It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Monday, July 05, 2021
Belowground microbial solutions to aboveground plant problems
IMAGE: PRIORITY TO MICROBIOTA-INDUCED PLANT GROWTH OVER DEFENSE, UNDER LOW LIGHT CONDITIONS. IMAGE CREATED WITH BIORENDER. view more
CREDIT: STÉPHANE HACQUARD
Land plants - plants that live primarily in terrestrial habitats and form vegetation on earth - are anchored to the ground through their roots, and their performance depends on both the belowground soil conditions and the aboveground climate. Plants utilize sunlight to grow through the process of photosynthesis where light energy is converted to chemical energy in chloroplasts, the powerhouses of plant cells. Therefore, the amount and quality of light perceived by chloroplasts through light absorbing pigments, such as chlorophyll, is a defining factor in plant growth and health. A substantial amount of the chemical compounds produced during the conversion of light energy to chemical energy, termed photoassimilates (mainly sugars), is translocated to the plant root compartment and invested in the surrounding soil to sustain microbial growth. Consequently, roots harbour complex microbial communities of bacteria and filamentous eukaryotes (i.e., fungi and oomycetes), and the composition of these communities profoundly influences plant performance. However, the extent to which plants can take advantage of belowground microbes to orchestrate aboveground stress responses remains largely unexplored. Now, in a new study published in Nature Plants, Stéphane Hacquard and his colleagues from the Department of Plant-Microbe Interactions at the MPIPZ in Cologne, Germany, shed light on these aboveground-belowground connections.
To tackle this question, the first author of the study Shiji Hou performed experiments where the aboveground light conditions and the belowground microbial conditions could be controlled. By comparing the growth of Arabidopsis thaliana (Thale Cress) grown in the absence of root microbes (i.e., germ-free) to those colonized by a complex community of 183 bacteria, 24 fungi and 7 oomycetes, the researchers observed that the presence of microbes rescued the plant growth-deficiency observed under low light conditions. Inoculation experiments with leaf pathogens further indicated that plants colonized by microbes were also more resistant to aboveground leaf pathogens than germ-free control plants, indicating that the presence of root microbes can promote both plant growth and defense under low light.
By comparing growth and defense responses of colonized plants between the two light conditions, the scientists observed that investment in growth under low light conditions came at the cost of defense, since microbiota-induced defense responses were reduced and plants were more susceptible to leaf pathogens under low light. Based on this observation, the authors of the study then hypothesized that when light conditions are suboptimal, plants favor microbe-induced growth over microbe-induced defense responses. To test this hypothesis, the researchers screened different A. thaliana mutants to identify those that failed to invest in growth under low light. Consistent with their hypothesis, the identified mutants were better at resisting leaf pathogens instead. Furthermore, the scientists found that the presence of the host transcription factor MYC2 was crucial to tip the balance in favor of microbiota-induced growth instead of microbiota-induced defense under low light conditions.
The researchers then went on to investigate whether the make-up of the microbial community belowground could explain aboveground investment in growth at the expense of defense under low light. To do this, they analyzed the composition of the root microbiota across the different A. thaliana mutants and observed that the bacterial community composition was markedly different depending on whether the different plants invested in growth under low light. This experiment led to the identification of 67 bacterial strains that were predicted to be associated with plant growth rescue under low light. To test a potential causal link, the researchers prepared three different bacterial communities composed of either: 1) all 183 strains, 2) the 183 strains lacking the 67 strains predicted to be important for growth rescue or 3) the 67 strains alone. Remarkably, A. thaliana wild-type plants colonized with the 67-member community invested in growth under low light, whereas those colonized by the community lacking these bacterial strains did not, instead favoring better resistance to leaf infection by pathogens.
In the words of study lead Stéphane Hacquard: "Our results suggest that plant growth and defense responses are engaged in different feedback loops with the root microbiota depending on aboveground light conditions. It is likely that light-induced change in root exudation profiles is an important mechanism that stimulates the growth of particular beneficial bacterial root commensals that boost plant growth, in the expense of defense responses under low light". The observation that microbiota-root-shoot-circuits exist in plants is reminiscent of recent results obtained in the context of the microbiota-gut-brain axis in animals, where a direct link between gut commensals and brain functions was uncovered. The results suggest that bacterial root and gut commensals have important functions in modulating stress responses not only locally, but also in distant host organs.
