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Archive for June, 2013

Biological Traits Influence Vulnerability to Climate Change in Birds, Amphibians and Corals

By Claire Asher, on 25 June 2013

Climate change is fast becoming a reality, and with temperature rises of between 0.8°C and 2.6°C predicted over the next 35 years, biodiversity will certainly be impacted, with many species set to suffer declines or potential extinction. But all species are not equal and certain traits may make species more or less vulnerable to climate change than others. A new model presented in PLOS one this month investigates the impact of biological traits, such as physiology, ecology and evolutionary history, on vulnerability to climate change for some of the most threatened groups: birds, amphibians and corals. Around 10% of species are both highly vulnerable to climate change, and already listed as threatened with extinction. The model also identifies potentially vulnerable species for future conservation priorities, giving biologists a head-start in trying to slow the inevitable loss of biodiversity that climate change will bring.

Many researchers are interested in predicting how biodiversity might respond to climate change. Biodiversity is essential to human survival – diverse, functional ecosystems provide us with food, water and medicine. However, predicting how ecosystems might respond to changes in temperature and rainfall is a complicated matter. Most previous models have considered the availability of suitable habitat for species based upon their current range and predictions of temperature changes. However, not all species are created equal – biological traits of individual species are likely to play an important role in determining species survival. For example, some species are adapted to a very specialised habitat or are poor as dispersal and so may struggle to find alternative habitats even if they are available. Other species have long generation times and produce few young, or have very limited genetic diversity in the population, making adaptation to new habitats more difficult. Species like these are likely to be more vulnerable to climate change than generalists who are good at dispersal and produce lots of offspring. Not considering the biology of a species when modelling responses to climate change can lead to under- or over-estimations of how vulnerable a species actually is.

Accounting for Biology
To address this issue, Foden and colleagues, working in collaboration with Professor Georgina Mace from CBER, developed a systematic framework for assessing species vulnerability to climate change, and applied this model to three of the best-studied, and most endangered groups of animals: birds, amphibians and corals. They considered three factors – sensitivity (whether a species can survive where it is), exposure (the predicted extent of change under climate models), and adaptive capacity (whether a species can avoid the negative impacts of climate change by moving or evolving).

The components of species vulnerability - sensitivity, adaptive capacity and exposure

Components of species
vulnerability – sensitivity, adaptive
capacity and exposure

In consultation with extinction risk specialists, they identified 90 biological, ecological, phsyiolocial and environmental traits which are likely to influence vulnerability to climate change. In particular, they identified habitat specialisation, rarity, environmental tolerance, disruption of environmental triggers and interactions with other species as key components of species sensitivity to climate change. Adaptive capacity is composed of dispersal ability, barriers to dispersal, genetic diversity, generation length and reproductive output. They assessed these traits for each of 16,857 species of bird, amphibian and coral, across the globe.

The proportion of species in a region that are sensitive or have limited adaptive capacity (blue), high exposure to climate change (yellow) or both (maroon).

The proportion of species in a region that are sensitive or have limited adaptive capacity (blue), high exposure to climate change (yellow) or both (maroon).

Armed with these traits, they were able to determine sensitivity, adaptive capacity and exposure for each species, and generated maps of where species may be particularly vulnerable. They found that around 24% – 50% of bird species, 22% – 44% of amphibians and 15% – 32% of corals are both sensitive and exposed, and have limited capacity to adapt. They identified the Amazon region as containing many highly vulnerable birds and amphibians. Many bird species were also highly vulnerable in central Eurasia, the Congo basin, the Himalayas, Malaysia and Indonesia, with amphibians most vulnerable in north Africa, eastern Russia, and the northern Andes. The waters around Malaysia, Indonesia and the Philippines were hot-spots for highly vulnerable corals.

A Silver Lining
It’s not all doom and gloom, though. The study also identified some species and regions where species’ traits may make them more able to cope with climate change. Around 28% -53% of bird species, 23% – 59% of amphibians and 30% – 55% of corals may survive projected climate changes because of their inherent ability to disperse or adapt to change. In particular, southern Asia and North America may see less severe biodiversity declines than previously thought.

The interplay between climate change and biodiversity is complex, and unlikely to be uniform across taxonomic groups. It is important to consider the physiological, ecological and evolutionary traits of individual species when making predictions about the impact of climate change. This study considered the effects of temperature and rainfall changes, as well as ocean acidification and sea-level rise, on global biodiversity. However, many other factors will influence whether species survive over the long-term – habitat destruction, invasive species and pollution are also major drivers of extinction which need to be taken into account when predicting the future of a species. Taking into account the biology of a species, and it’s interaction with other species, is a major step forward in our understanding of how biodiversity will respond to the impending climate changes that are now inevitable.

