Tag: evolution

Fish shrinkage probed in lab

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Scientists are starting a novel project to investigate whether overfishing alters fish behaviour and changes their pattern of development.

Overexploitation of stocks has already been shown to select for smaller fish.

A team reporting at the meeting of the European Society for Evolutionary Biology in Germany will deliberately remove the largest individuals from populations of lab-bred guppies.

The experiment is designed to uncover what is happening in our oceans.

“There are clear indications that almost all… commercial fish are shrinking,” said marine biologist Carl Lundin, who directs the International Union for the Conservation of Nature’s Global Marine and Polar Program.

For mass spawning fish such as cod, there is a great advantage to maintaining older, larger females because they are very efficient at restocking the population.

And if industrial fishing selectively removes the largest individuals, explained Dr Lundin, the industry suffers as populations are reduced to the smallest fish.

However, smaller seafood is unlikely to be the only consequence of industrial fishing; research has also shown that fish in the oceans are reproducing earlier.

Experimental evolution

Now evolutionary biologist Beatriz Diaz Pauli and her colleagues from the University of Bergen, Norway have begun an experiment that they hope will help uncover what other changes we can expect to see in the oceans’ fishes.

The team established nine populations of guppies, each comprising 500 to 900 individuals. Over the next few years, Ms Diaz plans to remove all the fish that measure over 16mm from three of her tanks.

In the remaining tanks, Ms Diaz will purge fish under 16mm, or take fish independent of their size – regimens that will act as a control for the effects of changing the density of fish in the tanks.

The team will then painstakingly record the changes that they see in the fish’s growth rate, age and size of maturation, reproductive effort, and mating and feeding behaviours.

The team hopes to unpick whether the shifts they see are a result of fish moulding themselves to a new environment – a so-called plastic response – or are a consequence of genetic changes.

Plastic responses are not inherited. For example, an organism might reach a smaller body size if it gets little food as a juvenile, but its young would not inherit this propensity to be small.

Genetic responses, by contrast, are inherited, and even if a future generation is returned to an environment where food is plentiful, it would remain small.

Determining the nature of the changes in the fish will help scientists understand how stocks might recover if overexploitation stopped or breeding grounds were protected.

“If we set aside 20-30% of the habitat where reproduction… of key commercial fish stocks [occurs], we are much more likely to avoid these types of problems,” said Dr Lundin.

He added that carrying out experiments of this type allows researchers to control other factors that could affect the fishes’ survival and concentrate on just the consequences of overexploitation.

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Plant has evolved a specialist bird perch

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New research sheds light on the world’s most specialised bird perch.

The researchers suspect that the vertical, branchless stem of a South African plant – locally called the Rat’s Tail – has evolved to encourage pollinating birds to visit its flowers.

The birds hang upside down from this perch and fertilise the plant when they thrust their beaks into the red flowers to drink nectar.

The international team reports the findings in the Annals of Botany.

Plants go to great lengths to attract animals to pollinate them; they seduce insects, birds and small mammals with colourful, shapely, sweet-smelling flowers.

Some plants even wave at passing pollinators.

On first seeing the deep red, long-tubular flowers of Babiana ringens in 2003, botanist Spencer Barrett from the University of Toronto, Canada, suspected that he was dealing with a plant that was pollinated exclusively by birds.

But the position of the flowers at the base of the plant perplexed him.

Most birds avoid feeding on or close to the ground to keep clear of ground-dwelling predators; plants reliant on bird-pollination tend to keep their flowers up high.

Dr Barrett and his colleague Bruce Anderson from University of Stellenbosch in South Africa, wondered if the curious perch-like structure had evolved to give pollinating birds a foothold from which to feed.

Crouching among the shrubs of the Cape of South Africa, binoculars in hand, Dr Barrett and his team confirmed that the flowers were exclusively pollinated by sunbirds.

“When we saw a bird visiting we were completely enchanted,” said Dr Barrett.

Relaxed selection

Still unconvinced that the stick-like protrusion had evolved as a perch, the team set about to gather further evidence.

They set out to look at the full distribution of B. ringens across the Cape, and found that in the east, where sunbirds have a greater variety of flowering plants to choose between, B. ringens‘ perches were smaller than in the west, where plants can rely on regular visits from sunbirds.

Dr Barrett suspects that in the absence of pollinating birds, the plants do not need to invest in maintaining the perch, and so it shrinks over many generations – an example of what is called relaxed selection.

With time, this branch might return to its ancestral form, which the researchers suspect was a central stem with flowers at its top, much like many of B. ringens’close relatives.

“It’s a fascinating piece of work,” said plant biologist Professor Simon Hiscock from the University of Bristol.

