New genetic research has potentially cracked the long unsolved mystery of the turtle's evolutionary origins by determining that turtles and lizards have a close common ancestor, according to biology professor Kevin Peterson.
Peterson, who coauthored the study published in the July 2011 issue of Biology Letters, said he and his colleagues used a new methodology that involved analysis of microRNAs short regulatory genetic molecules found in cells that develop over millions of years of evolution instead of fossil evidence, as previous researchers have done.
"My interest is trying to address difficult phylogenetic problems and that would include turtles," Peterson said. "Their morphology is so enigmatic and they are so derived that it is difficult to see who their nearest relative is."
Previous research methods involved studying morphological data, or the physical characteristics of fossils and current species of turtles, to determine their phylogenetic tree, according to the study.
Categorizing turtles is difficult because they display great physical differences from other reptiles such as lizards, according to the study.
Most lizards fall in the Diapsida clade, which consists of animals with temporal fenestrae, which are two holes in their skull that house muscles attached to the jaw, according to the study. Turtles' lack of this feature in fossil evidence suggests that they are likely related to another reptile that is older than the Diapsids. Additionally, they could be related to another earlier taxonomical group entirely, such as anapsids or synapsids.
Developments in the field of evolutionary biology have led to a better understanding of how the genome evolved, according to Alysha Heimberg GR '11 who researched with Peterson. The long-standing question of whether turtles were anapsids, syapsids or diapsids will be answered by microanalysis, she said.
Peterson's analysis of MicroRNAs allowed him to categorize turtles more accurately than the previous fossil analysis methods, he said. MicroRNAs have a very specific set of properties in the way that they evolve that make them good phylogenetic markers, Heimberg said.
"Animals that are more closely related to one another share the same microRNA genes, so you can start to determine the relationship between animals just based on what microRNAs they share," she said. "They are the most highly conserved genes in the genome."
Because of this conservation, Peterson, Heimberg and their colleagues decided that microRNA analysis would be the best methodology in determining where turtles should be grouped among tetrapods. Previously, it was unclear whether turtles should be in the anapsid taxa, in which animals have ancestors with unfenestrated skulls, the syapsid taxa, in which many mammals with one major fenestra are placed, or in the diapsid taxa, from which they were originally eliminated.
Genetic analysis of turtles and lizards yielded results that implied that the turtles' ancestors had temporal fenestrae in their skulls, though they were lost in the evolutionary process, Heimberg said.
"We found that there were four microRNA genes that were shared among turtles and lizards that weren't shared in another taxonomical group," Heimberg said. "This suggested that turtles and lizards were most closely related."
MicroRNA data had never before been applied to this question of turtles' relationships with other groups, Heimberg said.
Now that this new research method has proved successful, Peterson said he is currently working on "three or four" more projects that involve microRNA analysis.
This study was titled "MicroRNAs Support a Turtle + Lizard Clade."