Original Article:

This research was made possible by funding from the MacArthur Foundation.

How Energy Shapes Life: Our Mitochondrial Partners

By Claire Asher, on 21 June 2013

Every cell in our bodies is a collaborative effort. A collaboration that dates back to just after the dawn of life itself, and marks one of the deepest divides in the tree of life. This symbiosis has had remarkably far-reaching effects, and is now thought to be key to understanding sex, ageing and death. The symbiotic entity in question is the mitochondrion; the power house of the cell. Biologists are increasingly realising the centrality of energy in understanding life, and a special issue of Philosophical Transactions of the Royal Society, introduced by Nick Lane, focuses on the synthesis of energy, genes and evolution.

Mitonucleus in Blue
Artwork by Odra Noel www.odranoel.eu
Image Used with Permission

Around 1.5 billion years ago, a simple, single-celled bacterium engulfed another one. But it didn’t eat it. Instead, the two cells started working together, and over the following millennia developed a cooperative relationship that laid the foundation for multicellular life and for diversification on a scale never seen before. This symbiotic relationship founded a new type of life – the eukaryote. Unlike their prokaryote ancestors, the eukaryotes had a special advantage: they now had internal membranes. This enabled them to generate energy more efficiently, utilising a force generated across a membrane, known as the proton-motive force. This allowed them to support bigger cells, a wider variety of diets, and ultimately led some eukaryotes to group together into multicellular, sexual organisms such as plants and animals.

Hand Over Your Genes!
Since mitochondria originated as free-living single celled organisms, they came complete with their own genetic information. As the cooperative partnership between the eukaryote host and its mitochondrial partners developed, the majority of these genes were transferred to the nuclear (host cell) genome. Bizarrely, though, a few genes remained with the mitochondria. In fact, the same few genes stayed put in almost all eukaryotes, even though this process happened many times independently. So what is so special about these genes? Why keep them in the mitochondria when it is costly to do so?

A Mitochondrion

A Rapid Response System
It is thought these genes remain in the mitochondria because they are needed there to allow individual mitochondria to respond rapidly to changes in the energy demands of the cell. When mitochondria are out-of-sync with their surroundings, they start generating dangerous ‘reactive oxygen species’ (ROS), also known as free radicals. Free radicals are famous for causing cancer, and this is because they act as mutagens, causing changes to DNA they encounter. Mutations caused by ROS can cause all sorts of problems, not just cancer, and many of the diseases of old age are thought to be caused by ROS mutations.

So, we need to keep a small set of genes – those that code for key components of the respiratory chain that generates energy in mitochondria – in close proximity to their site of use so that they can quickly respond to the ever-changing cellular environment. This is also true for chloroplasts in plants, and recent research by Puthiyaveetil and colleagues in GEE lends support for this explanation. They have found evidence for a chemical partnership, unique to eukaryotes, which may be important in regulating chloroplast function to minimise ROS production.

Both mitochondria and chloroplasts have held onto a few genes in order to allow a rapid response to changes in demand. Trouble is, by keeping those genes there, they are vulnerable to attack from any free radicals that are generated. Mutations in these genes can often cause the production of more free radicals, causing a downward spiral that eventually leads to the death of the cell.

Unusual Inheritance

Fertilisation

Mitochondrial genes are not inherited in the same way as normal nuclear genes. The majority of our genes (residing in the nucleus) are inherited biparentally, that is, we inherit half from our mother and half from our father. Mitochondrial genes, on the other hand, are inherited only in the maternal line, through the egg. Mitochondria are also present in sperm cells – they have to be, since they are crucial for providing energy to power all that intense swimming – but sperm mitochondria are killed shortly after fertilisation. Recent research by John Allen in GEE suggests that uniparental inheritance of mitochondrial genes relates back to those pesky free radicals.