This study poses questions about the influence of pollinators on the structures of flowers and on plants’ reproductive strategies, he added.

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Waving robotic crab arm attracts females

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A vigorous wave of the claw can be the key to mating success for male fiddler crabs, report researchers at the 13th Congress of the European Society for Evolutionary Biology.

Male crabs advertise their quality as a potential mate to passing females by waving their large yellow claws.

Using robotic arms, researchers evaluated how the size and speed of the waving claw affected mating success.

The results may help explain why males protect their smaller neighbours.

To the fiddler crab Uca mjoebergi, the Australian mudflats in the north of the country are a heaving dance floor, where a male must rely on his moves to attract a mate.

Males stand outside their burrows and use their enlarged claw to attract females by moving it in circles.

If a female likes the look of a male, she will come closer and disappear down his burrow in the sand, possibly staying to mate.

Wave of waving

When a female wanders through a neighbourhood, “you see part of the mudflat light up” with waving yellow claws, said ecologist Sophie Callander from the Australian National University in Canberra.

Dr Callander and her colleagues used a fully adjustable robotic arm – called Robocrab – to determine what female crabs are looking for in a mate.

Dr Callander set up three robotic arms around a female crab, and sat beneath the unforgiving Australian sun for many hours recording the females’ reactions to different combinations of wave speeds and claw size.

Females approaching from 20cm preferred males with a higher wave rate and larger claws. Intriguingly, this preference increased in strength when the male was flanked by more slowly waving, smaller-clawed crabs.

Fiddler crabs also use these claws in displays of dominance and fighting prowess.

Previous work has shown that larger males sometimes go to the aid of smaller males when an intruder is trying to steal a smaller male’s burrow.

This behaviour is unlikely to be an altruistic form of neighbourhood watch, and Dr Callander thinks that her experiment could offer an explanation.

“If larger males can retain smaller neighbours they might… increase their mating success,” she told BBC News.

For fiddler crabs at least, it pays to keep close to the small and weak.

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Bed bugs protect their sperm from bacteria

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Bed bugs protect their sperm against sexually transmitted infections by producing germ-busting ejaculates, scientists have found.

Bacteria covering bed bugs’ bodies are transmitted to the female, along with the sperm, during mating.

The new work shows that without the protection of antibacterial agents in the bug’s ejaculate, 40% of sperm die.

The results were presented at the 13th Congress of the European Society for Evolutionary Biology.

Bed bugs, and the related bat bugs that live in African caves, are renowned for their bizarre sex lives.

‘Traumatic insemination’

Males, instead of penetrating the female’s vagina, pierce her underside and deposit sperm inside the female, where it swims through the insect’s blood system to the ovaries to fertilise the eggs.

Female bed bugs protect themselves against the diseases that males transmit with a structure on their bellies that guides the penis into a mass of germ-fighting cells.

Males, it seems, have also evolved a way to fend off the effects of sexually transmitted infections, evolutionary biologist Oliver Otti from the University of Sheffield, UK, told conference attendees in Germany.

Suspecting that males load their ejaculates with proteins that protect sperm, Dr Otti carefully extracted sperm from a number of male bed bugs, being sure not to mix it with the seminal fluid that usually makes up the rest of the ejaculate.

He then mixed the sperm with a “soup” of micro-organisms that he had collected from the outer skin of the bed bugs.

To half of these samples he added lysozyme, a bacteria-killing enzyme known to be active in bed bugs, and saw that 40% more sperm survived in its presence.

Females didn’t gain any protection from these introduced bacteria-busting enzymes, he explained; the presence of lysozyme in the ejaculate seemed to be purely to protect sperm.

But other work by Dr Otti’s colleague Michael Siva-Jothy, who is also based at the University of Sheffield, shows that females protect themselves from the infections introduced during sex with their own lysozymes.

In fact, females ramp up their lysozyme activity just before they are about to feed. Dr Siva-Jothy explained that this is probably because in the bed bug world, feeding is generally always followed by mating.

“Wounding is a very frequent event during and after copulation, and generally genitals are not that clean, ” Dr Otti told BBC News.

He explained that the research that has focused on human sexual transmitted diseases has tended to ignore the microbes that coexist with us on our skin; these microbes are likely also transferred during sex.

“It is not clear what the cost of having them around is,” Dr Otti added.

The advantage of studying bed bugs, he said, is that we share many components of our immune system. As a result, scientists can learn much from manipulating the bugs’ sex lives to study the consequences on lifespan and offspring production – some of these trade-offs could be relevant to humans.

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Insects use antibacterial secretions to protect young

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Scientists have confirmed that so-called burying beetles coat their young’s food with an antibacterial substance to guarantee their survival.

Burying beetles lay their eggs on the carcasses of small animals, such as birds and rodents.

