May 21, 2013
For nearly 150 years, starting in the late 17th century, millions of people living in Ireland subsisted largely off one crop: the potato. Then, in 1845, farmers noticed that their potato plants’ leaves were covered in mysterious dark splotches. When they pulled potatoes from the ground, most were shrunken, mushy and inedible. The blight spread alarmingly quickly, cutting yields from that year’s harvest in half. By 1846, harvest from potato farms had dropped to one quarter of its original size.
The disease—along with a political system that required Ireland to export large amounts of corn, dairy and meat to England—led to widespread famine, and nearly all of the few potatoes available were eaten, causing shortages of seed potatoes that ensured starvation would continue for nearly a decade. Ultimately, over one million people died, and another million emigrated to escape the disaster, causing Ireland’s population to fall by roughly 25 percent; the island has still not reached its pre-famine population levels today.
At the time, the science behind the blight was poorly understood, and most believed it was caused by a fungus. During the twentieth century, scientists determined that it was caused by an oomycete (a fungus-like eukaryote) called Phytophthora infestans. However, without access to the 1840s-era specimens, they couldn’t identify exactly which strain of the organism was responsible.
Now, an international group of scientists has gone back and sampled the DNA of Irish potato leaves preserved in the collections of London’s Kew Gardens since 1847. In doing so, they discovered that a unique, previously unknown strain of P. infestans that they call HERB-1 caused the blight.
The researchers, from the Sainsbury Laboratory in the UK and the Max Planck Institutes in Germany, came to the finding as part of a project sequencing DNA from 11 different preserved historial samples and 15 modern ones to track the evolution of the pathogen over time, published today in the journal eLife [PDF].
Currently, P. infestans is distributed worldwide, with the vast majority comprised of the destructive. strain US-1. Most of the other strains of P. infestans occur only in Mexico’s Toluca Valley, where wild potato varieties are indigenous, so scientists long believed that US-1 had been responsible for the 1840s famine.
But when the researchers extracted small pieces of intact DNA from the old dried-out potato leaves, originally collected from from Ireland, Great Britain, Europe and North America, and compared them with present-day P. infestans specimens, they found that the strain responsible for the famine differed slightly from today’s US-1.
Based on their analysis of the genetic variation between the two strains and the other historical samples, they suggest that sometime in 1842 or 1843, the ancestor of the HERB-1 strain of P. infestans made it out of Mexico to North America and then to Europe, perhaps contained within the potatoes that ships carried as food for their passengers. Soon, it spread across the world, triggering famine in Ireland, and persisting until the 1970s, when it died out and was largely replaced by the US-1 strain. The two strains likely split apart sometime soon after their common ancestor made it out of Mexico.
The study is the first time that the genetics of a plant pathogen have been analyzed by extracting DNA from dried plant samples, opening up the possibility that researchers can study other plant diseases based on the historical collections of botanical gardens and herbaria around the world. Better understanding the evolution of plant diseases over time, the team says, could be instrumental in figuring out ways to breed more robust plant varieties that are resistant to the pathogens that infect plants today.
May 13, 2013
Humans drew the short end of the toothbrush when it comes to our pearly whites’ longevity. Other animals such as reptiles and fish frequently lose and replace their teeth by growing new ones, but people are stuck with the same set of mature adult teeth their entire lives. If they lose a tooth–or all 32–dentures are usually the only option.
Oddly enough, alligators’ deadly chomps may hold a clue for how scientists could coax humans into regrowing teeth. These reptiles belong to the order Crocodilia, who, with their famous cheerful grins, caused songwriters to warn that you should never smile at a crocodile. To the bane of Captain Hook and other victims of gator and croc attacks, the large reptiles often regrow their razor teeth multiple times. Researchers think that, given time, technology may advance so that we can borrow these reptilian smiles. But first, scientists need to understand just how these animals keep their smiles toothy.
In a paper published this week in the Proceedings of the National Academy of Sciences, an international team of researchers attempted to get at the mechanisms behind the superior tooth regenerating abilities of one species of Crocodilia–the American alligator–in the hopes of applying the results to humans.
In humans, organs such as hair, scales, nails and teeth “are at the interface between an organism and its external environment and therefore, face constant wear and tear,” the researchers write. But alligators have evolved ways to deal with these challenges. The carnivores can replace any of their 80 teeth up to 50 times throughout their 35 to 75-year lives. Small replacement teeth grow under each mature alligator tooth, ready to spring into action the moment a gator loses a tooth.