These findings have important applications for utilizing belowground microbes to promote aboveground stress responses in plants. By applying the knowledge gained in this study it would now be conceivable to design synthetic microbial communities with modular functions that could be used to promote plant resistance to particular biotic or abiotic stresses, and ultimately promote plant health in nature.
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Seabird colony creates 'halo' of depleted fish stocks
A vast seabird colony on Ascension Island creates a "halo" in which fewer fish live, new research shows.
Ascension, a UK Overseas Territory, is home to tens of thousands of seabirds - of various species - whose prey incudes flying fish.
The new study, by the University of Exeter and the Ascension Island Government, finds reduced flying fish numbers up to 150km (more than 90 miles) from the island - which could only be explained by the foraging of seabirds.
The findings - which provide rare evidence for a long-standing theory first proposed at Ascension - show how food-limited seabird populations naturally are, and why they are often so sensitive to competition with human fishers.
"This study tells us a lot about large colonies of animals and how their numbers are limited," said Dr Sam Weber, of the Centre for Ecology and Conservation on Exeter's Penryn Campus in Cornwall.
"These birds are concentrated at Ascension Island during the breeding season, and the intensity of their foraging is naturally highest near the island.
"As they use up the most accessible prey located near to the island, they have to travel increasingly long distances to feed, causing the 'halo' to expand outwards.
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Masked booby feeding a chick
CREDIT
Sam Webe
"Once individuals can't find enough food to break even with the energy they expend finding it, the colony stops growing.
"Human impacts such as fisheries can interfere with this natural balance and have negative effects on populations of marine top predators like seabirds, even if they don't directly harm the birds.
"What was particularly surprising is the large scale of the footprint we found.
"It shows that Marine Protected Areas may need to be very large because some predators rely on prey stocks across a huge area."
The pattern of prey depletion revealed by the study is known as "Ashmole's halo", after British ornithologist Philip Ashmole, who first proposed it about 60 years ago after a visit to Ascension Island.
For the study, the researchers counted flying fish, tracked seabirds' foraging trips and examined their regurgitated food.
The nesting seabird species on Ascension that prey on flying fish include frigatebirds, masked boobies and brown boobies.
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Masked booby
CREDIT
Sam Weber
The research team included the RSPB and the Royal Netherlands Institute for Sea Research.
The study was funded by UK Government's Conflict, Security and Stability Fund and by a Darwin Initiative grant.
The paper, published in the journal Proceedings of the National Academy of Sciences, is entitled: "Direct evidence of a prey depletion 'halo' surrounding a pelagic predator colony."
Male dragonflies lose their 'bling' in hotter climates
Less pigmentation keeps them cool, but could make it difficult to find a mate
A study published the week of July 5 in the Proceedings of the National Academy of Sciences led by Michael Moore at Washington University in St. Louis finds that dragonfly males have consistently evolved less breeding coloration in regions with hotter climates.
"Our study shows that the wing pigmentation of dragonfly males evolves so consistently in response to the climate that it's among the most predictable evolutionary responses ever observed for a mating-related trait," said Moore, who is a postdoctoral fellow with the Living Earth Collaborative at Washington University.
"This work reveals that mating-related traits can be just as important to how organisms adapt to their climates as survival-related traits," he said.
Many dragonflies have patches of dark black pigmentation on their wings that they use to court potential mates and intimidate rivals.
"Beyond its function in reproduction, having a lot of dark pigmentation on the wings can heat dragonflies up by as much as 2 degrees Celsius, quite a big shift!" Moore said, noting that would roughly equal a 3.5 degrees Fahrenheit change. "While this pigmentation can help dragonflies find mates, extra heating could also cause them to overheat in places that are already hot."
The researchers were interested in whether this additional heating might force dragonflies to evolve different amounts of wing pigmentation in different climates.
For this study, the scientists created a database of 319 dragonfly species using field guides and citizen-scientist observations. They examined the wing ornamentation shown in photographs submitted to iNaturalist and gathered information about climate variables in the locations where the dragonflies were observed. The researchers also directly measured the amount of wing pigmentation on individual dragonflies from almost 3,000 iNaturalist observations in a focused group of 10 selected species. For dragonflies in each of these 10 species, the scientists evaluated how populations differed in the warm and cool parts of their geographic ranges.
