Molecular phylogeny of elasmobranchs inferred from mitochondrial and nuclear markers

Abstract

The elasmobranchs (sharks, rays and skates) being the extant survivors of one of the earliest offshoots of the vertebrate evolutionary tree are good model organisms to study the primitive vertebrate conditions. They play a significant role in maintaining the ecological balance and have high economic value. Due to over-exploitation and illegal fishing worldwide, the elasmobranch stocks are being decimated at an alarming rate. Appropriate management measures are necessary for restoring depleted elasmobranch stocks. One approach for restoring stocks is implementation of conservation measures and these measures can be formulated effectively by knowing the evolutionary relationship among the elasmobranchs. In this study, a total of 30 species were chosen for molecular phylogeny studies using mitochondrial cytochrome c oxidase subunit I, 12S ribosomal RNA gene and nuclear Internal Transcribed Spacer 2. Among different genes, the combined dataset of COI and 12S rRNA resulted in a well resolved tree topology with significant bootstrap/posterior probabilities values. The results supported the reciprocal monophyly of sharks and batoids. Within Galeomorphii, Heterodontiformes (bullhead sharks) formed as a sister group to Lamniformes (mackerel sharks): Orectolobiformes (carpet sharks) and to Carcharhiniformes (ground sharks). Within batoids, the Myliobatiformes formed a monophyly group while Pristiformes (sawfishes) and Rhinobatiformes (guitar fishes) formed a sister group to all other batoids.

A Review of the Mitogenomic Phylogeny of the Chondrichthyes

“…The analysis revealed the formation of four well-defined groups-Torpediformes, Rajiformes, Pristiformes, and Myliobatiformes-with branches supported by high bootstrap values (Figure 1). These results are consistent with previous studies based on the analysis of morphological and molecular data [21][22][23][24][25]. One other important finding of the present study was the arrangement of the orders Torpediniformes and Rajiformes close to the root of the tree (Figure 2).…”

“…However, the principal phylogenies based on mitochondrial and nuclear markers indicate that the Rajoidei is the sister group of all the other members of the Batoidea [3,6,16,21,22].…”

“…[20] based on a number of mitochondrial markers and the nuclear RAG1 gene, which contradicts the hypothesis of monophyly for the Triakidae. Similarly, the relationships among the members of the Batoidea remain unclear [15,[21][22][23]. In general, for example, four groups are recognized, the stingrays (Myliobatiformes), electric rays (Torpediniformes), sawfish (Pristiformes), skates, and guitarfish (Rajiformes), although the relationships among these groups are still undefined [21][22][23][24][25].…”

“…Similarly, the relationships among the members of the Batoidea remain unclear [15,[21][22][23]. In general, for example, four groups are recognized, the stingrays (Myliobatiformes), electric rays (Torpediniformes), sawfish (Pristiformes), skates, and guitarfish (Rajiformes), although the relationships among these groups are still undefined [21][22][23][24][25]. In addition, a number of new species of ray have been described recently, and the radiation patterns of this group have been better identified [25][26][27][28][29].…”

Introductory Chapter: The Elasmobranchs as a Fishery Resource

“…The elasmobranchs (sharks, rays, and skates) represent one of the most ancient vertebrate lineages, which have survived a number of diferent mass extinction events over the past 400 million years [1]. Approximately 1200 elasmobranch species are currently found in the world's oceans [2].…”