Researchers at Washington State University’s Mount Vernon Northwestern Washington Research & Extension Center demonstrated in a new paper that some strains of wild spinach are resistant to Fusarium wilt, a fungal disease that is a persistent problem for growers of commercial spinach seed, and they identified regions of the plants’ genome associated with that resistance.

The findings are important for seed growers in western Washington and Oregon, where a significant portion of the world’s spinach seed is grown and where the pathogen has long been a problem due to the acidic soils.

“We were very, very pleased we found some excellent resistance when we did the screening and then we followed up with the DNA sequencing and looking at where that resistance might be lying,” said Lindsey du Toit, a plant pathologist who has worked on fighting disease in seed crops for 25 years at WSU’s Mount Vernon NWREC.

Though the new paper, published this month in Scientific Reports, identified several varieties of wild spinach associated with resistance to Fusarium wilt, more study is needed to understand the genetic nature of the resistance. However, seed companies don’t have to wait to apply the findings — they can begin breeding hybrids with the wild spinach varieties that showed resistance.

“You don’t necessarily have to understand the mechanism of resistance in order to use it,” du Toit said. “This is a tool that’s available immediately to breeding programs.”

Spinach consumption has been growing dramatically around the world. In the U.S., the per-capita consumption of the vitamin-rich vegetable has more than doubled in the past 20 years, with a particularly strong market for baby leaf spinach.

Most of the domestic crop is grown in hot, dry regions such as California, Texas and Florida. But growing spinach seed requires a rare combination of seasonal conditions — long, dry summers that aren’t too hot. As a result, around a fifth of the world’s spinach seed is grown in the Pacific Northwest.

But those crops have little resistance to Fusarium wilt, which afflicts spinach by entering through the roots and blocking their ability to take up water. Seed growers have tried to manage this problem by rotating spinach crops on long timeline — a decade or more between plantings — and taking other measures to treat the soil with calcium carbonate to reduce the acidity.

Even so, the prospect of an expensive “wipeout” of an entire crop has remained a continual threat.

In the current study, du Toit and a former post-doctoral researcher in her lab, Sanjaya Gyawali, screened 68 varieties of wild spinach from the region where the plant originated — Uzbekistan and Tajikistan — and compared them to 16 cultivated varieties. Researchers from the University of Arkansas also participated in the study.

They found strong resistance to the pathogen in several wild varieties. They then identified the chromosomal locations associated with the most powerful resistance. Those locations — known as quantitative trait loci — can be used by breeders to introduce more resistance to Fusarium wilt into commercial lines using marker-assisted selection, a technique that uses DNA markers to select for desirable traits.

The work was funded in part by the Specialty Crop Research Initiative of the U.S. Department of Agriculture’s National Institute of Food and Agriculture. The project was also supported by WSU CAHNRS Hatch Projects, and the Alfred Christianson Endowment in Vegetable Seed Science.

University of California, Berkeley, astronomers have now discovered the first instance of a massive black hole tearing apart a star thousands of light years from the galaxy’s core, which itself contains a massive black hole.

The off-center black hole, which has a mass about 1 million times that of the sun, was hiding in the outer regions of the galaxy’s central bulge, but revealed itself through bursts of light generated by the spaghettification of the star — a so-called tidal disruption event, or TDE. In a TDE, the immense gravity of a black hole tugs on a star — similar to the way the moon raises ocean tides on Earth, but a lot more violently.

“The classic location where you expect massive black holes to be in a galaxy is in the center, like our Sag A* at the center of the Milky Way,” said Yuhan Yao, a Miller Postdoctoral Fellow at UC Berkeley who is lead author of a paper about the discovery recently accepted for publication in The Astrophysical Journal Letters (ApJL). “That’s where people normally search for tidal disruption events. But this one, it’s not at the center. It’s actually about 2,600 light years away. That’s the first optically discovered off-nuclear TDE discovered.”

The galaxy’s central massive black hole, about 100 million times the mass of our sun, is also gorging itself, but on gas that has gotten too close to escape.

