Tiny molecules drove the evolution of the vertebrates

Blogging on Peer-Reviewed ResearchThe spinal column that runs down your back is an identity badge that signifies your membership among the vertebrates – animals with backbones. Vertebrates have arguably the most complex bodies and genomes of any animal group and certainly, our lineage has come a long way from its last common ancestor.

TigerThe closest evolutionary cousins of the vertebrates are simple aquatic creatures such as the jawless lancelets and the sac-like, immobile sea squirts. How did these simple body plans diversify into the vast array of sophisticated forms wielded by today’s fish, amphibians, reptiles and mammals?

Gene number

Many scientists have suggested that the answer lies in the number of our genes. At three different points, the vertebrate genome (its full suite of genes) experienced a massive jump in size as huge chunks of genes – possibly the entire lot – were duplicated. The first of these coincided with the origins of the group itself and the second happened alongside the rise of the first jawed fish, setting them and their descendants aside from more ancient jawless forms like the lampreys.

So far, there seems to be a tidy connection between gene number and complexity, but the third round of duplication is a bit of a stumbling block. It happened at some point during the evolution of the bony fishes and while this group proceeded to radiate into a multitude of different shapes, their basic body plan stayed essentially the same. No big jump in complexity there.

Indeed, as the full genome sequences of more and more species are revealed, it’s becoming clear that the basic genetic toolkit that controls the development of animal bodies is remarkably consistent across the kingdom. Even the genome of a sea anemone, one of the simplest and most ancient animals on Earth, is strikingly similar to that of vertebrates.

In this light, it’s looking increasingly unlikely that the advent of new genes can account for the large rise in vertebrate complexity. Now, Alysha Heimberg and colleagues from Dartmouth College have proposed a new theory, centred around tiny molecules called microRNAs.

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Earliest bat shows flight developed before echolocation

Blogging on Peer-Reviewed ResearchTheir heads and bodies of bats have amassed an extraordinary array of adaptations that have make them lords of the night sky. Today, the thousand-plus types of bats make up a fifth of living mammal species. Richard Dawkins once described the evolution of bats as “one of the most enthralling stories in all natural history” and as of this week, the story has a clearer beginning.

OnychonycterisThe success of bats hinges on two key abilities: their mastery of flight, a feat matched only by birds and insects; and echolocation, the ability to navigate their way through pitch-blackness by timing the reflections of high-pitched squeaks. For evolutionary scientists, the big question has always been: which came first?

The ‘clawed bat’

Until now, fossil bats haven’t provided any clues for all of them show signs of both echolocation and flight. But a stunning new fossil, discovered by Nancy Simmons from the American Museum of Natural History is an exception and it provides a categorical answer to the long-running debate – the earliest bats could fly but could not echolocate.

The new creature hails from the Green River in Wyoming and is known as Onychonycteris, meaning “clawed bat”. Its fossils date back to about 52.5 million years ago and by comparing it to other prehistoric bats, Simmons found that it is the most ancient member of this lineage so far discovered. It acts as a ‘missing link’ in bat evolution, much like the famous Archaeopteryx hinted that birds may have evolved from dinosaurs.

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Malawi cichlids – how aggressive males create diversity

Blogging on Peer-Reviewed ResearchCertain groups of animals show a remarkable capacity for quickly evolving into new species to seize control of unexploited niches in the environment. And among these ecological opportunists, there are few better examples than the cichlids, a group of freshwater fishes that are one of the most varied group of back-boned animals on the planet.

Malawi cichlidsIn the words of Edward O. Wilson, the entire lineage seems “poised to expand.” The Great Lakes of Africa – Tanganyika, Malawi and Victoria – swarm with a multitude of different species; Lake Malawi alone houses over 500 that live nowhere else in the world.

All of these forms arose from a common ancestor in a remarkably short span of time. Now, a new study suggests that this explosive burst of diversity has been partly fuelled by rivalry between hostile males.

Michael Pauers of the Medical College of Wisconsin found that male cichlids have no time for other males that look like them and will bite, butt and threaten those who bear the same colour scheme. In doing so, they encourage diversity in the lake since mutant males with different tints are less likely to be set upon by territorial defenders.

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Cross-breeding restores sight to blind cavefish

Blogging on Peer-Reviewed ResearchIn the caves of Mexico lives a fish which proves that a million years of evolution can be undone with a bit of clever breeding.

