Next Monday, Madonna will present a paper on phylogeny and global phylogeography of Angel sharks: 17 (out of 22) species in the genus Squatina. In this paper, the authors first obtained a comprehensive phylogenetic reconstruction and tested biogeographic patterns using a molecular clock. The genus was found to be monophyletic and composed of four main clades. The authors found supports for the effect of both the Thetys Sea
From Stelbrink B, vonRintelen T, Cliff G, Kriwet J; Molecular Phylogenetic and Evolution (2010) 54:395-404
Title: Molecular systematics and global phylogeography of angel sharks (genus Squatina)
Abstract.
Angel sharks of the genus Squatina represent a group comprising 22 extant benthic species inhabiting continental shelves and upper slopes. In the present study, a comprehensive phylogenetic reconstruction of 17 Squatina species based on two mitochondrial markers (COI and 16S rRNA) is provided. The phylogenetic reconstructions are used to test biogeographic patterns. In addition, a molecular clock analysis is conducted to estimate divergence times of the emerged clades. All analyses show Squatina to be monophyletic. Four geographic clades are recognized, of which the Europe–North Africa–Asia clade is probably a result of the Tethys Sea North Atlantic ), probably related to the rise of the Panamanian isthmus. The molecular clock analysis show that both lineage divergences coincide with the estimated time of these two geological events.
From Amanda:
ReplyDeleteThe present paper presents the first comprehensive phylogeography of the squatinid sharks based on two mitochondrial (COI and 16S rRNA) different tree reconstruction methods (Mr Bayes and Beast). In addition, to this they estimated divergence times among the Squatina species and tested biogeographic patterns of inferred species groups (i.e. influence of the rise of the Panamanian isthmus and the Tethys Sea closure on speciation).Interspecific relationships of angel sharks have not been investigated and their position based on morphological characters is still widely debated. The angel shark genus Squatina is comprised of 22 extant, morphologically homogenous, benthic species. Some members of the group have a wide geographic range, whilst some are restricted to smaller areas. They found that evolutionary rates in sharks are generally slower in comparison with warm-blooded vertebrates. The results in the paper base all their findings on the Beast analysis of CO1 and disregard the 16S rRNA data because of saturation for which resulted in asymmetrical trees.
Although it’s a first of its kind they could have used mitochondrial genes in conjunction with nuclear genes to substantiate their results.
The approach to the paper was sound but the dating was not (always better to use several dates and not rely on 1 fossil record, they could have used the sea level changes or tectonic changes to help with the dating). The authors did recognise that specimens from key sampling areas were not collected, which contributed to the difficulties in interpreting their results. The numbering and chronology in the figures are wrong and the standard errors reflected for the statistical analysis are small. The legends are coloured wrongly.
Their hypothesis testing was not solid, the authors discussed the trends/topologies that might have been the cause of the different speciation events but they fail to tie these major events into their dating of divergence times between the different species.