Whether they compared species with hotter versus cooler geographic ranges, or compared populations of the same species that live in warmer areas versus cooler areas, the researchers saw the same thing: male dragonflies nearly always responded to warmer temperatures by evolving less wing pigmentation.
Sorting the observations another way, the researchers determined that male dragonflies spotted in warmer years tended to have less wing pigmentation than male dragonflies of the same species in cooler years (the database included observations recorded during the time period from 2005-19).
"Given that our planet is expected to continue warming, our results suggest that dragonfly males may eventually need to adapt to global climate change by evolving less wing coloration," Moore said.
The study includes projections, based on climate warming scenarios, that indicate it will be beneficial for male wing pigmentation to shrink further as the Earth warms over the next 50 years.
But the changes are not happening the same way for both sexes.
"Unlike the males, dragonfly females are not showing any major shifts in how their wing coloration is changing with the current climate. We don't yet know why males and females are so different, but this does show that we shouldn't assume that the sexes will adapt to climate change in the same way," Moore said.
Dragonflies have different amounts of pigment on their wings that help males and females of the same species identify each other. One of the interesting implications of this research is that if the male wing pigmentation evolves in response to rapid changes in climate and the female pigmentation evolves in response to something else, females may no longer recognize males of their own species.
This could cause them to mate with males of the wrong species.
"Rapid changes in mating-related traits might hinder a species' ability to identify the correct mate," Moore said. "Even though our research suggests these changes in pigmentation seem likely to happen as the world warms, the consequences are something we still really don't know all that much about yet."
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This work was completed in collaboration with: Kim Medley, director of Washington University's environmental field station, Tyson Research Center; Kasey Fowler-Finn, an associate professor of biology at Saint Louis University; and Washington University undergraduates Kaitlyn Hersch, Chanont Sricharoen, Sarah Lee, Caitlin Reice, Paul Rice and Sophie Kronick.
How can 'shark dandruff' contribute to coral reef conservation?
VIDEO: BASED ON MICROSCOPIC SHARK SCALES FOUND ON FOSSIL- AND MODERN CORAL REEFS IN CARIBBEAN PANAMA, SMITHSONIAN SCIENTISTS REVEAL THE CHANGING ROLES OF SHARKS DURING THE LAST 7000 YEARS, BOTH BEFORE...view more
CREDIT: STRI
For 400 million years, shark-like fishes have prowled the oceans as predators, but now humans kill 100 million sharks per year, radically disrupting ocean food chains. Based on microscopic shark scales found on fossil- and modern coral reefs in Caribbean Panama, Smithsonian scientists reveal the changing roles of sharks during the last 7000 years, both before and after sharks in this region were hunted. They hope this new use for dermal denticles will provide context for innovative reef conservation strategies.
Microscopic scales covering a shark's body--dermal denticles--reduce drag as sharks swim and protect from abrasion with hard substrates and ectoparasite attachment.
"If you have ever petted a shark in an aquarium touch tank," said Erin Dillon, who began this study as an intern at the Smithsonian Tropical Research Institute (STRI) and is now wrapping up her doctoral work at the University of California, Santa Barbara, "denticles are the reason why shark skin is rough like sandpaper if you rub it in one direction yet smooth in the other direction. Sharks are essentially covered by millions of tiny teeth."
Just as humans shed dry skin and dandruff, sharks shed their denticles, which accumulate in marine sediments. The oldest denticles found so far, in the Harding Sandstone of Colorado, are about 455 million years old.
STRI paleobiologist, Aaron O'Dea, pieces together clues from fossil- and modern coral reefs to reconstruct baseline conditions before human colonization, and to understand how ecological and evolutionary processes change through time.
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Erin Dillon collecting bulk surface sediment samples from a modern coral reef in Bocas del Toro, Panama.
CREDIT
STR
"Placoderms in the Paleozoic, and then marine reptiles in the Mesozoic, were larger and ate sharks." O'Dea explained. "Placoderms ruled the oceans for around 70 million years and marine reptiles of the Mesozoic ruled for more than 100 million years. Sharks are only top predators now because extinction events preferentially took out other groups but allowed sharks to survive. Sharks seem to have remarkable evolutionary resilience and I was fascinated to work on a technique that would help us explore how sharks have fared more recently when humans step into the picture."
His team sampled material from a 7000-year-old fossilized reef in Bocas del Toro, Panama and nearby modern Caribbean reefs. O'Dea asked Erin Dillon to see if she could find shark denticles in the samples.