Studies of massive black holes at galactic centers tell astronomers about the evolution of galaxies like our own, which has one central black hole — called SagA* because of its location within the constellation Sagittarius — weighing in at a puny 4 million solar masses. Some of the largest galaxies have central black holes weighing several 100 billion solar masses, presumably the result of the merger of many smaller black holes.

Finding two massive black holes in the center of a galaxy is not surprising. Most large galaxies are thought to have massive black holes in their cores, and since galaxies often collide and merge as they move through space, large galaxies should occasionally harbor more than one supermassive black hole — at least until they collide and merge into an even bigger black hole. They typically hide in stealth mode until they reveal their presence by grabbing nearby stars or gas clouds, creating a short-lived burst of light. These are rare events, however. Astronomers calculate that a massive black hole would encounter a star once every 30,000 years, on average.

The new TDE, dubbed AT2024tvd, was detected by the Zwicky Transient Facility, an optical camera mounted on a telescope at Palomar Observatory near San Diego, and confirmed by observations with radio, X-ray and other optical telescopes, including NASA’s Hubble Space Telescope.

“Massive black holes are always at the centers of galaxies, but we know that galaxies merge — that is how galaxies grow. And when you have two galaxies that come together and become one, you have multiple black holes,” said co-author Ryan Chornock, a UC Berkeley associate adjunct professor of astronomy. “Now, what happens? We expect they eventually come together, but theorists have predicted that there should be a population of black holes that are roaming around inside galaxies.”

The discovery of one such roaming black hole shows that systematic searches for the signature of a TDE could turn up more rogue black holes. The find also validates plans for a space mission called LISA — the Laser Interferometer Space Antenna — that will look for gravitational waves from mergers of massive black holes like these.

“This is the first time that we actually see massive black holes being so close using TDEs,” said co-author Raffaella Margutti, a UC Berkeley associate professor of astronomy and of physics. “If these are a couple of supermassive black holes that are getting closer together — which is not necessarily true — but if they are, they might merge and emit gravitational waves that we’ll see in the future with LISA.”

LISA will complement ground-based gravitational wave detectors, such as LIGO and Virgo, which are sensitive to the merger of black holes or neutron stars weighing less than a few hundred times the mass of our sun, and telescopic studies of pulsar flashes, such as the Nanograv pulsar timing array experiment, which are sensitive to gravitational waves from the mergers of supermassive black holes weighing billions of solar masses. LISA’s sweet spot is black holes of several million solar masses. LISA is slated to be launched in the next decade.

Transient outbursts

Because black holes are invisible, scientists can only find them by detecting the light produced when they shred stars or gas clouds and create a bright, hot, rotating disk of material that gradually falls inward. TDEs are powerful probes of black hole accretion physics, Chornock said, revealing how close material can get to the black hole before being captured and the conditions necessary for black holes to launch powerful jets and winds.

The most productive search for TDEs has used data from the Zwicky Transient Facility, originally built to detect supernova explosions, but also sensitive to other flashes in the sky.

The ZTF has discovered nearly 100 TDEs since 2018, all within the cores of galaxies. X-ray satellites have also detected a few TDEs, including two in the outskirts of a galaxy that also has a central black hole. In those galaxies, however, the black holes are too far apart to ever merge. The newly discovered black hole is close enough to the core’s massive black hole to potentially fall toward it and merge, though not for billions of years.

Yao noted that two alternative scenarios could explain the presence of the wandering black hole in AT2024tvd. It could be from the core of a small galaxy that merged with the larger galaxy long ago and is either moving through the larger galaxy on its way out or has become bound to the galaxy in an orbit that may, eventually, bring it close enough to merge with the black hole at the core.

Erica Hammerstein, another UC Berkeley postdoctoral researcher, scrutinized the Hubble images as part of the study, but was unable to find evidence of a past galaxy merger.

AT2024tvd could also be a former member of a triplet of black holes that used to be at the galactic core. Because of the chaotic nature of three-body orbits, one would have been kicked out of the core to wander around the galaxy.

Searching galaxies for off-center black holes

Because the ZTF detects hundreds of flashes of light around the northern sky each year, TDE searches to date have focused on flashes discovered near the cores of galaxies, Yao said. She and Chornock created an algorithm to distinguish between the light produced by a supernova and a TDE, and employed it to search through the 10,000 or so detections by ZTF to date to find bursts of light in the galactic center that fit the characteristics of a TDE.