The researchers show that without the anti-microbial secretions the young fail to gain weight and die.

The results were presented the 13th Congress of the European Society for Evolutionary Biology.

Most animals try to do the best for their young, but burying beetles, in the genus Nicrophorus, which are found in temperate regions in Europe and North America, are truly doting parents.

As a prospective parent, burying beetles find a dead animal, such as a mouse or bird, and roll the carcass into a ball.

They then bury the carcass, hiding it from predators that might eat it or fancy it for a nursery for their own young – no small feat for a beetle that is only 15mm long.

The beetles then lay their eggs in the flesh of the animal and wait to welcome their young into the world.

But a buried carcass is not going to stay fresh for very long, and the bacterial communities that colonise it are likely to threaten the beetle’s developing larvae.

Germ-free

So burying beetles use secretions from their anal glands to coat the fur or feathers with substances that guarantee the carcass stays germ-free and fresh for longer.

Now scientists from the University of Manchester have worked out what makes these secretions so good at killing germs.

The researchers extracted secretions from the anal glands of a species of burying beetle called Nicrophorus vespilloides, and showed that when this substance was added to bacterial cells, they were destroyed.

Evolutionary biologists Andres Arce, who led the study, and his colleagues, suspecting that they were dealing with a enzyme that “chops up microbial cell walls”, investigated and confirmed that the secretions were rich in lysozymes.

These are anti-microbial enzymes, and a common component of animals’ immune systems.

Lysozymes are also secreted in mammals’ breast milk and in human tears.

The team showed that larva raised in the absence of either their parents secretions were 40% more likely to die before adulthood.

Dr Arce explained that for a non-social insect, these burying beetles are already known to show quite substantial levels of parental care.

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Age-related brain shrinking is unique to humans

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The brains of our closest relatives, unlike our own, do not shrink with age.

The findings suggest that humans are more vulnerable than chimpanzees to age-related diseases because we live relatively longer.

Our longer lifespan is probably an adaptation to having bigger brains, the team suggests in their Proceedings of the National Academy of Sciences paper.

Old age, the results indicate, has evolved to help meet the demands of raising smarter babies.

As we age, our brains get lighter. By 80, the average human brain has lost 15% of its original weight.

People suffering with age-related dementias, such as Alzheimer’s, experience even more shrinkage.

This weight loss is associated with a decline in the delicate finger-like structures of neurons, and in the connections between them.

Alongside this slow decline in its fabric, the brain’s ability to process thoughts and memories and signal to the rest of the body seems to diminish.

Researchers know that certain areas of the brain seem to fare worse; the cerebral cortex, which is involved in higher order thinking, experiences more shrinkage than the cerebellum, which is in charge of motor control.

Yet despite the universality of ageing, scientists do not fully understand why our brains experience this continuous loss of grey matter with age.

Intriguingly, the brains of monkeys do not seem to undergo the same weight loss, raising the question of whether it is a distinctively human condition.

Now, a team of neuroscientists, anthropologists, and primatologists have pooled their expertise and datasets to reveal the answer.

Comparing magnetic resonance images from more than 80 healthy humans between the ages of 22 and 88 with those of a similar number of captive-bred chimps, the researchers found that chimps’ brains do not shrink with age.

The results suggest that the estimated 5-8 million years of evolutionary history that separate chimps from humans have made all the difference in the way that the species age.

It takes a village…

Anthropologist Chet Sherwood from George Washington University in Washington DC, who led the study, thinks that humans live longer to “pay for” their larger-brained children.

Humans live relatively long compared to other great apes. The majority of this extended life is post-menopausal, while chimps are reproductively viable right up to their death.

A human brain is three times the size of chimpanzee’s.

And it is not such a stretch, Dr Sherwood suggests, to conclude that grandparents’ extended lives are in an evolutionary sense there to relieve mothers from being solely responsible for raising their big-brained, energetically costly infants.

“I say this right now, as my seven year old daughter is being looked after by my mother,” he told BBC News.

“Because neurons cannot regenerate, aging, he thinks, is just the stress of living long enough to lend a helping hand to some relatives.”

“[The study] provides very good evidence that the patterns of brain ageing in humans are quite different from other animals,” commented neuroscientist Tom Preuss from Emory University in Atlanta, US, who was not involved in the research.

However, Dr Preuss was clear that these differences do not make other animals useless as models for studying age-related diseases.

Instead, the differences could help to explain why humans suffer more from these diseases than other animals seem to.

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Worms’ sex life yields advantage over parasites

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Sex gives worms the power to fight off parasites, report researchers this week in the journal Science.

Worms forced to reproduce asexually succumbed to a nasty bacterial infection and died.

