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American Mastodon Mitochondrial Genomes Reveal Multiple Lineages

NEW YORK – An international team of researchers has conducted a phylogeographic study of the extinct American mastodon (Mammut americanum) based on 35 complete mitochondrial genomes, finding that there were multiple lineages that existed within the species, including two distinct clades from eastern Beringia.

In a study published on Tuesday in Nature Communications, the researchers further noted that their data showed these clades arose at different times, suggesting a pattern of repeated northern expansion and localized extinctions in response to glacial cycling. They also found that there were lower levels of genetic diversity among northern mastodons than in endemic clades south of the continental ice sheets.

"The results of our study highlight the complex relationships between population dispersals and climate change, and can provide testable hypotheses for extant species expected to experience substantial biogeographic impacts from rising temperatures," the authors wrote.

According to recent paleontological research, mastodons and mammoths displayed contrasting responses to cyclical glacial-interglacial climatic shifts, the investigators said. Temporal analyses of mastodon distribution patterns within the American midcontinent and eastern Beringia, which is known as present-day unglaciated areas of Alaska and the Yukon, have inferred that American mastodons briefly expanded into high latitudes during the last interglaciation, but underwent regional extinction when climates became much colder during the last glaciation, surviving thereafter only in lower-latitude temperate regions in North America. These local extinction events for mastodons were likely caused by climate-driven changes in vegetation at the onset of glaciation, which favored the spatial expansion of mammoths and other grazing species adapted to steppe-tundra geographies.

In order to test these inferences more empirically, the researchers used detailed phylogeographic analysis and Bayesian clock dating of American mastodon mitochondrial genomes. They obtained subsamples from fossil bones and teeth of American mastodons from museums, universities, and government institutions across North America and sequenced complete mitochondrial genomes from 33 of 122 specimens. The researchers also obtained partial sequences from another 12 specimens, but these were excluded from all subsequent analyses.

They identified five well-supported major clades, which they named by the approximate geographic provenance of their constituent specimens (A Alaska, Y Yukon, G Great Lakes, M Mexico, L Alberta/Missouri). They also included two specimens from Virginia in Clade G, due to the lower support for their monophyly, and tentatively assigned a single specimen from Nova Scotia to a separate Clade N.

After further analyses, the researchers identified two independent and genetically divergent clades (A and Y) that consisted primarily of specimens from eastern Beringia. Clade Y was grouped with Clades G, L, and N, and diverged from Clade A between 1.37 million years ago and 609,000 years ago.

The researchers also found that American mastodons repeatedly expanded into northern latitudes in response to interglacial warming.  However, they noted, the northern clades had extremely low levels of genetic diversity.

"Under a model of repeated expansion and extirpation, northern clades of mastodons would be expected to have lower levels of genetic diversity," the authors wrote. "This pattern would be consistent with repeated expansion of small founder matriarchal herds in response to climatic warming during interglaciations, and the transient nature of their occupation of northern latitudes. Genetic diversity is expected to be higher among samples from regions south of the continental ice sheets that were likely to have been inhabited by populations of mastodons throughout the Pleistocene."

In order to test this hypothesis, they examined levels of nucleotide diversity within their genomic dataset. They found that Clade Y had low nucleotide diversity, and that the genetic distance between the two mastodons in Clade A was also very small. By comparison, there were much higher levels of nucleotide diversity in clade G, which contained mastodons from south of their endemic range on the ice sheets.

This was consistent with expectations of small numbers of matrilocal mastodons expanding northward in response to glacial retreat, and also supported previous paleo-ecological models of environments inhabited by northern mastodons, the investigators noted.

"The phylogeographical history of Pleistocene megafauna can serve as a useful example for understanding the ecological responses of present-day species, and can generate testable hypotheses about the consequences of anthropogenic environmental impacts," the authors concluded.