“Supernovae cool down after they peak, and their color becomes redder,” Yao said. “TDEs remain hot for months or years and have consistently blue colors throughout their evolution.”

TDEs also exhibit broad emission lines of hydrogen, helium, carbon, nitrogen and silicon.

Last August, the Berkeley team discovered a burp of light that looked like a TDE, but its location seemed off-center, though within the resolution limits of the ZTF. The researchers suspected the black hole was indeed off center, and immediately requested time on several telescopes to pinpoint its location. These included NASA’s Chandra X-ray Observatory, the Very Large Array and the Hubble Space Telescope. They all confirmed its off-nucleus location, with HST providing a distance of about 2,600 light years — about one-tenth the distance between our sun and Sag A*.

Though close to the central black hole, the off-nuclear black hole is not gravitationally bound to it. Because the black hole at the core spews out energy as it accretes infalling gas, it is categorized as an active galactic nucleus.

Yao and her team hope to find other roaming TDEs, which will give astronomers an idea of how often galaxies and their core black holes merge, and thus how long it takes to form some of the extreme, supermassive black holes.

“AT2024tvd is the first offset TDE captured by optical sky surveys, and it opens up the entire possibility of uncovering this elusive population of wandering black holes with future sky surveys,” Yao said. “Right now, theorists haven’t given much attention to offset TDEs. They primarily predict rates for TDEs occurring at the centers of galaxies. I think this discovery really motivates them to compute rates for offset TDEs, as well.”

The 34 co-authors who contributed to the paper come from institutions in the United States, United Kingdom, Sweden, Russia, Germany, Australia and the Netherlands. ZTF is a public-private partnership, with equal support from the ZTF Partnership and the U.S. National Science Foundation.

Peering back in time to when the universe was younger than the Earth is now, the images span the period from around twelve billion years ago until one billion years ago. The new catalogue of images, soon to be published in the journal Astronomy and Astrophysics (A&A), includes nearly 1,700 galaxy groups. The research group’s impressive image of a galaxy cluster over 6 billion light years away is currently showcased as the European Space Agency’s (ESA) picture of the month.

‘We’re able to actually observe some of the first galaxies formed in the universe,’ says Ghassem Gozaliasl of Aalto University, and head of the galaxy groups detection team who led the study. ‘We detected 1,678 galaxy groups or proto-clusters — the largest and deepest sample of galaxy groups ever detected — with the James Webb Space telescope. With this sample, we can study the evolution of galaxies in groups over the past 12 billion years of cosmic time.’

The James Webb Space Telescope began operating in 2022. The largest telescope in space, its higher resolution and greater sensitivity have enabled astronomers to see farther and better than ever before. Because light travels at a finite speed, the further away an object is, the further back in time our image of it. By observing very faint, very distant galaxies — the faintest galaxies in this dataset are one billion times dimmer than the human eye can see — the team got a glimpse of what galaxies looked like in the early universe.

Galaxy groups and clusters are rich environments filled with dark matter, hot gas, and massive central galaxies that often host supermassive black holes, explains Gozaliasl. ‘The complex interactions between these components play a crucial role in shaping the life cycles of galaxies and driving the evolution of the groups and clusters themselves. By uncovering a more complete history of these cosmic structures, we can better understand how these processes have influenced the formation and growth of both massive galaxies and the largest structures in the universe.’

Cosmic family history

Galaxies aren’t scattered evenly throughout the universe. Instead, they cluster in dense regions connected by filaments and walls, forming a vast structure known as the cosmic web. Truly isolated galaxies are rare — most reside in galaxy groups, which typically contain anywhere from three to a few dozen galaxies, or in larger galaxy clusters, which can include hundreds or even thousands of galaxies bound together by gravity. Our own Milky Way is part of a small galaxy group known as the Local Group, which includes the Andromeda Galaxy and dozens of smaller galaxies.

‘Like humans, galaxies come together and make families,’ explains Gozaliasl. ‘Groups and clusters are really important, because within them galaxies can interact and merge together, resulting in the transformation of galaxy structure and morphology. Studying these environments also helps us understand the role of dark matter, feedback from supermassive black holes, and the thermal history of the hot gas that fills the space between galaxies.’