In sperm cells, mitochondria are working overtime to make sure the sperm has the energy to swim and find the egg. In the process they are churning out free radicals, and damaging the mitochondrial DNA. The mitochondria inside sperm start to suffer from the ravages of old age. But we don’t want to pass on old mitochondria to our offspring. It seems evolution has found an elegant solution – switch off the mitochondria in egg cells and pass these young mitochondria on to your offspring. De Paula and colleagues looked at the mitochondria in eggs, sperm and the bell of moon jellyfish (Aurelia aurita) and found egg mitochondria were largely inactive and simple in structure. Egg mitochondria also showed reduced levels of gene expression, an almost non-existent membrane potential and produced no free radicals, indicating that they were not actively generating energy. Eggs don’t need that much energy compared to sperm, and they can borrow what they do need from other cells. By switching off the mitochondria in eggs, and making sure these are the only ones passed on to the next generation, eukaryotes can reduce the build-up of harmful age-related mutations in mitochondria from generation to generation.

The Moon Jellyfish (Aurelia aurita)

Energy, Mitochondria and the Meaning of Life
The generation of energy is central to life. Mitochondria generate energy in all eukaryotic cells (animals, plants, fungi), and the symbiotic event that brought them into the eukaryotic cell changed life on Earth forever. Mitochondria proved massively beneficial, allowing eukaryotes to try new things, and ultimately produce the huge diversity of multicellular life we see today. However, housing another genome inside the cell came with nuances of its own, which led us to have sex, to age and die. Recent research in GEE highlights one way in which evolution has overcome one of these nuances – by switching off mitochondria in egg cells, offspring can inherit an undamaged copy, and because of this, ageing is not heritable. Other research in plants is broadening our understanding of why these genes are stored inside the mitochondria and chloroplasts in the first place. Throughout a plethora of different disciplines, mitochondria are proving crucial to understanding the fundamentals of life.

Original Articles:

() Philosophical Transactions of the Royal Society B

() Philosophical Transactions of the Royal Society B

Introduction to the Special Issue:

() Philosophical Transactions of the Royal Society B

This research was made possible by funding from the Natural Environment Research Council (NERC) and the Leverhulme Trust

Wanted: Field Assistant for Social Evolution Project in Panama – Immediate Start

By Claire Asher, on 18 June 2013

paperwasp A joint project between ZSL, UCL and the University of Bristol, investigating social evolution in Panamanian paper wasps, is urgently seeking a volunteer field assistant for this summer’s field season. Flights and accommodation are paid for. Please check out the advert below if you are interested, and contact emily.bell@ioz.ac.uk if you are interested in the position.

Social Behaviour in Tropical Paper Wasps – Volunteer Field Assistant Required

Farewell to One of Darwin’s Frogs
…but it’s not too late for another

By Claire Asher, on 18 June 2013

Amphibian declines are one of the biggest conservation concerns of the 21st century. In a paper last week in PLOS ONE, Claudio Soto-Azat at the Universidad Andrés Bello in Chile, in collaboration with Ben Collen from GEE’s Centre for Biodiversity and Environmental Research (CBER), and colleagues at ZSL, reported some sad news about two species of Darwin’s frog in South American. They announce the exctintion of the Chile Darwin’s Frog and substantial declines in the closely related species, Darwin’s Frog.

Using published data and archived specimens, Soto-Azat and colleagues reconstructed the historical range of these two species across Chile and Argentina, and went looking for the frogs right across their original range. Not a single Chilea Darwin’s frog (Rhinoderma rufum) was found. They used modeling based upon recorded sightings to predict whether the Chile Darwin’s frog truly is extinct. Unfortunately, this model confirmed the worst, that Chile Darwin’s frog is now extinct in the wild, and probably vanished over 30 years ago.

Darwin’s Frog (Rhinoderma darwinii)
Image from ARKive. Photograph used with permission from (Universidad Andrés Bello)

Populations of the closely related Darwin’s frog (Rhinoderma darwinii) were found, however these populations are much smaller and more fragmented than previously thought. The authors recommend classifying Darwin’s only remaining frog as Endangered. No frogs were found in or near urban environments – the surviving populations exist in primary forest. The populations that have gone extinct had been put under strain from human pressures. In Chile, loss of native forest as it is chopped down to make way for pine plantations to satisfy a growing global demand for wood and paper may be reducing the availability of habitat for these colourful little frogs.

The same frog captured twice during the study - identifiable by it's unique underbelly patterning

Darwin’s Frog (Rhinoderma darwinii)
The same frog captured twice – identifiable by it’s unique underbelly patterning

Darwin’s frogs aren’t just colourful, they also have a rather remarkable way to care for their young. The male waits patiently until the eggs he fertilised are ready to hatch, when he swallows the eggs and holds them in his vocal sac as they hatch. The tadpoles remain their at least until they are able to feed by themselves, but in Darwin’s frog (R.darwinii), the young stay until they can hop out, as tiny froglets. Chile Darwin’s frog is lost forever, but it may not be too late to save Darwin’s frog, and this unusual strategy for rearing offspring.