"What started as a three-month-long internship turned into a two-year stay in Panama and then expanded into part of my PhD thesis," Dillon said. "I've grown with this project as my role shifted from first exploring and processing the samples as an intern to leading the project, analyzing and interpreting the data, and spearheading the writing."
For each week or so of collecting samples from fossil reefs and modern coral rubble, it took about a year of lab work to recover and identify the denticles. In total, Dillon and colleagues had to sift through around 300 kilograms of reef sediments, enough to fill two bathtubs, to find the denticles they needed to know how many and what kind of sharks were in Bocas del Toro in the past. First, they used acetic acid to dissolve away the chalky sediments and then they sorted the residue under a microscope using a paintbrush to isolate the denticles.
"Finding the first denticles was thrilling," exclaimed Dillon. "They were beautifully preserved and abundant enough to provide insights into millennia-old shark communities."
But before using these fossils to uncover past shark communities, Dillon wanted to better understand the relationship between shark numbers and denticle abundances, how denticles fossilize in reef sediments, and which sharks possess which types of denticles. After publishing these studies, she could finally return to the fossil record.
She found that denticle accumulation rates, and therefore, shark abundances, were over three times higher before humans began using marine resources in the region. All denticle types declined over time, but those found on commercially valuable, fished species declined the most. The oldest samples contained a higher proportion of denticles from fast swimming, pelagic sharks like hammerheads and requiem sharks. In contrast, Dillon found that nurse shark denticles are relatively more common today than they were in the past.
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False color scanning electron microscope images of dermal denticles (scales) from several shark species found on Caribbean reefs. Denticle morphology varies across sharks with different ecological life modes.
To complement this evidence from the fossil record, Dillon combed through archaeological studies and historical narratives to better understand the forces that might have caused these declines. She found that the steepest decline in shark abundance occurred in the late 20th century according to these historical records. This timing coincided with the development of a shark fishery in Panama, which selectively targeted pelagic sharks. Yet, the decline in denticles belonging to nurse sharks, which are infrequently harvested both today and historically, suggested that indirect factors like the loss of coral reef habitat or prey items were also to blame.
"When the Spanish arrived in the Americas, they wrote fantastic accounts of seas swarming with sharks," Dillon said. "But these days we see very few sharks: we are lucky to see the occasional nurse shark. Our data show that sharks in Bocas del Toro have been depleted both by long-term harvesting, which accelerated in the second half of the 20th century, and by habitat degradation, which began even earlier with the expansion of banana cultivation and coastal development. There is so much land-based runoff from the coast today that in some locations it's like swimming through limeade."
"Taken together, Erin's body of work shows that denticle assemblages can be used with care to help reconstruct past shark communities through time," O'Dea said. "Sharks are an integral part of ocean health and play important roles in the great diversity and functioning of coral reefs. Having empirical estimates of past shark abundances and community composition helps us frame our understanding of what is natural in the seas."
"We hope to extend this method to other locations to examine broader geographic patterns of change in reef shark communities over long ecological timescales," Dillon said. "For example, we're currently reconstructing trends in denticle accumulation along Panama's Pacific coast over the last several thousand years using sediment cores. These baseline data will help us explore the causes and consequences of changes in shark abundance and functional diversity. Our work can also help tailor shark management goals to this local region."
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This study was made possible due to generous support from Panama's Secretariat for Science and Technology (SENACYT), SNI, STRI, the Association of Marine Laboratories of the Caribbean, the Save Our Seas Foundation, the International Coral Reef Society, the Schmidt Family Foundation, the Alfred P. Sloan Foundation, M. Selin, J. Bilyk, V. and B. Anders and J. and M. Bytnar.
The Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is a unit of the Smithsonian Institution. The institute furthers the understanding of tropical biodiversity and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems. Promo video.
NEWS RELEASE
Fossil shark scales provide a glimpse of reef predator populations before human impact
The results indicate that shark abundance in the region declined roughly three-fold since prehistoric times
IMAGE: SPIKEY, RIDGED SCALES REDUCE DRAG IN SWIFT-SWIMMING SHARKS, WHILE THICKER, ROUNDER SCALES OFFER PROTECTION FROM ABRASION. THE THREE-PRONGED SCALE ON TOP MAY SERVE A DEFENSIVE FUNCTION. FALSE COLOR ELECTRON MICROSCOPE... view more
CREDIT: ERIN DILLON, AARON O'DEA AND JORGE CEBALLOS
Scientists recently made news by using fossil shark scales to reconstruct shark communities from millions of years ago. At the same time, an international team of researchers led by UC Santa Barbara ecologist Erin Dillon applied the technique to the more recent past.