Because the new catalog includes observations that span from one billion to twelve billion years ago, scientists can compare some of the earliest structures in the universe with relatively modern ones to learn more about galaxy groups and how they evolve. Studying the history of galaxy groups can also help astronomers understand how the giant, brightest group galaxies (BGGs) at their centres form through repeated mergers — an area explored in depth across several of Gozaliasl’s recent publications.

‘When we look very deep into the universe, the galaxies have more irregular shapes and are forming many stars. Closer to our time, star formation is what we refer to as ‘quenched’ — the galaxies have more symmetric structures, like elliptical or spiral galaxies. It’s really exciting to see the shapes changing over cosmic time. We can start to address so many questions about what happened in the universe and how galaxies evolved,’ says Gozaliasl.

For years, scientists have spied strange streaks running down Martian cliffsides and crater walls. Some have interpreted those streaks as liquid flows, suggesting the possibility of currently habitable environments on the Red Planet. But this new study, which used machine learning to create and analyze a massive dataset of slope streak features, points to a different explanation: dry process related to wind and dust activity.

“A big focus of Mars research is understanding modern-day processes on Mars — including the possibility of liquid water on the surface,” said Adomas Valantinas, a postdoctoral researcher at Brown who coauthored the research with Valentin Bickel, a researcher at Bern. “Our study reviewed these features but found no evidence of water. Our model favors dry formation processes.”

The research was published in Nature Communications on Monday, May 19.

Scientists first saw the odd streaks in images returned from NASA’s Viking mission in the 1970s. The sinewy features are generally darker in hue than the surrounding terrain and extend for hundreds of meters down sloped terrain. Some last for years or decades, while others come and go more quickly. The shorter-lived features — dubbed recurring slope lineae (RSL) — seem to show up in the same locations during the warmest periods of the Martian year.

The origin of the streaks has been a hot topic among planetary scientists. Modern Mars is remarkably dry, and temperatures rarely peak above freezing. Still, it’s possible that small amounts of water — perhaps sourced from buried ice, subsurface aquifers or abnormally humid air — could mix with enough salt to create a flow even on the frozen Martian surface. If true, RSLs and slope streaks could mark rare, habitable niches on a desert world.

Other researchers haven’t been convinced. They contend the streaks are triggered by dry processes like rock falls or wind gusts, and only appear liquid-like in orbital images.

Hoping for new insights, Bickel and Valantinas turned to a machine learning algorithm to catalog as many slope streaks as they could. After training their algorithm on confirmed slope streak sightings, they used it to scan more than 86,000 high-resolution satellite images. The result was a first-of-its-kind global Martian map of slope streaks containing more than 500,000 streak features.

“Once we had this global map, we could compare it to databases and catalogs of other things like temperature, wind speed, hydration, rock slide activity and other factors.” Bickel said. “Then we could look for correlations over hundreds of thousands of cases to better understand the conditions under which these features form.”

This geostatistical analysis showed that slope streaks and RSLs are not generally associated with factors that suggest a liquid or frost origin, such as a specific slope orientation, high surface temperature fluctuations or high humidity. Instead, the study found that both features are more likely to form in places with above average wind speed and dust deposition — factors that point to a dry origin.

The researchers conclude that the streaks most likely form when layers of fine dust suddenly slide off steep slopes. The specific triggers may vary. Slope streaks appear more common near recent impact craters, where shockwaves might shake loose surface dust. RSLs, meanwhile, are more often found in places where dust devils or rockfalls are frequent.

Taken together, the results cast new doubt on slope streaks and RSLs as habitable environments.

That has significant implications for future Mars exploration. While habitable environments might sound like good exploration targets, NASA would rather keep its distance. Any Earthly microbes that may have hitched a ride on a spacecraft could contaminate habitable Martian environments, complicating the search for Mars-based life. This study suggests that the contamination risk at slope streak sites isn’t much of a concern.

“That’s the advantage of this big data approach,” Valantinas said. “It helps us to rule out some hypotheses from orbit before we send spacecraft to explore.”