Original Article:

() PLOS ONE

Continental Catfish Show No Sign of Stopping

By Claire Asher, on 14 June 2013

The tree of life is a vast tangle of branches and twigs representing around 9 million species. Understanding the patterns and processes by which these species originated is a fundamental topic in evolutionary biology. Previous studies have suggested that the generation of new species may not be uniform over time, often with an initial burst of speciation, slowing as the lineage expands to fill the available niches. However, recent research in the department of GEE suggests this may not always be the case, and a previous focus on island species may have skewed our perspective of species accumulation.

When a new lineage begins to diverge, adapting to exploit new habitats or lifestyles, this often leads to an adaptive radiation, with many new species appearing over a short space of time. This initial rush of new species eventually begins to slow, as all the available niches become saturated, and slowly speciation grinds to a halt. This is the traditional view of how lineages have diversified to produce the vast array of species we see today (as well as many who have long since gone extinct). However, much of the research in this area has focussed on species confined to islands and lakes, where different processes may influence the appearance of new species. Continental species have a larger area to expand into, and a wider array of climatic conditions to cope with, providing a different set of parameters which may influence speciation. Recent research in the department of Genetics, Evolution and Environment suggests the pattern of species generation in continental lineages may differ from that of islands.

Tree of Life image © 2007 Tree of Life Web Project. Image of rose © 1999 Nick Kurzenko. Image of annelid worm © 2001 Greg W. Rouse

In their recent paper in Systematic Biology, Day and colleagues determined species relationships for 81 species of squeaker catfish from Africa in order to investigate the patterns of species generation and diversification during their 35 million year evolution. Using genetic data along with fossil evidence, they constructed a squeaker catfish tree and estimated divergence dates and historical geographic distributions. This tree revealed an almost constant rate of species generation over time. Across 35 million years of evolution, through major climatic and environmental changes, the squeaker catfish have been churning out new species at approximately 1 every 8 million years, and look set to keep going for another 90 million years, or so.

Syndontis 'squeaker' catfish. Photograph by Roger Bills

Syndontis ‘squeaker’ catfish . Photograph by

So why don’t the continental, river-dwelling catfish follow the same rules developed through years of studying island and lake-dwelling species? Continents are, of course, generally much larger than islands, and rivers provide more opportunity for movement than lakes. Continents allow for more constant species generation, as there is more space and habitat diversity available for species to move into and exploit. The climatic changes in Africa over the last 30 million years may have also helped the catfish maintain constant production of new species – creating a variable environment which is much less likely to become saturated with species. The squeaker catfish have survived through major tectonic activity in the East African Rift valley, including volcanoes and earthquakes, as well as huge climatic changes that influenced sea level and temperature. By combining phylogenetic and biogeographical data, Day et al suggest these environmental variations may have played an important role in African catfish diversification.

The processes shaping species diversification are varied and complex, and differ markedly between geographically isolated lineages (island / lake ecosystems) and wider ranging ones (e.g. continental rivers). Continental groups may take longer to reach species saturation, with environmental fluctuations facilitating continued species generation at a relatively constant rate. Moving beyond ‘model organisms’ and well-studied systems has the potential to reveal new processes and patterns, and illuminate old ones.

Original Article:

() Systematic Biology

This project was made possible by funding from the Natural Environment Research Council (NERC)

Skulls and Teeth: Constraints on Mammalian Evolution

By Claire Asher, on 7 June 2013

When we think about evolution, we tend to imagine the appearance and behaviour of animals as infinitely malleable, changing and adapting to perfectly suit the whim of natural selection. However, in many cases, evolution is actually surprisingly constrained. The potential for a species to evolve into new environments, diets or lifestyles may be limited if the necessary variation isn’t available in the population. In this case, the raw materials for natural selection to act upon are missing and a species cannot evolve in a particular direction. Evolutionary constraints may be developmental or genetic in origin, and may in part explain why some groups of species are more diverse than others.

Early in mammalian evolution, around 160 million years ago, mammals diverged into two main groups; placental mammals (e.g. humans, rats, elephants) and marsupials (e.g. kangaroos, possums, koalas). Placental and marsupial mammals use a different strategy for early development; mammalian infants develop either internally (placental mammals) or in a pouch (marsupials). Despite a similar period of evolutionary history, there are far fewer species of marsupials, and as a group they show less diversity in terms of locomotion and diet. New research suggests their unusual developmental strategy may have constrained their evolution, limiting the variety of present-day marsupials.

