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Archive for May, 2014

Finding a Place to Call Home:
Translocation and the Plight of the Hihi

By Claire Asher, on 16 May 2014

Climate change alters how climate is distributed both geographically and temporally. Over the coming decades, for species sensitive to climatic variables, it may become a case of ‘relocate or die’ – those species that are not able to shift their populations from old, unsuitable habitat into newly emerging suitable habitat, in line with climate change, will likely go extinct. Conservationists can provide a helping hand to species in this position, however – translocation programs aim to establish populations in appropriate habitat when the species is unlikely to reach it on their own. Determining whether translocations are likely to be necessary in the future, what populations to move and where to move them are complex questions to answer, however. Recent work by researchers at the Institute of Zoology (part of the Center for Ecology and Evolution and affiliated with UCL’s GEE department) developed a framework for understanding species’ relationships with climate and identifying potential translocation sites which will provide suitable habitat through future climate change. For one of New Zealand’s endemic birds, the Hihi, translocation to sites further south may be it’s best chance of long-term survival.

Hihi, endangered bird endemic to New Zealand

The climate is changing. Changes in temperature, rainfall and seasonality are occurring globally, and we are already measuring the effects on wildlife. Often, conditions are shifting geographically, and many species will find that their current range no longer overlaps with any suitable habitat (human land-use change isn’t helping!). In these cases, some species will be able to shift their ranges to account for this, but many species will be unable to do change quickly enough to keep up and instead face extinction. Humans can intervene here by moving endangered species to more suitable habitat, but translocation is expensive and it is crucial to select the new location carefully if the population is to have a chance of succeeding. IoZ researchers set out to develop a statistical framework for determining suitable translocation habitat, using one of New Zealands most endearing but endangered endemics, the Hihi (Notiomystis cincta), to test the framework.

The population of Hihis in Tiritiri Mantangi island offers a special opportunity to study the direct effects of climate change without other variables such as food ability confounding the results. This is because they have been provided supplementary food for nearly two decades. Using data on the reproductive success of females in this population, combined with climate data, Dr AliƩnor Chauvenet and Dr Nathalie Pettorelli from the Institute of Zoology, along with colleagues at Imperial College London and Massey University in New Zealand, were able to show that Hihi populations are effected by the climate even when food availability is removed from the equation.

Next, using mathematical modelling, the authors tried to predict the future of Hihi populations, using different simulated changes in climate based upon the variables that were found to be most important in influencing current Hihi populations on Tiritiri Mantangi. Changes in temperature, as well as increases in climate variability had a significant influence on the survival of simulated Hihi populations. The final step was to again use mathematical modelling to predict and map suitable Hihi habitat both now, and in the future. Again, this modelling showed that current Hihi populations are most strongly influenced by temperature, a key variable in determining habitat suitability, with rainfall as another important influence.

Looking forward, under models of predicted future climate change, suitable Hihi habitat is expected to move south. The north of New Zealand, which currently offers highly suitable habitat, is predicted to become almost entirely unsuitable over the next few decades. The most successful reintroduced population of Hihis, as well as the largest and last remaining natural Hihi population both stand to lose suitable habitat by 2050. New suitable habitat is expected to emerge in the southern end of the North Island, as well as the northern part of the South Island, where historically conditions have not been suitable for Hihis.

Because Hihis show population declines as temperatures warm even when we control for food availability, even careful management of existing population may prove ineffective under future climate change. Instead, translocation may provide the only solution to guarantee the long-term survival of the Hihi in New Zealand. Although translocations traditionally perform the role of reintroduction – returning a species to part of it’s historical range – future plans for endangered species like the Hihi need to take climate change into consideration. We should opt for ‘assisted colonisation’ – introducing populations to new habitat that is likely to persist (and perhaps even become more suitable) through future climate change. In this way we can attempt to ‘future-proof’ our conservation efforts and hopefully ensure the survival of many species which might otherwise go extinct as the climate changes.

Original Article:

() Journal of Applied Ecology

This research was made possible by funding from AXA Research and Research Councils UK .

Two’s Company, Three’s a crowd:
The Evolution of Two Sexes

By Claire Asher, on 6 May 2014

You’ve probably never given much consideration to why there are men and women. Or, more specifically, why there are two sexes, rather than one, three or 50. But this is a question that has been keeping some scientists awake at night for decades. Recent research in the department of Genetics, Evolution and Environment used mathematical models of evolution to investigate how the evolution of the two sexes was influenced by the inheritance patterns of the energy-producing organelle, mitochondria. The results of this model contradict previous work supporting the idea that inheritance of mitochondria through only one parent might explain the emergence of two sexes. The evolutionary dynamics of mitochondrial inheritance are more complex than previously thought.

Sexual reproduction is a beneficial thing, in evolutionary terms, but this benefit doesn’t depend upon there being different sexes, only on there being two individuals sharing their genes to produce an offspring. This system would also work with no sexes at all (everyone can mate with everyone), or with many sexes. In fact, two is actually the worst number you could have picked – with two sexes any individual is limited to an available pool of mates just 50% of the population. With three sexes, this pool would increase to 66% of the population, with four 75%, and so on. So why have most sexually-reproducing species on settled on two sexes?

