In This Issue

doi:10.1073/iti5214111

Mastodon disappearance in Arctic latitudes

Megafaunal mammals, including the American mastodon, during the last interglacial period, around 125,000 years ago. Image courtesy of George “Rinaldino” Teichmann.

Previous radiocarbon dating studies estimated that the last mastodons in Arctic latitudes of northwestern North America disappeared around 18,000 years ago, after the onset of the most recent glacial maximum. Because evidence suggests that mastodons preferred woody forests and lowland swamps over sparse tundra, Grant Zazula et al. (pp. 18460–18465) reexamined the ages of mastodon bones from the Beringia region of eastern Alaska and western Yukon. Using collagen ultrafiltration and single amino acid dating methods, the authors found that mastodon specimens from Beringia consistently dated to more than 50,000 years old, beyond the limits of radiocarbon dating. The authors suggest that mastodons may have been extirpated from Beringia around 75,000 years ago, near the end of an interglacial period. Further ecological and climate changes in temperate latitudes may have led to total extinction of the species around 10,000 years ago. The results suggest that mastodons may have disappeared from the region around the Bering Land Bridge more than 50,000 years before humans migrated from Asia to the Americas, and that the previously derived young radiocarbon ages may have been due to museum preservation techniques or other forms of modern contamination, according to the authors. — P.G.

Policy goals for the Gulf of Mexico’s hypoxic zone

Targeted pattern of cropland conservation investments to reduce the Gulf’s hypoxia.

The Gulf of Mexico’s summer hypoxic zone is the second largest in the world, driven by the flow of nitrogen and phosphorus from cropland in the Mississippi–Atchafalaya River Basin. To reduce the size of the zone to 5,000 km², a goal set by the national Gulf of Mexico Task Force’s Action Plan, researchers seek strategies to mitigate the flow of these nutrients into the Gulf. Sergey Rabotyagov et al. (pp. 18530–18535) present an integrated assessment model that links a hypoxic zone model with the water quality effects of cropland conservation investment decisions, examined across more than 550 agricultural subwatersheds that deliver nutrients into the Gulf. The authors simulated the impact of cropland conservation investments on nutrient delivery to the Gulf and applied an evolutionary algorithm to select the most cost-effective location and treatment extent within each subwatershed. The authors identified the tradeoffs between the expected costs of agricultural conservation treatments and the expected size of the Gulf’s hypoxia across the historical range of variability. Based on their findings, the Action Plan goal can be achieved on average for an estimated cost of $2.7 billion annually, assuming established conservation practices and a recent pattern of cropping practices, according to the authors. — T.J.

Equid genomes and speciation theories

Although the richly detailed equine fossil record is often cited as an example of evolution, the succession of events that led to the diversity of modern-day equids remains unclear. To fill in these animals’ evolutionary histories, currently represented only by the genetic histories of the horse and domestic donkey, Hákon Jónsson et al. (pp. 18655–18660) sequenced the genomes of each living species of ass and zebra, thereby completing the genomic record for extant species within the genus. Additionally, the authors used a museum specimen to characterize the genome of the quagga zebra, extinct since the early 1900s. By scanning the genomes, the authors identified 48 adapted genes involved in characteristics such as olfaction, immune response, and locomotion. Furthermore, the genome sequences reveal a highly dynamic demographic history with synchronous expansions and collapses on different continents following climate change. The authors identified instances of hybridization despite extremely divergent chromosomal structures, a feature thought to prevent reproduction between species. The findings suggest that equids may serve as a model for how chromosomal structure and gene flow interact to influence speciation. — T.J.

How the plague bacterium coopted fleas

SEM of Yersinia pestis on proventricular spines of a flea. Image courtesy of National Institute of Allergy and Infectious Diseases.

The flea-borne plague bacterium Yersinia pestis blooms into a biofilm in a valve in the gut of fleas, inducing infected fleas to feed repeatedly by impeding their intake of blood meals from human hosts; the successive feeding attempts help transmit the bacterium to the host. Iman Chouikha and B. Joseph Hinnebusch (pp. 18709–18714) explored how Y. pestis adopted a flea-borne transmission route, given that its close evolutionary cousin, the enteric pathogen, Yersinia pseudotuberculosis, from which Y. pestis diverged less than 6,400 years ago, is toxic to fleas and transmitted through the fecal–oral route. The authors report that a urease enzyme, which hydrolyzes urea, is responsible for the oral toxicity of Y. pseudotuberculosis to fleas; the corresponding gene in Y. pestis strains is mutated, abrogating the enzyme’s activity. Deleting the gene from Y. pseudotuberculosis rendered the bacterium nontoxic to fleas, whereas restoring a functional urease ureD allele to Y. pestis made the plague bacterium toxic to fleas, suggesting the enzyme’s pivotal role in the plague bacterium’s ability to propagate through fleas. Because urease-induced mortality can wipe out more than one-third of infected flea vectors, the mutant gene may have been subject to strong positive selection in favor of transmissibility during the evolution of the plague bacterium, according to the authors. — P.N.

Hippocampus storage capacity and memory retrieval

A brain region called the hippocampus is thought to store and accurately retrieve a large number of similar experiences through the activation of combinations of neurons. However, evidence for high-capacity storage in this brain region has been lacking. To determine how animals can accurately remember specific experiences without being confused by similar memories, Charlotte Alme et al. (pp. 18428–18435) tested the extent to which exposure to a large number of similar environments generated activity in distinct sets of hippocampal neurons called place cells, which fire when an animal is at a certain location. Over 2 consecutive days, the authors recorded the activity of place cells as rats chased food crumbles in 11 square boxes located in different rooms, which resembled one another in shape and size. While exposure to the same room at different times produced highly similar patterns of neuronal activity, exposure to different rooms activated distinct, largely nonoverlapping sets of place cells. According to the authors, the findings suggest that the storage capacity of the hippocampus is large, and that the lack of overlap between neuronal representations of different environments could enable animals to accurately remember an experience without interference from similar memories. — J.W.

Genetics of horse domestication

Przewalski’s horse.

Investigations into the genetic underpinnings of horse domestication have been limited by a lack of closely related wild horse species for comparison. Mikkel Schubert et al. (pp. E5661–E5669) sequenced the genomes of ancient horse specimens from Russia that date to around 16,000 and 43,000 years before present, well before horses were domesticated around 5,500 years ago. The authors compared the genomes with five modern domesticated breeds and the distantly related Przewalski’s horse, the only living wild horse species. The comparisons revealed that the ancient specimens share more genetic similarities with modern domesticated breeds than with the Przewalski’s horse, suggesting that domesticated breeds derived at least partially from the ancient populations. The authors also identified groups of genes that were favored during domestication, including genes involved in muscle, limb, joint, and cardiac system development that illustrate physical adaptations that might have resulted from human use. In addition, changes in genes associated with social behavior, learning capabilities, fear response, and agreeableness may illustrate the taming process. Modern horse breed genomes also contained a large number of deleterious gene mutations, likely due to inbreeding, which represents the cost of domestication. According to the authors, the results help understand the genetic basis of horse domestication. — J.P.J.