Human activities have caused shark populations to plummet worldwide since records began in the mid-20th century. However, the scientists were concerned that these baseline data may, themselves, reflect shark communities that had already experienced significant declines. Dillon compared the abundance and variety of shark scales from a Panamanian coral reef 7,000 years ago to those in reef sediments today to discern how reef-associated shark communities have changed since humans began using marine resources in the area.
The results, published in the Proceedings of the National Academy of Sciences(link is external), indicate that shark abundance in the region declined roughly three-fold since prehistoric times, with swifter-swimming species taking a harder hit. Much of this decrease is echoed in historical records, suggesting that sharks in Caribbean Panama were most heavily impacted within the past century.
"These results give us new insight into what a 'healthy' shark community might look like on a coral reef before human exploitation," said Dillon, a doctoral student in the Department of Ecology, Evolution, and Marine Biology. "And they can help us set more appropriate and location-specific baselines for management and conservation."
With their cartilaginous skeletons, sharks don't readily fossilize. Often seemingly all that remains of an ancient shark is its hard teeth. But under the right conditions, a closer look at the surrounding sediments will reveal hundreds of microscopic shark scales only a few times thicker than a human hair. Just like the animal's teeth, shark scales are composed of dentin with a hard enamel surface. Researchers call them dermal denticles, meaning "skin teeth," and believe the two are essentially the same structures -- just in different parts of the body.
Scientists often rely on microfossils to reconstruct ancient ecosystems. Items like scales, pollen grains and plankton shells can provide a wealth of information about the conditions and denizens of past ecosystems that aren't preserved in large fossils. What's more, sharks shed a lot more scales in their lifetime than teeth, so dermal denticles can offer paleo-ecologists much more material to analyze than teeth do.
Dillon and her team were fortunate to have access to a fossil reef in Bocas del Toro, on Panama's Caribbean coast. Normally, ancient reefs are entombed under the living coral, but construction had exposed the site, enabling the scientists to collect samples over several years before it was filled in.
They collected sediments that had accumulated within the fossil reef. Debris that settled between the fingers of branching coral was protected from extensive mixing with sediments of different ages. This essentially preserved a time capsule of material from the ancient reef as it accreted.
The team used radiometric dating to estimate the age of the reef. Corals incorporate trace amounts of uranium, but not thorium, into their skeletons as they grow. Scientists can use the predictable rate at which uranium decays into thorium to determine the age of a coral sample. Using this method, the authors dated corals on the fossil reef to around 7,000 years ago.
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Shark scales are minute, appearing like ordinary sand until examined under a microscope.
CREDIT
ISABELLE LEE
Next came the arduous process of separating the denticles from the sediments. Using a solution of acetic acid, what Dillon referred to as "glorified vinegar," she tediously dissolved around 300 kg of carbonate sand -- enough to fill two bath tubs -- to a manageable 400 g of residual material, which she then sorted through under a microscope to find the scales.
Different denticle shapes correspond with different functions. For example, thin scales with points and ridges reduce drag, and are found on sharks like great hammerheads and silky sharks that swim fast. Ridge spacing also matters, with animals that reach fast burst speeds tending to sport narrower ridges. Meanwhile, animals like nurse and zebra sharks, which spend their time near tough substrates, tend to have thick, plate-like scales that offer abrasion protection. "They're sort of like armor," Dillon explained. Accounting for the form and abundance of different scales provided the team with a sense of what types of sharks inhabited the ancient reef as well as their relative numbers.
That said, just as different parts of the mouth sport differently shaped teeth, scale morphology also varies across a shark's body. Given this variability, it's nearly impossible to match an isolated scale to a specific species, as can often be done with teeth. That's why Dillon and her colleagues stuck to broad ecological categories of sharks in their paper.
The team's painstaking analysis ultimately paid off.
"We showed that tiny shark scales can be well-preserved and found in high enough abundances to reconstruct shark baselines over long ecological timescales," Dillon said, "and we found about a 71% decrease in total shark abundance between the mid-Holocene -- before major human impact in our study region -- and now." These prehistorical reefs would have had similar environmental conditions to those of today, she added, with the primary difference being that they predate the earliest evidence of human occupation in this part of Panama.