To figure out the molecules and cells responsible for replacement, the researchers used X-rays and small tissue samples from alligator embryos, hatchlings and 3-year old juveniles’ developing teeth. They also grew tooth cells in the laboratory and created computer models of the process. Alligator teeth appear to cycle continuously, they write, but in fact the animals’ teeth seem to go through three distinct phases: pre-initiation, initiation and growth.
Once an alligator loses a tooth, these three phases kick off. The dental lamina, or a band of tissue associated with the initial stages of tooth formation in many animals, begins to bulge. This triggers stem cells and an array of signaling molecules that direct the process of forming a new tooth.
These results may be applicable to humans’ pearly whites. Alligators’ flesh-chomping incisors are surprisingly similar to well-organized, complex vertebrate teeth such as ours. In humans, a remnant of the dental lamina–the structure crucial to tooth formation–still exists and sometimes wrongly activates and begins forming toothy tumors. If the researchers could better tease out the molecular signaling pathways behind alligator tooth replacement, they reason, they they may be able to induce those same chemical instructions in humans to coax the body into forming a new tooth after one gets kicked out in a soccer game or has to be removed after becoming infected.
Alternatively, doctors may be able to shut off the molecules responsible for conditions that cause uncontrolled tooth formation. Individuals suffering from cleidocranial dysplasia syndrome grow many unusually shaped, peg-like teeth, for example, and people with Gardner syndrome also grow supernumerary, or extra, teeth.
While the researchers still need to clarify more molecular details behind alligator tooth growth, this initial study does hint that doctors and dentists may someday be able to selectively bestow patients with the reptiles’ tooth-regenerating abilities.
“Based on our study, it may be possible to identify the regulatory network for tooth cycling,” the researchers conclude. “This knowledge will enable us to either arouse latent stem cells in the human dental lamina remnant to restart a normal renewal process in adults who have lost teeth or stop uncontrolled tooth generation in patients with supernumerary teeth.”
Either way, they note that “Nature is a rich resource from which to learn how to engineer stem cells for application to regenerative medicine.”
May 9, 2013
With their big, glossy black eyes and downy fluff, baby Weddell seal pups are some of the most adorable newborns in the animal kingdom. But these cute infants are far from helpless bundles of joy. New research published in the journal Marine Mammal Science reveals that Weddell seal pups likely possess the most adult-like brain of any mammal at birth.
The seal pups’ brains, compared to adult seals’ brain proportions, are the largest known for any mammal to date. The researchers write that this is “remarkable” considering that the pups are quite small at birth compared to many other newborn mammals.
To arrive at these findings, a team of researchers from the Smithsonian Environmental Research Center and the National Museum of Natural History traveled to Antarctica to collect fresh pups specimens. They took advantage of the fact that many pups never make it to adulthood due to stillbirths, abandonment and accidental death, such as being crushed by an adult. The researchers collected 10 dead seal pups (which quickly freeze in the Antarctic temperatures), conducted a few measurements and then decapitated and shipped the frozen heads back to the Smithsonian. They also tossed in a couple adult Weddell seal heads into the mix, one of which had died from acute toxemia–possibly from its gut being punctured by a fish spine–and the other whose cause of death could not be determined.
Back in the U.S., the researchers partially thawed the skulls in a lab and–like a well picked-over Thanksgiving turkey–manually peeled the tissue off of the baby seal faces. Then, they drilled into the skulls to extract the intact brains. Finally, they put the bones into a tank full of flesh-eating beetles to remove any remaining scraps of meat. Clean skulls and brains in hand, they went about taking measurements, and they also drew upon measurements of some older Weddell Seal skull specimens from the museum’s collection.
Remarkably, baby Weddell seal brains are already 70 percent developed at birth, the team found. Compare this to human infants, whose brains are a mere 25 percent of their eventual adult mass. As a Smithsonian statement explains, baby animals born with proportionally larger brains usually live in challenging environments in which they need to act quickly in order to survive. Other animals that share this trait include most marine mammals, zebras and wildebeest.
For Weddell seal pups, large brains likely help with diving under ice sheets and orienting themselves under water at less than three weeks old–an extremely dangerous task for any mammal, newborn or not. The pups must acclimate quickly since Weddell seal mothers abandon their young at about 6 weeks old, meaning they need to be able to completely fend for themselves when that day arrives.
