Third cousin couples have the most children and grandchildren

Marriage between closely related cousins is a heavy taboo in many cultures and its critics often cite the higher risk of genetic diseases associated with inbreeding. That risk is certainly apparent for very close relatives, but a new study from Iceland shows that very distant relatives don’t have it easy either. In the long run, they have just as few children and grandchildren as closely related ones.

Shuffling the genetic deck

Sex chromosomes aside, every person has two copies of each gene, one inherited from their father and one by their mother. Not every gene will be in correct working order, but there’s a good chance that a faulty copy will be offset by a functional one from the other parent.

However, if two parents are closely related, there’s a higher-than-average chance that they will already share some of the same genes and a similarly increased chance that their child will receive two defective copies. That can be very bad news indeed and in cases where important genes are affected, the results can include miscarriage, birth defects or early death.

Sex, then, is a shuffling of their genetic deck and theoretically the more closely related the partners are, the greater the chance that their child will be dealt a dud hand. And yet, some studies have found that some closely related couples actually do better than distant relatives in terms of the number of children they manage to raise. This trend is certainly unexpected and the big question is whether it is the result of biology or money.

Malawi cichlids – how aggressive males create diversity

Certain 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.

In 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.

Sex runs hot and cold – why does temperature control the gender of Jacky dragons?

Among Jacky dragons, females are both hot and cool, while males are merely luke-warm. For this small Australian lizard, sex is a question of temperature. If its eggs are incubated at low temperatures (23-26ºC) or high ones (30-33ºC), they all hatch as females; anywhere in the idle, and both sexes are born.

This strategy – known as ‘temperature-dependent sex determination (TSD) – seems unusual to us, with our neat gender-assigning X and Y chromosomes, but it’s a fairly common one for reptiles. Crocodiles are all-male at high temperatures and all-female at low ones, while turtles flip the rules around and produce more males in cooler climes. Now, a thirty-year old idea to explain this puzzling system has finally been confirmed.

Assigning gender based on temperature is not uncommon but it is nonetheless puzzling. Gender seems like an incredibly fundamental physical trait to leave to something as variable as the temperature of your surroundings. How has such a system evolved? What possible benefits could a species receive by switching control of from chromosomes to the environment?

The social life of our extinct relatives

One of our extinct evolutionary cousins, Paranthropus robustus, may have walked like a man but it socialised like a gorilla. Using only fossils, UCL scientists have found that P.robustus males were much larger than females, competed fiercely for mates and led risky lives under heavy threat from predators.

I wrote an article about the cool new finding for Nature Network. Here’s the opening and you can read the full article here.

A single fossil can tell you about the shape, diet and movements of an extinct animal but with enough specimens, you can reconstruct their social lives too.

Charles Lockwood of University College London used an unusually large collection of fossils to peer back in time at the social structures of one of our closest extinct relatives, Paranthropus robustus, which inhabited southern Africa between 1.2 million and 2 million years ago.

Drought drives toads to mate with other species

When it comes to sex, it makes sense to stick to your own species. Even putting aside our own innate revulsion, inter-species liaisons are a bad idea because they mostly fail to produce any young. In the few instances they do, the hybrid progeny aren’t exactly racing ahead in the survival stakes and are often sterile (think mules).

But having poor unfit young is still better than having no young at all and if an animal’s options are limited, siring a generation of hybrids may be a last resort. Karin Pfennig from the University of North Carolina found that the plains spadefoot toad uses just this strategy in times of need.

Female toads breed just once a year, so it pays for them to make the right choice. According to Pfennig’s work, they take their health and their environment into account when choosing mates. If their bodies are weak and their surroundings are precarious, the benefits that another species’ genes can provide to their young are enough to outweigh the risks.

Clock gene and moonlight help corals to co-ordinate a mass annual orgy

Every month, at the full moon, tourists and students gather on the beach at Koh Phangan, Thailand for a night of booze, dancing, and debauchery. But the moon-themed antics of these party-goers look positively tepid when compared to those of the Great Barrier Reef’s corals. With the help of two genes and a spot of moonlight, the corals synchronise one of the greatest spectacles of the natural world – a mass annual orgy.

When it comes to sex, corals play a numbers game. Encased in their rocky shells, direct contact is out of the question so they reproduce by releasing millions of eggs and sperm directly into the surrounding water.

This strategy only makes sense if all the corals release their sex cells en masse and sure enough, every individual within a third of a million square kilometres of reef does so during the days after the October full moon.

