having occurred in the Paleocene period, as about57.6Mya, but current species diversity of the East Asia bagrids is mainly the consequence of speciation which took place from Miocene to Pliocene period (20.3-2.6Mya). We speculate that the Himalayan-Tibetan plateau uplift and the onset of the monsoon in East Asia may effectively promote and maintain the diversification of fishes in this region. The Bagrus species were estimated as having first occurred in the late Oligocene (27.0Mya) that was highly congruent with the earliest fossil records of Bagrus known so far in Africa. However, current species diversity of Bagrus species is mainly the consequence of speciation which took place in Miocene period. The Mystus species were estimated to first emerge in the Eocene that was also highly congruent with the earliest fossil records known so far in China.