A Koala (Phascolarctos cinereus)
A Marsupial Mammal

Chipmunk

A Chipmunk (Neotamias umbrinus)
A Placental Mammal

Pouch-Living Limits Marsupial Diversity
Placental mammals give birth to reasonably well-developed young after a lengthy pregnancy. However, marsupials, such as kangaroos and koalas, do things a bit differently. The infant is ejected very early in development, and must crawl to a specialised pouch where it can complete the rest of its development, feeding by suckling. This developmental strategy is pretty demanding on the young marsupial, and requires that they are strong enough to crawl and suckle at a very young age. There is evidence that the shoulder has been constrained by the demands of crawling at a young age. Has suckling imposed similar constraints on the skull?

During development, embryonic connective tissue begins to form into bone through a process known as ossification. In marsupials, ossification in the skull and particularly in facial bones occurs much earlier in development compared to placental mammals. This pattern is probably necessary to allow marsupial young to suckle at such a young age. Early facial ossification has wide-ranging effects on brain, tooth and muscle development, and may have limited the diversity of facial shapes available to marsupials. Suckling may have been a major constraint on marsupial evolution.

Facial bones (viscerocranium)
and braincase (neurocranium)

In a recent study conducted in GEE, Bennett and Goswami (2013) measured a number of skull characteristics for 125 species of placental and marsupial mammals. Previous studies have failed to find evidence for evolutionary constraints affecting mammalian skulls, but these studies primarily focused on carnivores, and considered the whole skull at once. By looking at different regions of the skull separately, Bennett and Goswami found that marsupials exhibit less variation between species in the facial bones (viscerocranium) than placental mammals. This pattern is not present for the braincase (neurocranium), which is not involved in suckling behaviour, and where bones ossify much later. These results suggest that early suckling might have constrained the diversity of marsupial skull shapes, and may go some way to explaining why marsupials are less diverse.

Something to Get Your Teeth Into
Mammalian diversity has probably been constrained by a variety of factors, not just bone ossification. Another recent study from GEE and Earth Sciences by Halliday and Goswami (2013) showed that tooth growth patterns may have restricted the relative size and shape of mammalian teeth, potentially influencing diet. Mammalian tooth development is thought to proceed by an ‘inhibitory cascade’, whereby the growth of each developing tooth is controlled by a balance between stimulatory signals from the embryonic tissue, and inhibitory signals from the adjacent tooth. This results in the middle molar being intermediate in size between the first and last molar.

Coloured teeth

Lower right jaw of two early mammals, showing the first molar (yellow), second molar (green) and third molar (red).
Pentacodon occultus (left) – a very early insectivorous mammal
Hyracotherium (right) – the first horse, a browsing herbivore
Photograph by Thomas Halliday

Across 154 species of mammal, despite considerable variation in tooth shape and size, the relative sizes of the three molars conformed to the same basic pattern in about two thirds of species. The largest tooth was almost always either the first or last molar, in line with a common pattern of tooth development across all mammals. Diet is strongly related to tooth size and shape: mammals with a larger third molar (coloured red) tend to eat more plant matter, while mammals with a larger first molar (coloured yellow) tend to be meat-eaters. Across 180 million years of evolution, almost all mammals still conform to the same basic pattern of tooth development that their early ancestors used.

The Exception to the Rule
One extinct group of mammals, called condylarths, bucked the trend in molar shape. Their middle molar was largest, inconsistent with an inhibitory cascade. Perhaps this was an archaic adaptation to a new diet – condylarths were among the first omnivorous mammals, and a similar pattern is found in modern-day bears. The developmental constraint of the may not have been too strong for this particular deviation, though, and could have been achieved by a fairly simple modification to the inhibitory cascade.

Adaptation is not infinitely plastic. Natural selection can only work with the raw materials of variation in the population. Sometimes, one aspect of life constrains another, but if the pay-off is great enough, evolution will often find a way, despite the constraints.

Original Articles:

()Testing the Inhibitory Cascade Model in Mesozoic and Cenozoic Mammaliaforms BMC Evolutionary Biology 13: 79

 

This project was made possible by funding from the Natural Environment Research Council (NERC), as well as Abbey Research and Collaboration Awards and the University of London Central Research fund.