The corals’ co-ordination would put even the most organised flash-mobs to shame and until now, scientists had no idea how they did it, especially with neither eyes nor brains. Aside from the obvious contribution of moonlight, the only other available clue was that corals seem to be especially sensitive to blue light.

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.

While 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.

Butterflies evolve resistance to male-killing bacteria in record time

Six years ago, the males of a Samoan butterfly were being killed off by a bacteria and made up only 1% of the population. Now, the males have returned in a dramatic comeback and the sex ratio is equal again. In just ten generations, they evolved resistance to the parasite, a powerful example of natural selection in action.

In our world, there is (roughly) one man for every woman. Despite various social differences, our gender ratio remains steadfastly equal, so much so that we tend to take it for granted. Elsewhere in the nature, things are not quite so balanced.

Take the blue moon butterfly (Hypolimnas bolina). In 2001, Emily Dyson and Greg Hurst were studying this stunningly beautiful insect on the Samoan islands of Savaii and Upolu when they noticed something strange – almost all the butterflies were females. In fact, the vastly outnumbered males only made up 1% of the population.

Male-killers

The cause of this female-dominated world was an infection, an inherited bacterium called Wolbachia. Wolbachia is a strong candidate for the planet’s most successful parasite for it infects a huge proportions of the world’s arthropods, themselves a highly successful group. And it does not like males.

Wolbachia has an easy route of infection – it can be passed to the next generation through the eggs of an infected female. But it can’t get into sperm, and for that reason, male insects are useless to it and it has a number of strategies for dealing with them.

Sometimes, it allows females to reproduce without male fertilisation. At other times, it forces males to undergo sex changes to become females. But in cases like the blue moon butterfly, it simply kills the males outright before they’ve even hatched from their eggs.

In 2001, Dyson and Hurst noted that the islands with the fewest males were the ones with the most prevalent Wolbachia infections.

The butterflies fight back

But by 2005, things had changed. Sylvain Charlat from University College London, along with Hurst and others, found that males were increasing in number all around Upolu Island. A year later, a formal survey confirmed the males’ amazing comeback.

On Upolu, they equalled in the females in number. Within just 10 generations, the male butterflies had gone from being outnumbered a hundred to one to an equal footing with the females. “To my knowledge, this is the fastest evolutionary change that has ever been observed,” said Charlat.

Charlat found the same story at a site on Savaii Island close to neighbouring Upolu. On the other side of the island, the males were still in the minority and many failed to hatch. But at 7% of the population, they were doing better than they had done in five years before.

All the butterflies were still infected with the same Wolbachia strain that had slaughtered their males just a few years back. And the bacteria themselves had not changed – when Charlat mated infected females with uninfected males from a different island, the parasite’s male-killing nature resurfaced within a few generations.

Evolution in action

Charlat believes that the Upolu butterflies had gained a resistance gene (or several) that allowed them to shrug off the male-killing bacteria. It either evolves the trait itself, or gained it from South-east Asian populations that had already become resistant.

Whatever the origin, the mutation spread like genetic wildfire across the Upolu and onto neighbouring Savaii. Most mutations carry small benefits and spread slowly. But by levelling a populations sex ratio, a mutation that resists Wolbachia clearly provides a huge advantage.

Surviving males who carry the gene(s) would have had their pick of females, since most of their competition lay dead in their eggs. And females, that picked up the mutation would have had twice the number of surviving young.

Arms race

Charlat’s work highlights just how powerful an influence parasites have in the course of evolution. Just how powerful parasites can be in the course of evolution. Events like this may be very commonplace, but at such speed, they may have happened before researchers could spot them.

The butterflies’ newfound resistance is also marvellous example of the Red Queen hypothesis, where parasites and hosts are caught in an evolutionary arms race. Each is forced to acquire new adaptations and counter-adaptations just to stay in the same place.

In this particular arms race, the butterflies have won the battle against Wolbachia. But the war isn’t over. The parasite now faces renewed pressures to find innovative ways of doing away with the dead-end males. How long will it be before it evolves a retaliatory strike?