Blind cavefishThe blind cavefish (Astyanax mexicanus) is a sightless version of a popular aquarium species, the Mexican tetra. They live in 29 deep caves scattered throughout Mexico, which their sighted ancestors colonised in the middle of the Pleistocene era. In this environment of perpetual darkness, the eyes of these forerunners were of little use and as generations passed, they disappeared entirely. They now navigate through the pitch-blackness by using their lateral lines to sense changes in water pressure.

But there is a deceptively simple way of restoring both the eyes and sight that evolution has taken, and Richard Borowsky from New York University’s Cave Biology Research Group has found it. You merely cross-bred fish from different caves.

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Evolutionary arms race turns ants into babysitters for Alcon blue butterflies

Blogging on Peer-Reviewed ResearchIn the meadows of Europe, colonies of industrious team-workers are being manipulated by a master slacker. The layabout in question is the Alcon blue butterfly (Maculinea alcon) a large and beautiful summer visitor and its victims are two species of red ants, Myrmica rubra and Myrmica ruginodis.

Myrmica rubra and an Alcon blue butterfly caterpillarThe Alcon blue is a ‘brood parasite’ – the insect world’s equivalent of the cuckoo. David Nash and European colleagues found that its caterpillars are coated in chemicals that smell very similar to those used by the two species it uses as hosts. To ants, these chemicals are badges of identity and so similar are the caterpillars that the ants adopt them and raise them as their own. The more exacting the caterpillar’s chemicals, the higher its chances of being adopted.

The alien larvae are bad news for the colony, for the ants fawn over them at the expense of their own young, which risk starvation. If a small nest takes in even a few caterpillars, it has more than a 50% chance of having no brood of its own. That puts pressure on the ants to fight back and Nash realised that the two species provide a marvellous case study for studying evolutionary arms races (which I’ve blogged about before here).

Theory predicts that if the parasites are common enough, they should be caught in an ongoing battle with their host, evolving to become more sophisticated mimics, while the ants evolve to become more discriminating carers. The two species make a particularly good model for this because their geographical ranges overlap in a fractured mosaic.

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Mud time capsules show evolutionary arms race between host and parasite

Blogging on Peer-Reviewed ResearchEvolution can sometimes be seen as a futile contest. Throughout the natural world, pairs of species are locked in an evolutionary arms race where both competitors must continuously evolve new adaptations just to avoid ceding ground. Any advantage is temporary as every adaptive move from a predator or parasite is quickly neutralised by a counter-move from its prey or host. Coerced onward by the indifferent force of natural selection, neither side can withdraw from the stalemate.

Mud time capsules show evolutionary arms race between host and parasiteThese patterns of evolution are known as Red Queen dynamics, after the character in Lewis Carroll’s Through the Looking Glass who said to Alice, “It takes all the running you can do, to keep in the same place.”

These arms races are predicted by evolutionary theory, not least as an explanation for sex. By shuffling genes from a mother and father, sex acts as a crucible for genetic diversity, providing a species with the raw material for adapting to its parasites and keep up with the arms race.

Watching the race

We can see the results of Red Queen dynamics in the bodies, genes and behaviours of the species around us but actually watching them at work is another matter altogether. You’d need to study interacting species over several generations and most biologists have neither the patience nor lifespan to do so.

But sometimes, players from generations past leave behind records of the moves they made. Ellen Decaestecker and colleagues from Leuven University found just such an archive in the mud of a Belgian lake.

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Bdelloid rotifers – 80 million years without sex

Sex is, on the whole, a good thing. I know it, you know it, and natural selection knows it. But try telling it to bdelloid rotifers. These small invertebrates have survived without sex for some 80 million years.

Bdelloid rotifers under the microscopeWhile many animals, from aphids to Komodo dragons, can reproduce asexually from time to time, it’s incredibly rare to find a group that have abandoned sex altogether. The bdelloid rotifers (pronounced with a silent b) are an exception.

They live in an all-female world and since their discovery, not a single male has ever been found. Genetic studies have confirmed that they are permanently asexual, and females reproduce by spawning clone daughters that are genetically identical to them.

The bdelloids pose a problem for evolutionary biologists, who have struggled to explain how they could make do without a strategy that serves the rest of the animal kingdom very well. Now, Natalia Pouchkina-Stantcheva, Alan Tunnacliffe and colleagues from the University of Cambridge have found out how they do it.

Sexual animals have two copies of each gene that have only minimal differences between them. But the asexual bdelloid lifestyle has uncoupled the fates of each copy in a gene pair, allowing them to evolve in new directions. They get two genes for the price of one.

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