The authors also discovered that the types of sharks found on these reefs shifted between prehistoric times and today. Midwater swimmers, like requiem and hammerheads, declined more than demersal species, like the nurse shark. "If you went snorkeling on these reefs a couple thousand years ago, not only would sharks have been a more common sight but there would have been relatively more fast-swimming pelagic sharks," she said.
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Dillon was able to identify groups of sharks on the ancient reef based on the scales they left behind.
CREDIT
ERIN DILLON, ASHLEY DIEDENHOFEN, AND JORGE CEBALLOS
Yet, Dillon was struck by the fact that sharks of all types declined over this time period. "If fishing were the only driver, then we wouldn't expect to see such a big drop in nurse sharks over time because they have low commercial value and are rarely targeted by fisheries in the region," she said. "But we did." This suggests that the observed shark declines weren't simply the result of direct impacts on the animals, like overfishing, but might also have stemmed from indirect factors like the loss of reef habitat or available prey.
Dillon and her co-authors also looked at historical accounts of shark abundance through time. "We found that the biggest decline in shark abundance, according to these records, occurred in the latter half of the 20th century," she said. Between these accounts and the results from the fossil record, the evidence suggests that most of the shark declines in this location happened within the past 100 years.
The study's findings provide insight into shark ecology as well as important context for the numbers of sharks observed on reefs today. Most modern time-series data of shark abundance come from places with well-studied commercial fisheries, and often data collection starts well after fishing had commenced. This makes it difficult to be certain how many sharks were present before human activities began impacting the ocean, as well as the long-term ecological consequences of shark declines.
Dillon plans to continue investigating dermal denticles. She is currently studying variation in the rates at which different shark species shed their scales at the Aquarium of the Pacific. If one species sheds much faster than another, that species will leave behind more scales even if there the two populations are the same size.
She and her colleagues are also collecting sediment cores from regions with different human and ecological histories to track high-resolution trends in scale types and abundances over the last several millennia.
Using shark scales to reconstruct past abundances and diversity is a relatively new methodology, and this is the first time it's been applied to questions related to shark management and conservation. "Before this, we didn't really know just how to answer the question of how abundant sharks were on intact coral reefs before human impact," Dillon said, adding that she hopes other researchers take advantage of this powerful technique and apply it to other locations around the world.
How fish got their spines
Scientists from Konstanz unravel the genetic mechanisms controlling fin spine formation across fish lineages
IMAGE: TWO MALES OF THE CICHLID FISH ASTATOTILAPIA BURTONI, THE MODEL ORGANISM USED TO STUDY SPINE AND SOFT-RAY DEVELOPMENT IN HOECH ET AL. view more
CREDIT: JOOST WOLTERING
In the movie "A Fish Called Wanda", the villain Otto effortlessly gobbles up all the occupants of Ken`s fish tank. Reality, however, is more daunting. At least one unfortunate fan who re-enacted this scene was hospitalized with a fish stuck in the throat. At the same time this also was a painful lesson in ichthyology (the scientific study of fishes), namely that the defense of some fishes consists of needle-sharp fin spines.
Two types of fin elements Indeed, many fish species possess two types of fin elements, "ordinary" soft fin rays, which are blunt and flexible and primarily serve locomotion, and fin spines, which are sharp and heavily ossified. As fin spines serve the purpose to make the fish less edible, they offer a strong evolutionary advantage. With over 18.000 members, the spiny-rayed fish are the most species-rich fish lineage. These fishes even evolved separate "spiny fins" consisting of spines only. Therefore, the evolution of fin spines is considered a major factor in determining diversity and evolutionary success amongst fishes.
In the study published in PNAS, researchers at the University of Konstanz from a team led by Dr Joost Woltering, who - together with his PhD student and first author of the study Rebekka Höch - works in the laboratory of Professor Axel Meyer, show how fin spines arise during embryonic development. They also explain how the spines could evolve out of ancestral soft-rays independently in different lineages of fish. The study focuses on a model species for the spiny-rayed fish, the cichlid Astatotilapia burtoni, which possesses well-developed soft-rayed and spiny fin parts.
