Turtles ‘closer relative to birds than snake and lizards’

Washington, May 27: Researchers have confirmed that the turtles are most closely associated to crocodilians and birds rather than to lizards, snakes, and tuataras.

Paleontological and morphological studies place turtles as either evolving from the ancestor of all reptiles or as evolving from the ancestor of snakes, lizards, and tuataras. Conflictingly, genetic studies place turtles as evolving from the ancestor of crocodilians and birds.

By showing that turtles are closely related to crocodiles and birds (Archosaurs) than lizards, snakes and tuatara (Lepidosaurs), the study by a team of Boston University researchers has challenged previous anatomical and Paleontological assessments.

Nick Crawford, a post-graduate researcher in biology in BU’s Graduate School of Arts ‘n’ Sciences and lead author of the study, attained these findings by using computational analysis to examine regions of the different animals’ genomes.

“Turtles have been an enigmatic vertebrate group for a long time and morphological studies placed them as either most closely related to the ancestral reptiles, that diverged early in the reptile evolutionary tree, or as closer to lizards, snakes, and tuataras,” said Crawford.

The study is the first genomic-scale analysis, addressing the Phylogenetic position of turtles, using over 1000 loci from representatives of all major reptile lineages which includes tuatara (lizard-like reptiles found only in New Zealand).

Previous studies of morphological traits positioned turtles at the base of the reptile tree with lizards, snakes and tuatara (Lepidosaurs), whereas molecular analyses typically allied turtles with crocodiles and birds (Archosaurs).

The BU researchers have challenged a recent analysis of shared microRNA families that suggested that turtles are more closely linked to Lepidosaurs.

They did this with data from many single-copy nuclear loci dispersed throughout the genome, using sequence capture, high-throughput sequencing and published genomes to obtain sequences from 1145 ultra conserved elements (UCEs) and their variable flanking DNA.

The resulting phylogeny provides overwhelming support for the hypothesis that turtles have evolved from a common ancestor of birds and crocodilians, rejecting the hypothesized association between turtles and Lepidosaurs.

The researchers used UCEs because they are easily aligned portions of extremely divergent genomes, and allow many loci to be interrogated across evolutionary timescales.

Because sequence variability within UCEs increases with distance from the core of the targeted UCE, it suggests that Phylogenetically informative content in flanking regions can inform hypotheses spanning different evolutionary timescales.

The combination of taxonomic sampling, the genome-wide scale of the sampling and the robust results obtained, regardless of the analytical method, points that the turtle-Archosaur relationship is unlikely to be caused by long-branch attraction or other analytical artifacts.

The BU study is the first to design a well-resolved reptile tree that includes the tuatara and multiple loci, and also is the first to investigate the placement of turtles within reptiles using a genomic-scale analysis of single-copy DNA sequences and a complete sampling of the major relevant evolutionary lineages.

Because UCEs are conserved across most vertebrate groups and found in groups including yeast and insects, this framework can be generalized beyond this study and relevant to resolving ancient Phylogenetic enigmas throughout the tree of life.

This approach to high throughput Phylogenomics-based on thousands of loci-is likely to basically change the way that systematists gather and analyse data. (ANI)

This study has been published in the Royal Society journal Biology Letters. (ANI)