In a few previous GEE blog articles (see here and here), I have discussed the phenomenon known as ‘uniparental mitochondrial inheritance’ (UPI), in which mitochondria, organelles found in our cells that are responsible for generating energy, are inherited only through the maternal line – that is, you inherit all of your mitochondria from your mother and none from your father. UPI is found in many living things, although some species do things a bit differently and there are many different ways to achieve the same result. Work by GEE researcher Professor John Allen has previously shown that the mitochondria within egg cells in jellyfish, fruit flies and fish are largely inactive; this inactivity allows for a perfect ‘mitochondrial template’ to be passed on to the offspring and prevent the accumulation of mutations through the generations. Essentially, this is why aging isn’t heritable. It wouldn’t work to inactivate sperm mitochondria because they need so much energy for all that swimming, so if we did inherit mitochondria from our fathers they would probably be mutated. UPI is also thought to help evolution remove harmful mutations from the population and reduce conflict and promote coadpatation between the mitochondrial symbiont and its host cell.

Fertilisation So, UPI makes a lot of sense, evolutionarily, and some scientists think it might also explain why we have two sexes, as opposed to any other mating system. It’s important to be clear, when we talk about having two sexes we’re saying nothing about the external differences between the sexes (sexual dimorphism) observed in many multicellular organisms. We’re talking about the existence of two ‘mating types’, such that individuals cannot mate with members of the same type. Recent research by another group of GEE academics including Professor Andrew Pomiankowski, Dr Nick Lane and Professor Robert Seymour, investigated the evolution of UPI and in particular it’s relationship with the evolution of a two-sex mating system. We might expect a strong link between UPI and the existence of two sexes, since uniparental inheritance immediately generates differences between the two mating partners, and ensures that reproduction is not possible unless one member of each ‘type’ is present. Although UPI is often thought to have been a key driver in the evolution of mating types, there have been few investigations of what conditions are needed for the fitness benefits of UPI to actively drive the emergence of two mating types. So the authors developed a new mathematical model of mitochondrial inheritance and the evolution of UPI in a population where biparental inheritance (BPI) is the norm. They incorporated mitochondrial mutation (which might sometimes be selfish) and selection into the model, and included different mating types.

The model agreed with a great deal of previous work that indicates that UPI tends to increase fitness. It does so slowly, with selection acting cumulatively across many generations to remove less fit mitochondrial variants and increase fitness for UPI individuals. In a population of individuals where mitochondria is inherited biparentally, a new mutation causing UPI exists in a single individual. Slowly UPI improves the fitness of cells by reducing the number of mutated mitochondria they carry, and the UPI mutation might start to spread in the population. The problem is, as it spreads the benefits of UPI are inevitably leaked into the rest of the BPI population – UPI individuals mate with BPI individuals producing some BPI offspring who carry the fitter mitochondria from their UPI parent. This leaking of benefits means that the fitness benefits of UPI are frequency-dependent; the more common UPI becomes in a population, the less each UPI individual benefits from the reproductive strategy. This makes it hard for UPI to fully take over a population – their model tended instead to produce mixed populations with some UPI and some BPI individuals interbreeding.

Mitochondrial Evolution - Leaking of beneficial mutatations

Leaking of fit mitochondrial (blue) into BPI cells (a)

If the researchers included mating types in the model at the start of it’s evolutionary run, then UPI could become associated with specific mating types and in this situation, so long as mutation rates were high or each cell carried many mitochondria, UPI could spread to fixation in the population. But UPI itself was not able to alter the number or existence of mating types. The authors suggest that this may explain the continuum of UPI levels we observe in nature. For any given species, the occurrence of UPI will depend upon the evolutionary starting point, energetic demands, mutation rates and the selfish (or unselfish) nature of mutations.

Although most people never even consider why we have two sexes, male and female, the evolution of a two mating-type system is seemingly paradoxical and many theories and hypotheses have been proposed to explain it. One such explanation is that uniparental inheritance, which is critical for stabilising the mitochondria-cell symbiosis and preventing the accumulation of harmful mutations, may have driven the evolution of two sexes. However, mathematical modelling by scientists in GEE suggests this is not the case, and UPI more likely evolved after the two mating-type system emerged. In their model, although UPI initially spreads through populations, it’s fitness benefits are frequency-dependent, meaning it only rarely takes over an entire population. Populations in which all members inherit mitochondrial uniparentally are only possible when a mutation causing UPI becomes tighly linked to genes that determine mating type. The initial emergence of two mating types still requires an explanation independent from mitochondrial inheritance patterns.

Original Article:

() Proceedings of the Royal Society B: Biological Sciences

This research was made possible by funding from the Natural Environment Research Council (NERC), the Engineering and Physical Sciences Research Council (EPSRC) and the Leverhulm Trust.