Different developmental genes for spines and soft-rays As a first step, the team determined the genetic profiles of soft-ray and spiny fins during embryonic development. "What became clear from these first experiments was that a set of genes that we already knew from fin and limb development becomes differently activated in spines and soft-rays," says Rebekka Höch. These genes correspond to so-called master regulator genes and are known to determine morphology in the axial and the limb skeleton. In the fish fins, these genes appear to provide a genetic code that determines whether the emerging fin elements will develop looking like a spine or like a soft-ray.
Soft-rays can change into spines and vice versa Next, the team identified genetic pathways that switch on these master regulator genes and that determine their activity at different positions across the fins. "Importantly, we were able to address the roles of these pathways using chemical tools, so-called inhibitors and activators, as well as the 'gene scissors' CRISPR/Cas9 and thereby test how spiny and soft-rayed fin domains are established during development," says Joost Woltering, assistant professor in the Department of Biology at the University of Konstanz and senior author of the study.
In their experiments, the scientists were able to alter the number of spines or soft-rays in the fins. This effect was most striking when the so-called BMP (bone morphogenetic protein) signaling was modulated. "We did not only see changes in the activation of the master regulatory genes, but we also observed so-called homeotic transformations, in which soft-rays had become spines, or the other way around, spines had turned into soft-rays," Joost Woltering explains.
An additional observation was that in these fish not only the morphology of the fin elements changed, but also the accompanying fin coloration. "Male cichlids have bright yellow spots on their fins, but these are restricted to the soft-rayed part. What we observed was that when a soft-ray changed into a spine, the fin also lost the yellow spots at this position," says Joost Woltering. This observation shows that in spiny-rayed fish, spines and soft-rays are integrated parts of a larger developmental module that determines a number of the visible features of the fins.
The same principle in different fish lineages As the puzzle was put together, the team came to realize that a deeply conserved patterning system had become redeployed during evolution of the spiny fin. "In fact, the genetic code that determines the fin domain where spines will appear is also active in fins that do not have spines. This indicates that an ancestral genetic pattern was redeployed for making spines," says Rebekka Höch.
With this newly gained insight in mind the authors set out to investigate fin patterning in catfish, a group of fish of which members have independently evolved spines in the fins. Indeed, the genetic code identified for spines in the cichlid matched the one of the catfish spines. Although some differences exist between the different spiny fish species, it altogether suggests the existence of a deeply conserved fin pattern that is relied on to make spines when this is favored by evolutionary selection.
The next steps
For its future research the team will focus on the genes that act downstream of the identified spine and soft-ray control genes to find out how exactly they alter fin morphology by controlling ossification and cellular growth pathways. "In the end we want to gain a better understanding of how new anatomical structures arise that make some species more successful than others, and how this contributed to the incredible evolutionary diversity of the fish lineages," concludes Joost Woltering.
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Key facts:
THE EMBARGO ON THE PAPER WILL LIFT ON THE 5TH OF JULY (2021) AT 3:00 PM U.S. EASTERN TIME (9:00 PM CEST)!
Original study: Rebekka Höch, Ralf F. Schneider, Alison Kickuth, Axel Meyer, and Joost M. Woltering (2021) Spiny and soft-rayed fin domains in acanthomorph fish are established through a BMP-gremlin-shh signaling network. PNAS; DOI: 10.1073/pnas.2101783118
Journalists can access the embargoed article through EurekAlert!
All authors of the study are affiliated with the Department of Biology at the University of Konstanz. Ralf F. Schneider currently works at the Helmholtz Centre for Ocean Research Kiel (Geomar), Alison Kickuth at the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG).
The BMP (bone morphogenetic protein) and shh (sonic hedgehog) signaling pathways are crucially involved in the formation of the fin pattern of fish during development. They control the activity of master regulatory genes (hoxa13 and alx4) that determine whether the developing fin elements will become soft or spiny fin rays respectively.
A genetic comparison between fishes with spines from different lineages suggests that fin spines have evolved independently several times through repeated redeployment of a highly conserved genetic pattern.
Funding: Deutsche Forschungsgemeinschaft (DFG; especially #WO-2165/2-1), European Research Council (ERC; #293700) and Young Scholar Fund of the University of Konstanz.