More on evolution in action:
Natural selection does a handbrake turn – quick evolution at work
Of flowers and pollinators – a case study in punctuated evolution

More on parasites and evolutionary arms races:
Parasites can change the balance of entire communities
Viruses evolve to be more infectious in a well-connected population
Beetle and yeast vs. bee – how American bees are losing the evolutionary arms race

More on animal sex and reproduction:
Virgin birth by Komodo dragons
When the heat is on, male dragons become females
Chimerism, or How a marmoset’s sperm is really his brother’s
Aphids get superpowers through sex

 

Reference: Charlat, Hornett, Fullard, Davies, Roderick, Wedell & Hurst. 2007. Extraordinary flux in sex ratio. Science 317: 214.

 

 

Technorati Tags: butterfly, Wolbachia, male-killing, sex ratio, blue moon butterfly, Sylvain Charlat, Gregory Hurst, evolutionary arms race

Photos by Sylvain Charlat and Comacontrol

When the heat is on, male dragons become females

It seems almost fashionable now to blame everything on climate change, but the most unusual claim yet is that it could lead to sex-changing lizards.

For humans and other mammals, sex is neatly determined by the X and Y chromosomes. If you have a Y you are male, and without it you are female. Reptiles however, use a variety of strategies, and the mammalian X/Y system is just one of them.

In some species, the female is the one with different chromosomes, in this case Z and W, and the male has two Zs. And some reptiles ignore sex chromosomes altogether. For them, an individual’s sex is determined by the temperature that their eggs were incubated at.

Scientists had long believed that these strategies were mutually exclusive with each species choosing one of the other.

But Alexander Quinn and colleagues form the University of Canberra have found that an Australian lizard, the central bearded dragon (Pogona vitticeps) flouts this rule. It has become the first animal known to use two separate methods to determine the sex of individuals.

The dragon uses the Z/W system, where the males carry two Z chromosomes and the females have a Z and a W. But Quinn found that these genes are only the dominant influence on gender if eggs are incubated between 20 and 32 degrees Celsius.

At higher temperatures, males ignore their genetic heritage and become females instead. When Quinn incubated broods of eggs between 34 and 37 degrees Celsius, the hatchlings were almost invariably female. And as predicted, about half of these sisters were genetically male. For dragons at least, when the heat is on, the men turn into women

Quinn believes that the key to the manliness of boy dragons lies in a temperature-sensitive protein produced by the Z chromosome. The protein’s activity needs to surpass a certain threshold before a dragon can become male. For that, there need to be two copies of Z, and the temperature must be just right.

Reptiles that use temperature to assign gender must have fine-tuned their systems over time to cope with an ever-changing environment. But Quinn fears that the current pace of climate change may be too rapid for these animals to adapt to.

If temperatures rise far enough to bias an entire species over to a single gender, extinction would be all but inevitable. These warnings have been sounded before, and Quinn’s work suggests that they should be shouted a little bit louder.

More about animal sex and reproduction: 
Virgin birth by Komodo dragons
Butterflies evolve resistance to male-killing bacteria in record time 
Chimerism, or How a marmoset’s sperm is really his brother’s
Aphids get superpowers through sex

More on the effects of climate change: 
Icebergs are hotspots for life
Climate change responsible for decline of Costa Rican amphibians and reptiles
Hope for corals – swapping algae improves tolerance to global warming
Corals survive acid oceans by switching to soft-bodied mode

 

 

Reference: Quinn, Georges, Sarre, Guarino, Ezaz & Graves. 2007. Temperature sex reversal implies sex gene dosage in a reptile. Science 316: 411.

Technorati Tags: bearded dragon, sex determination, temperature sex, temperature lizards, science

Chimerism, or How a marmoset’s sperm is really his brother’s

Marmosets twins often exchange cells as embryos. As a result, individuals can carry tissues that are genetically identical to their siblings. And because these tissues include sperm, marmoset males sometimes fertilise females with the genes of their brothers.

Imagine you are a man who has just learned, through a genetic test, that your son carried your brother’s genes instead of your own. You might well have some stern words to exchange with your partner. But if you were a marmoset, this would all be part and parcel of life.

In a striking new study, scientists from the University of Nebraska have shown that marmosets inherit genes not only from their parents, but from their monkey uncles and aunts too. Each individual is a genetic chimera.

In Greek mythology, the chimera was a monstrous mixture of lion, goat and dragon (see below). But in the world of genetics, the word has much less grotesque overtones – it simply means an animal whose body contains two or more groups of cells with distinct sets of genes.

Most species of marmoset give birth to non-identical twins. At first, each embryo is surrounded by its own protective sac ­– the chorion – but after the first month of development, these sacs fuse together.

