Effect of taxon sampling on recovering the phylogeny of squamate reptiles based on complete mitochondrial genome and nuclear gene sequence data

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The complete nucleotide sequences of the mitochondrial (mt) genomes of three species of squamate lizards: Blanus cinereus(Amphisbaenidae),Anguis fragilis(Anguidae), andTarentola mauritanica(Geckkonidae) were determined anew. The deduced amino acid sequences of all 13 mt protein-coding genes were combined into a single data set and phylogenetic relationships among main squamate lineages were analyzed under maximum likelihood (ML) and Bayesian Inference (BI). Within Squamata, the monophyly of Iguanidae, Anguimorpha, Amphisbaenia, Gekkota, Serpentes, and Acrodonta received high statistical support with both methods. It is particularly striking that this is thefirst molecular analysis that recovers the monophyly of Scincomorpha (including Scincidae, Xantusiidae, Cordylidae, and Lacertidae), although it is only supported in the Bayesian analysis, and it is sensitive to changes in the outgroup (see below). Phylogenetic relationships among the main squamate lineages could not be resolved with ML but received strong support with BI (above 95%). The newly reconstructed phylogeny of squamates does not support the Iguania–Scleroglossa split. Acrodonta and Serpentes form a clade, which is the sister group of the remaining squamate lineages.

Within these, Gekkota were thefirst branching out, followed by Amphisbaenia, and a clade including Anguimorpha as sister group of Scincomorpha + Iguanidae. The recovered topology differed substantially from previously reported hypotheses on squamate relationships, and the relative effect of using different outgroups, genes, and taxon samplings were explored. The sister group relationship of Serpentes + Acrodonta, and their relative basal position within Squamata could be due to a long-branch attraction artifact. Phylogenetic relationships among Scincomorpha, Amphisbaenia, and Anguimorpha were found to be rather unresolved. Future improving of squamate phylogenetic relationships would rely onfinding snake and acrodont species with slower mt evolutionary rates, ensuring thorough taxon coverage of squamate diversity, and incorporating more nuclear genes with appropriate evolutionary rates.

Introduction
The molecular phylogeny of land vertebrates is presently among the best documented (Meyer and Zardoya, 2003) owing to newlycompiled large sequence data sets based on mitochondrial (mt) and/ or nuclear genes, as well as on rather thorough lineage samplings. This is particularly true for recently reported amphibian (San Mauro et al., 2005; Frost et al., 2006; Roelants et al., 2007), and mammal (Murphy et al., 2001a,b; Springer et al., 2001) molecular phylogenies, which are relatively robust from a statistical point of view, and will be essential as a framework to any future comparative study pertaining these taxa.
In contrast, our understanding of phylogenetic relationships within the third main lineage of tetrapods, i.e. sauropsids (reptiles + birds) is still emerging because thus far accumulated molecular data for this group are limited as compared to mammals and amphibians. The
classic hypothesis on sauropsid phylogenetic relationships is based on the absence or presence of two skull temporal fenestrae, and
considers a basal split into Anapsida (turtles) and Diapsida (other reptiles + birds), respectively (Meyer and Zardoya, 2003). The latter are further divided into Lepidosauria (squamates + the New Zealand living fossil, the tuatara) and Archosauria (crocodiles + birds). The traditional view of turtles as anapsids (Lee, 2001) has been challenged by several morphological studies suggesting diapsid affinities of turtles (Rieppel and deBraga, 1996; Hill, 2005). Molecular phylogenies (Zardoya and Meyer, 1998; Hedges and Poling, 1999; Kumazawa and Nishida, 1999;

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