IMAGE: ONE OF THE 2.7 BILLION YEAR-OLD DIAMONDS USED IN THIS WORK view more
CREDIT: CREDIT: MICHAEL BROADLEY
A unique study of ancient diamonds has shown that the basic chemical composition of the Earth's atmosphere which makes it suitable for life's explosion of diversity was laid down at least 2.7 billion years ago. Volatile gases conserved in diamonds found in ancient rocks were present in similar proportions to those found in today's mantle, which in turn indicates that there has been no fundamental change in the proportions of volatiles in the atmosphere over the last few billion years. This shows that one of the basic conditions necessary to support life, the presence of life-giving elements in sufficient quantity, appeared soon after Earth formed, and has remained fairly constant ever since.
Presenting the work at the Goldschmidt Geochemistry conference, lead researcher Dr Michael Broadly said, "The proportion and make-up of volatiles in the atmosphere reflects that found in the mantle, and we have no evidence of a significant change since these diamonds were formed 2.7 billion years ago".
Volatiles, such as hydrogen, nitrogen, neon, and carbon-bearing species are light chemical elements and compounds, which can be readily vaporised due to heat, or pressure changes. They are necessary for life, especially carbon and nitrogen. Not all planets are rich in volatiles; Earth is volatile rich, as is Venus, but Mars and the Moon lost most of their volatiles into space. Generally, a planet rich in volatiles has a better chance of sustaining life, which is why much of the search for life on planets surrounding distant stars (exoplanets) has focused on looking for volatiles.
On Earth, volatile substances mostly bubble up from the inside of the planet, and are brought to the surface through such things as volcanic eruptions. Knowing when the volatiles arrived in the Earth's atmosphere is key to understanding when the conditions on Earth were suitable for the origin and development of life, but until now there has been no way of understanding these conditions in the deep past.
Now French and Canadian researchers have used ancient diamonds as a time capsule, to examine the conditions deep inside the Earth's mantle in the distant past. Studies of the gases trapped in these diamonds show that the volatile composition of the mantle has changed little over the last 2.7 billion years.
Lead researcher, Michael Broadley (University of Lorraine, France) said "Studying the composition of the Earth's modern mantle is relatively simple. On average the mantle layer begins around 30km below the Earth's surface, and so we can collect samples thrown up by volcanoes and study the fluids and gases trapped inside. However, the constant churning of the Earth's crust via plate tectonics means that older samples have mostly been destroyed. Diamonds however, are comparatively indestructible, they're ideal time capsules".
We managed to study diamonds trapped in 2.7 billion year old highly preserved rock from Wawa, on Lake Superior in Canada. This means that the diamonds are at least as old as the rocks they are found in - probably older. It's difficult to date diamonds, so this gave us a lucky opportunity to be sure of the minimum age. These diamonds are incredibly rare, and are not like the beautiful gems we think of when we think of diamonds. We heated them to over 2000 C to transform them into graphite, which then released tiny quantities of gas for measurement".
The team measured the isotopes of Helium, Neon, and Argon, and found that they were present in similar proportions to those found in the upper mantle today. This means that there has probably been little change in the proportion of volatiles generally, and that the distribution of essential volatile elements between the mantle and the atmosphere are likely to have remained fairly stable throughout the majority of Earth's life. The mantle is the part between the Earth's crust and the core, it comprises around 84% of the Earth's volume.
Dr Broadley continued "This was a surprising result. It means the volatile-rich environment we see around us today is not a recent development, so providing the right conditions for life to develop. Our work shows that these conditions were present at least 2.7 billion years ago, but the diamonds we use may be much older, so it's likely that these conditions were set well before our 2.7 billion year threshold".
Commenting, Dr Suzette Timmerman (University of Alberta, Canada) said:
"Diamonds are unique samples, as they lock in compositions during their formation. The Wawa fibrous diamonds specifically were a great selection to study - being more than 2.7 billion years old - and they provide important clues into the volatile composition in this period, the Neoarchean period. It is interesting that the upper mantle already appears degassed more than 2.7 billion years ago. This work is an important step towards understanding the mantle (and atmosphere) in the first half of Earth's history and leads the way to further questions and research".
Dr Timmerman was not involved in this work, this is an independent comment.
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Diagram of the Earth's layers, showing the position the diamonds were formed in the Upper Mantle
CREDIT
Credit: Michael Broadley
The Goldschmidt Conference is the World's main geochemistry conference. It is hosted alternately by the European Association of Geochemistry (Europe) and the Geochemical Society (USA). The 2021 conference (virtual) takes place from 4-9 July, https://2021.goldschmidt.info/. The 2022 conference takes place in Hawaii.
