Phylogenetic Relationships Of Eurasian Catfishes (Otocephala: Siluriformes) And Divergence Time Estimates For Major Otocephalan Clades

The Otocephala, a clade including ostariophysan and clupeomorph teleosts, represents about a quarter of total fish species diversity, with about 1,000 genera and more than 7,000 species. A series of recent papers have defended that the origin of this clade and of its major groups may be significantly older than the oldest fossils of each of these groups suggest. Some of these recent papers explicitly defend a Pangean origin for some otocephalan groups such as the Siluriformes or Cypriniformes. To know whether or not the otocephalans as a whole, and particularly the mainly freshwater, cosmopolitan otophysans could have originated before the splitting of the Pangean supercontinent is of extreme importance, since otophysan fishes are among the most useful animal groups for the determination of historical continental relationships. Whilst as one of the member in otocephala, catfishes represent approximately 1/20 of total vertebrate species diversity, however, the interrelationships among many of the monophyletic catfish families are unclear, and a natural classification remains elusive. Therefore, this thesis is divided into two parts, one is the molecular phylogenetic relationships of Eurasian catfishes, and the other is the divergence time estimates for major otocephalan clades. The main conclusions and hypotheses of the present study were as follows: 1. We determined complete mitochondrial cyt b gene sequences (1,138 base pair (bp)) for 76 individuals of 46 catfish species and 906 bp recombination activation gene 2 sequences for 79 individuals of 35 catfish species. Cytochrome b and RAG2 gene sequences for other related catfish groups and outgroup were retrieved from GenBank. Phylogenetic trees were constructed using Bayesian inference and maximum likelihood methods with representatives from Characiformes, Cypriniformes, Clupeiforms, Gonorynchiformes, and Gymnotiformes used as outgroups. Our results show that (1) the representatives of catfish species form a monophyletic group; (2) family Schilbidae is not a monophyletic group with some representatives from China having close relationships with Bagridae and/or Siluridae; (3) families of Cranoglanididae from East Asian and Ictaluridae from North America form a sister-group relationship; (4) families of Akysidae, Amblycipitidae, and Sisoridae form a monophyletic group; (5) Xiurenbagrus should be a valid genus within Amblycipitidae; (6) glyptosternoids forms a monophyletic group with genus Glaridoglanis having a basal position within it. 2. The mitochondrial genome DNA of Cranoglanis bouderius was amplified in its entirety using a long PCR technique. Protein-coding, rRNA, and tRNA genes were identified by comparison with the corresponding known sequences of other catfish taxa. The complete nucleotide sequence of the L-strand of C. bouderius mtDNA was determined to be 16,539 bp long. The structural organization of the mitochondrial genes and non-coding regions is identical to that of fish and higher vertebrates, consisting of two rRNAs, 22 tRNAs, and 13 protein-coding genes with a control region. Most of the C. bouderius mitochondrial genes are encoded on the H-strand, although the ND6 gene and eight tRNA genes are encoded on the L-strand. Of the 13 protein-coding genes, three parts (ATP8 and ATP6, ND4L and ND4, ND5 and ND6) were partially overlapped on the same strand clockwise and anticlockwise. The extent of overlap differs in the various fish mitochondrial genomes, as the protein-coding genes have similar lengths but the lengths of the non-coding regions differ.3. Divergence times of major otocephalan clades were estimated using Bayesian relaxed molecular clock method with 29 teleost species. With respect to the divergence times obtained in the present study, they support the scenario defended in recent papers dealing with the origin and biogeography of otocephalan fishes: that the origin of the major otocephalan groups is probably much older than a direct, exclusive analysis of the oldest fossil of each of these groups might indicate. The origin of the Otocephala, for example, is estimated here as about 239 Mya. Of particular interest for biogeographical studies, the origin of all otophysan orders, with the exception of the Gymnotiformes, is estimated as having occurred in the late Jurassic (Cypriniformes: 156 Mya; Characiformes: 164 Mya; Siluriformes: 157 Mya). The results show that it is likely that there were still some Pangean connections between Gondwana and Laurasia in the late Jurassic, and perhaps even in the early Cretaceous. In this case, this would help to explain the Pangean distribution of cypriniforms, characiforms and siluriforms, groups of primary freshwater fishes with relatively few, and rather derived, marine members (although characiforms are today only found in Africa and South America, fossils of this order have been found in northern parts of the globe such as Europe). The more restricted distribution of the Gymnotiformes (extant and fossil members only found in South America) could thus possibly be related to a more recent origin of this group 139 Mya. It is hoped that the present work will inform future phylogenetic, palaeontological, tectonic, biogeographical and molecular studies that might clarify the origin and distribution of otocephalans, and of teleostean fishes in general.