Researchers use genomic data to map "refuges" where North American trees survived the ice age

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During the last Ice Age, which peaked about 21,500 years ago, glaciers covered large parts of North America, including the Great Lakes region. Once the ice was removed, the land was gradually repopulated with trees that eventually formed dense forests.

But what was the source of the trees that carried out this vast postglacial recolonization? Identify the exact location of these so-called glacial refuges – the places where the ancestors of the forest species of today survived the last ice age – has proved difficult and is currently doing the trick. # 39; object of debate between biologists.

University of Michigan researchers recently reported using a newly developed genetic technique to estimate the precise longitude and latitude of ice age refugia in two widely distributed hickory species, amaranth and amaranth. 'shrub.

Their results confirm the controversial idea that some trees probably survived much further north and closer to the ice cap than is commonly thought. The publication of the study is scheduled for the week of April 8 Proceedings of the National Academy of Sciences.

Researchers looked for signatures of past geographic migrations in the DNA of trees. Their findings for American Walnut confirmed the idea of ​​Nordic Microrefuge, places where local weather conditions may have allowed the persistence of isolated tree populations in an area with generally inhospitable climate. .

"The traditional view is that these tree species only survive in larger refuges located further south, where the regional climate is much warmer," said Jordan Bemmels, the first author of the article. PNAS paper.

"Our findings on American Walnut are among the strongest evidence to date of the existence of Nordic microrefuges and are important for the survival of some species of temperate trees throughout the Ice Age, "said Bemmels, who conducted the study for his doctoral dissertation at the UM Department of Ecology and Evolutionary Biology.

The northern microrefugium sits near the confluence of the Mississippi and Ohio rivers, in an area that today includes the extreme south of Illinois, southeastern Missouri, northeastern Ontario, and northeastern Ontario. Arkansas and the far west of Kentucky.

It turns out that this place is only 160 miles from a site in southwestern Tennessee, near Memphis, where rare, preserved remains of Ice Age hickories have been discovered decades ago. .

Bemmels is now a postdoctoral researcher at the University of Georgia. His co-authors on the PNAS paper are Christopher Dick and Lacey Knowles of the Department of Ecology and Evolutionary Biology of U-M.

Identifying the locations of glacier refuges is important for biologists for several reasons. In addition to helping them understand the basic history of the forest, the information allows them to establish a baseline indicating the speed and distance at which tree species can migrate in response to climate change.

In addition, shelter locations help biologists identify tree populations that may be genetically unique and important for conservation efforts. Recently recolonized northern tree populations are often considered unimportant for the conservation of genetic diversity and the long-term survival of species compared to populations in the South that are believed to be reservoirs of unique genetic diversity .

But Bemmels and his colleagues conclude that "the growing evidence of non-Nordic microrefuge expansion suggests that misconceptions about the management of genetic diversity may need to be revised."

Many areas of refuge have been proposed in eastern North America, including the Gulf Coast, the Atlantic Coastal Plains, the Lower Mississippi Valley, the Appalachians of the south, the Florida Peninsula and central Texas.

Different research tools have been applied to the problem over the years, but they all have limitations. Climate-based models only identify large areas of potential habitat, and traditional studies on the geographic distribution of genealogical lineages provide poor spatial resolution. Fossil pollen records provide some clues about the location of shelters, but they are incomplete for most of eastern North America at the time of the last Ice Age and have been difficult to interpret.

In this study, Bemmels and colleagues exploited range expansion signals from large sets of genomic data, using a simulation-based framework to derive the precise latitude and longitude of glacial refuges in America. North for two hickory species.

The technique used, a data analysis pipeline called X-Origin, was developed by the laboratory of co-author Lacey Knowles of the University of Michigan. It was originally used to study the expansion of pica populations in Alaska, but it has many applications for understanding the geography of rangeland expansion in terrestrial species.

The researchers examined more than 1,000 genetic markers distributed on the genomes of almond hickory and shagbark, from datasets generated by Bemmels for another part of his doctoral dissertation. Genetic material was collected from approximately 150 individuals of each species across the range of both species.

The direction and distance at which trees migrated from their original source population left distinctive traces within their DNA – footprints that can be traced back to the geographical source.

The researchers used a computer-based simulation technique to model the range extension of different refuges and to obtain predictions of the genetic profiles likely to result from these "origins of expansion".

Next, they compared simulated genetic models to true genetic models extracted from the DNA of the tree to drown to identify the most likely scenarios. By repeating the process millions of times, they were able to statistically estimate the latitude and longitude where the ancestors of modern populations survived the last ice age – which was not the case previously for temperate trees.

The inferred location of the glacier refuge for the other species under study, hickory at shagbark, lies in the eastern coastal plain of the Gulf and includes most of the 39, Alabama, Mississippi and southeastern Louisiana. This location corresponds to more traditional proposals of southern refuge.

"The ability to extract details from genomic population data on where species have fled when changing climatic conditions have led to changes in their distribution means that the researcher's toolbox now 39, a powerful way to identify the geographical coordinates of these refuges in all species, "said Knowles.

Co-author Dick added, "Although the alleged northern refuge in the Mississippi Valley is generally harsh and inhospitable during the Ice Age, there were probably milder conditions near the glacier meltwater lakes. – called microclimates – in which some species of temperate trees could persist ".

the PNAS The document is entitled "Genomic evidence of survival near ice cap margins for some, but not all, trees in North America".