Blood vessels connect the developing embryos and embryonic stem cells can travel between them. These swapped stem cells can eventually set up groups of cells in one twin that contain the other’s genes. So the majority of marmosets have tissues descended from the stem cells of their siblings.

Up till recently, scientists thought that this chimerism only applies to blood cells. But Corinna Ross and colleagues proved otherwise.

They took DNA fingerprints of different organs from 36 twin pairs of Wied’s marmosets (Callithrix kuhlii) from fifteen different families. About three in four pairs had tissues that were genetic matches for their twins – a clear sign of chimerism.

Every single tissue type examined, including brain, skin, hair, muscle, liver and more, was chimeric in at least one set of twins. But the big surprise was over half of the male marmosets had chimeric sperm. Which means that these men were occasionally and unwittingly fertilising females with their sibling’s genes.

Ross found that parents from a third of the families examined passed on some genes to their children that they had themselves inherited from their brothers or sisters. She even speculates that a marmoset mother might be able to pass on a Y chromosome to her children if she was given one by her twin brother.

This unique way of passing on genes has many interesting consequences for marmoset siblings and parents. For a start, it means that marmoset brothers and sisters are more closely related to each other than human siblings are.

On average, a pair of human siblings (identical twins aside) shares 50% of their genes. But because marmoset twins often pass entire cell lines across to each other, some of their body parts carry the exact same genes. So on average, marmoset twins have more genes in common than human ones.

This increased relatedness could explain the strong social bonds that unite marmoset families, where parents and older siblings co-operate looking after younger ones. Marmoset fathers are particularly known in the mammal world for their devotion to their children, and again, chimerism might explain why.

Like many other animals, marmosets use certain chemical odours to work out whether a child is related to them, allowing them to care for their own genetic legacy and no one else’s. Ross speculates that chimeric children give off both their own odours and those of their twin, giving male marmosets even stronger evidence of their fatherhood. The purpose of chimerism then, could be to allow marmoset children to pass paternity tests with flying colours.

Indeed, Ross found that they spent more time caring for their children if they were chimeric and spent twice as much time giving them piggy-back rides.

So family trees in the marmoset world are strange ones. It is a world where individuals are a fusion of their own bodies and their sibling’s and where mothers can give birth to their own nephews and nieces. Thankfully, they don’t have to worry about geneaologies…

More about animal sex and reproduction:
Virgin birth by Komodo dragons
Butterflies evolve resistance to male-killing bacteria in record time 
When the heat is on, male dragons become females
Aphids get superpowers through sex

 

Reference: Ross, French & Orti. 2007. Germ-line chimerism and paternal care in marmosets (Callithrix kuhlii). PNAS 104: 6278-6282.

Technorati Tags: marmosets, monkeys, genetic chimerism, paternal care, genetic inheritance, science

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Christmas special – Virgin birth by Komodo dragons

A virgin birth is a key part of the Christmas story. Now, scientists have found that two Komodo dragons in English zoos have done the same. This ability helps dragon populations to recover in the wild, but it may push the remaining few closer to extinction.

According to Christian lore, Mary gave birth to baby Jesus without ever having had sex with Joseph. A biologist might describe this with the unwieldy word ‘parthenogenesis’, the Greek version of the more familiar term ‘virgin birth’(‘parthenos’ means virgin, and ‘genesis’ means birth.)

The New Testament aside, shunning fertilisation and giving birth to young through parthenogenesis is rare among higher animals, occurring in only one in every thousand species. Nonetheless, in a few short weeks, eight more virgin births are expected in the English town of Chester. The mother is called Flora, and she is a komodo dragon.

Komodo dragons are an endangered species in their island homes of Indonesia. Fifty-two zoos around the world co-operate in a dedicated breeding programme that aim to boost the natural populations of these largest of lizards.

In Europe, only two female dragons, both living in England, are sexually mature. One of these, Flora, lives at Chester Zoo where she has laid a clutch of 25 eggs despite never having been kept with a male.

Three of Flora’s eggs tragically collapsed while they were being incubated, but this provided Phillip Watts and colleagues from the University of Liverpool to trace their origins. They analysed the genetic make-up of the lost eggs using genetic fingerprinting and found that their genomes matched those of their mothers.

Children born through sex have two copies of every gene, one inherited from their father and one from their mother. But in the case of Flora’s babies, all of their genes were identical, suggesting that they all came from Flora alone.

