30,000-year-old bison bone DNA yields new clues about evolution
Thirty-thousand-year-old late Pleistocene bison bones discovered in permafrost at a Canadian goldmine have provided scientists the opportunity to establish means for determining how animals adapt to rapid environment change.
A study by the University of Adelaide's Australian Centre for Ancient DNA (ACAD), in collaboration with Sydney's Victor Chang Cardiac Research Institute, published by the online journal PLoS One
, carried out an analysis of special genetic states associated with turning genes on and off without changing DNA sequence. These type of changes that scientists term "epigenetic" changes may occur rapidly between generations, faster than the "standard evolutionary processes" change.
Some scientists believe epigenetic changes
may have played a key role in animal adaptation to rapid climate change.
According to the researchers
, epigenetic changes to gene expression may result in changes in heritable physical characteristics that are not encoded in DNA. Epigenetic changes can be influenced by environmental factors and can effect changes in physical characteristics for several generations. This, according to the researchers, raises the possibility that "epigenetic states," which do not involve changes in DNA sequences, being biochemical modifications (such as "base methylation") to DNA or associated proteins, may be acted upon by natural selection and may be involved in rapid evolutionary adaptation of species to changes in environment.
The researchers tested the hypothesis that epigenetic changes are stable and heritable and may, therefore, provide substrate for natural selection to act upon by using DNA from the 30,000-year-old bison (Bison priscus
) bones to measure stability of "epigenetic states" on evolutionary time scales. Their tests showed that epigenetic states defined by "methylation patterns" of the organic base cytosine in ancient bison DNA were identical to organic base methylation patterns in fresh bovine tissue, showing that epigenetic states can maintain stability over extensive time frames. This, according to researchers, provides the first evidence that cytosine methylation (epigenetic) states are retained in DNA from ancient specimens and provide a "powerful means to study the role of epigenetics in evolution" in future studies.
Epigenetics, a challenge to standard theories of evolution?
According to the project leader Professor Alan Cooper, Director of ACAD: "Epigenetics is challenging some of our standard views of evolutionary adaptation, and the way we think about how animals use and inherit their DNA. In theory, such systems would be invaluable for a wide range of rapid evolutionary adaptation but it has not been possible to measure how or whether they are used in nature, or over evolutionary timescales."
The challenge arises from the fact that the neo-Darwinian synthesis
, as it is called, has always maintained, as an aspect of "central dogma" that evolution is powered by random mutations occurring independent of environmental factors. The environment only serves to select those beneficial mutations that arise. But with the possibility that epigenetic changes may have played a major role in evolution arises in turn the possibility that evolution may occur by inbuilt environmental cue sensitive mechanisms. In the words of the researchers
: "Interposed between genes and environment, epigenetic modifications can be influenced by environmental factors to affect phenotype for multiple generations."
reports that Dr. Catherine Suter of the Victor Chang Institute is a specialist in epigenetics. She has been studying epigenetic adaptation in laboratory animals. The discovery of 30,000-year-old permafrost bison bones provided her opportunity to apply techniques used in epigenetic studies of laboratory animals to ancient DNA. Suter said: "This is the first step towards testing the idea that epigenetics has driven evolution in natural populations."
According to Cooper, "The climate record shows that very rapid change has been a persistent feature of the recent past, and organisms would need to adapt to these changes in their environment equally quickly. Standard mutation and selection processes are likely to be too slow in many of these situations." Cooper is interested in finding out the extent to which epigenetic changes have played a role in evolution of species.
Bastien Llamas, ACAD senior researcher said, "Standard genetic tests do not detect epigenetic changes, because the actual DNA sequence is the same. However, we were able to use special methods to show that epigenetic sites in this extinct species were comparable to modern cattle.There is growing interest in the potential evolutionary role of epigenetic changes, but to truly demonstrate this will require studies of past populations as they experience major environmental changes."