The researchers say the results are the most convincing evidence to date for a key theory in evolutionary biology.

The theory holds that sex evolved because it lets organisms reshuffle their genes into new combinations to stay a step ahead of parasites.

Sex has long troubled evolutionary biologists.

Reproducing asexually – where organisms clone themselves – makes much more sense; there is no need for an organism to search and seduce a mate, fight off competitors, or risk contracting a sexually transmitted disease.

What is more, given that an organism has survived long enough to reproduce, it is likely to have a first-rate set of genes under its pelt.

Why run the risk of diluting these good genes with potentially poorer ones from another organism?

And yet sex exists; the vast majority of animals and plants reproduce this way.

Fluctuating futures

Parasites, many biologists believe, might be the answer.

Parasites create a situation where, in spite of the disadvantages of sex, it is good for an organism to reshuffle its genome with that of another.

This reshuffling creates offspring with new gene combinations that are potentially better than older combinations at resisting a parasite’s advances.

The genetic “arms race” between a parasite and its host is often refered to as an example of Red Queen-style interaction – a term coined by biologist Leigh Van Valen who summoned the image of the constantly running Red Queen from Lewis Caroll’s Through the Looking-Glass.

The analogy seemed to him fitting for describing how species must continually evolve to keep up with each other.

But despite the theory’s popularity, there has been little hard evidence for it.

Out in the field, biologists have noted that organisms are more likely to reproduce sexually when there are more parasites loping around in their vicinity.

What has been missing is a direct manipulation to organisms’ sex lives to test if it makes them more or less resistant to parasites.

Direct evidence

Now researchers working at Indiana University in the US have used the round worm Caenorhabditis elegans to do just this.

The team engineered two types of worms – some that could only reproduce by having sex, and some that could only clone themselves.

The researchers watched the worms gorge themselves on a lawn of a nasty bacterium, Serratia marcescens, which invades the worms’ guts and from there multiplies into every crevice of their body, killing the worms from the inside.

Across five different populations, worms that reproduced sexually fared well over the 20 generations, while all animals that cloned themselves died quickly.

Testing theory

“What is really beautiful about these lab systems is that you can manipulate the system and show that [the theory] can work,” said evolutionary biologist Aneil Agrawal from the University of Toronto in Canada.

Dr Agrawal described the experiment as “elegant” because it allowed the researchers to demonstrate that it was not simply the presence of the parasite that spelled the end for the cloners, but the presence of a parasite that had co-evolved alongside the worms.

To do this, the team created two treatments: one used bacteria from an original stock kept in the freezer, and the other used bacteria that had lived alongside the worms for many generations and so had adapted along with them.

Clonally doomed

In essence, “the bacteria got more and more infective, but the [clonal worms] did not get more and more resistant, and that is why they went extinct,” explained lead author Levi Morran, an evolutionary biologist from University of Indiana in the US.

“I am really excited about this; I think this is really cool,” Dr Agrawal told BBC News.

“Whether this is actually happening in nature is another thing; we can’t know that from a lab system,” he explained.

But he adds that as a first step it is important to demonstrate that under conditions where you expect sex to alleviate the effects of parasites, it does.

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Safe Sex, Duck Style

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Male mallard ducks (Anas platyrhynchos) are famous for their long, spiraling genitalia. Now scientists have discovered that they have something else to crow about. Mixing duck ejaculate with a common bacteria, Escherichia coli, researchers have found that mallard duck semen kills bacteria. Semen from males with more colorful bills harbored the greatest antibacterial activity, killing up to three times more bacteria than those with duller bills, the team reports online today in Biology Letters. The finding suggests that female ducks may be drawn to brightly colored males not just because they’re more flashy, but because they spread fewer germs through sexual intercourse.

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Green Eggs and Salamanders

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It might sound like something out of a Dr. Seuss story, but biologists have long told tales of the green eggs of the spotted salamander. Ambystoma maculatum lays its brood in ponds each spring up and down North America. These marble-sized gelatinous sacs quickly turn green (bottom left and top right images) as photosynthesizing algae grow around the developing embryo and feast on its waste. In turn, the embryo enjoys the oxygen produced by the algae. Now scientists have discovered that the algae gets a little closer than they thought. Using long-exposure imaging, the researchers detected algal fluorescence (main image) inside the developing salamander. This is the first case of an algae living symbiotically within a vertebrate, the team reports online today in the Proceedings of National Academy of Sciences. How the photosynthesizing algae gets there, and how it survives inside the tissues and cells of this predominantly nocturnal amphibian is still baffling to scientists. But one thing’s for sure, the discovery means rewriting textbooks to add salamanders to a short list of organisms, including coral and bacteria, that form symbiotic relationships with plants.

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