Watts found a similar situation in London Zoo, where a late female called Sungai had given birth to four healthy dragon-lings (see left, image courtesy of Ian Stephen/Nature), over two years after she lost contact with a male.

Scientists had suspected that the babies were the result of sperm that Sungai had stored during that time, but genetic tests confirmed that she was the sole parent.

This double-sighting of parthenogenesis in Komodo dragons suggests that this unusual strategy is not so unusual in these lizards. It could even be used to help populations weather hard times.

Komodo dragons have Z and W chromosomes, rather than our Xs and Ys and in their case, it is the ones with a matching pair who are males (ZZ or WW), and the ones with a dual set who are females (WZ). As a result, parthenogenetic dragons are always male and when populations dwindle, they can kick-start numbers by mating with their own mothers.

This strategy could cause large problems for conservationists. By causing all an individual’s gene pairs to be identical, parthenogenesis achieves what inbreeding usually takes generations to do.

In rare cases, it could help struggling populations to recover, but if dragon numbers become so small that parthenogenesis becomes the norm, reduced genetic diversity could push the species further towards extinction

Zoos need to take heed as well. Females are usually kept apart from males, who are transferred between zoos to act as reptilian studs. This reduces the risk of aggression on the part of the larger males, but it could lead to a excessive number of virgin births.

Clearly, the key to saving this magnificent animal is more research into how best to account for its new-found ability, and breed a healthy diverse population.

More about animal sex and reproduction: 
Butterflies evolve resistance to male-killing bacteria in record time 
When the heat is on, male dragons become females
Chimerism, or How a marmoset’s sperm is really his brother’s
Aphids get superpowers through sex

Reference:  Watts, Buley, Sanderson, Boardman, Ciofi & Gibson. 2006. Nature 444: 1021-1022.

Technorati Tags: komodo dragons, virgin birth, parthenogenesis, zoos, breeding programmes, science

Aphids get superpowers through sex

 

When aphids have sex, the male often infect the females with beneficial bacteria that gives them useful powers, like the ability to fight off parasites.

As far as humans are concerned, sexually-transmitted infections are things to avoid. But imagine if these infections didn’t cause death and disease, but gave you superpowers instead. It may sound like a bizarre fantasy, but it’s just part of life for aphids.

Aphids mostly reproduce without sex, giving rise to many all-female generations that are exact copies (clones) of their parents. They only have sex once in autumn, the only time when mothers give birth to males.

Asexual reproduction makes sense for aphid mothers since they pass on all of their genes to their daughters. If they reproduced sexually, their offspring would only inherit half of their genes, diminishing their legacy. Why then would a female aphid choose to have sex at all?

Nancy Moran and Helen Dunbar at the University of Tuscon a surprising answer. They may be trying to receive sexually-transmitted infections from other aphids.

Sex is power

Aphids carry various strains of bacteria inside their bodies. These ‘symbionts’, far from causing disease, actually provide the aphids with useful abilities. Some strains allow them to feed off a greater variety of plants, while others give them the ability to withstand higher temperatures. Some can even save their lives.

Aphids are commonly targeted by parasitic wasps. These grisly creatures lay their eggs inside the aphids’ bodies and the developing grubs eat their hosts from the inside out. But aphids that carry the symbiont Hamiltona defensa avoid this cruel fate, because their bacterial partners destroy the developing wasp grubs. Clearly, these are friends worth having.

Mothers pass on the helpful symbionts to their children but they can also be transferred between unrelated individuals through sex. In fact, the only way for a female to get some symbionts in the first place, or to add to an existing collection, is to have sex with an infected male.

Sex, flies and symbionts

If other insects do this, we could use sexually-transmitted bacteria to our advantage. For example, the African tsetse fly, carrier of sleeping sickness, also harbours a symbiont that resembles the species found in aphids.

Moran and Dunbar suggest that we could infect flies with a genetically-engineered symbiont that disables or kills the parasite that causes sleeping sickness. Infected males would pass this killer symbiont through the population and reduce the spread of sleeping sickness.

More on aphids: 
Aphids defend themselves with chemical bombs

More about animal sex and reproduction:
Virgin birth by Komodo dragons
Butterflies evolve resistance to male-killing bacteria in record time 
When the heat is on, male dragons become females
Chimerism, or How a marmoset’s sperm is really his brother’s

 

Reference:  Moran & Dunbar. 2006. PNAS Epub