In nature, however, everything comes with a price. The Weddell seal pups may have the biggest, best developed brains on the block when compared to what they will be as adults, but this metabolically taxing organ requires excessive energy to maintain. A pup weighing just 65 pounds needs between 30 to 50 grams of glucose per day in order to survive, and the team estimates that the energetically hungry brain may account for a full 28 grams of that demand.
Luckily for the seal pups, their mothers’ milk is almost exactly matched to the babies’ caloric needs. Weddell seal milk supplies about 39 grams of sugar per day. Females seals, however, lose significant weight while tending to their young, which jeopardizes their own survival. At their mother’s cost, the babies’ brains are allowed to thrive. That is, until their mother decides she’s had enough with the nurturing and leaves her pups to survive on their own.
April 30, 2013
Baby animals may seem irresistibly adorable, but in reality many of them are calculating killers. Hyena, wolf or even dog litter runts are pushed aside by their larger siblings and left to go hungry; fuzzy white egret chicks will kick their weaker clutch mates out of the nest to certain doom; and baby golden eagles sometimes go so far as to snack on their smaller brothers and sisters while their mother looks on.
Perhaps most disturbing of all, however, is the case of the baby sand tiger shark. While sharks may not be the most snuggly animals to begin with, the sand tiger shark sets a new precedent for fratricide. This species practices a form of sibling-killing called intrauterine cannibalization. Yes, “intrauterine” refers to embryos in the uterus. Sand tiger sharks eat their brothers and sisters while still in the womb.
Even by nature’s cruel standards, scientists admit that this is an unusual mode of survival. When sand tiger sharks develop in their mother’s uteri (females have both a left and right uterus), some–usually the embryo that hatched first from its encapsulated, fertilized egg–inevitably grow faster and larger than others. Once the largest embryos cross a certain size threshold, the hungry babies turn to their smaller siblings as convenient meals. “The approximately 100 mm hatchling proceeds to attack, kill and eventually consume all of its younger siblings, achieving exponential growth over this period,” a team of researchers who investigated the phenomenon wrote this week in Biology Letters.
From what began as two uteri full of a dozen embryos results in just two dominating baby sand tiger sharks coming full term. What’s more, once the unborn babies consume all of the living embryos, they turn to their mother’s unfertilized eggs next, in a phenomenon called oophagy, or egg-eating. By the time those two surviving babies are finally ready to be introduced into the big, bright world, all of the pre-birth inner feasting has paid off. They emerge from their mother measuring in at about 95 to 125 centimeters long, or a bit longer than a baseball bat, meaning fewer predators can pick them off than if they had shared food with siblings and were smaller.
This peculiar situation has implications for the genetic makeup of the species. Female sand tiger sharks, like many animals, mate with multiple males. Oftentimes in nature, females determine which males will sire the next generation by selectively choosing to mate with the most impressive bachelor (or bachelors) around. If mating with multiple males at any given time–as sharks, insects, dogs, cats and many other animals sometimes do–the babies that the female eventually produces share the same womb with siblings that may have different fathers.
In this case, however, there are two modes of selection at work. Females may choose mates, but that does not guarantee those males’ genes will make the cut. The embryos the males sire will also have to survive the subsequent frenzy of cannibalism going on inside the female’s body.
To find out whether some males are mating but missing out on actually producing offspring, the authors of this new study undertook microsatellite DNA profiling of 15 sand tiger shark mothers and their offspring. The researchers collected the sharks from accidental mortality events near protected beaches in South Africa between 2007 to 2012. By comparing the embryo genetics, the researchers could determine how many fathers were involved in fertilizing the eggs.
Nine of the females, or 60 percent, had mated with more than one male, the researchers found. When it came to which embryos hatched and grew large first (and thus would have survived if their mothers hadn’t have been killed), 60 percent shared the same father. This means that even if a female mates with more than one male, there is no guarantee that the male has been successful in passing on his genes. Rather, he could have just provided a convenient entree for another male’s offspring.
This also explains some male sand tiger shark behavior and physiology. Male sand tiger sharks often guard their mates against other males just after copulation. Males of this species also produce a conspicuously large amount of sperm compared to other sharks. Both of these characteristics increase the likelihood that the embryo fertilized by that male will successfully implant in the female’s uterus earlier, giving it a significant head start for developing more quickly than its siblings, which makes it more likely that the recent mate’s offspring will eat the others that may come along.
As for the females sand tiger sharks, some researchers think they actually may not have much of a choice when it comes to mating with multiple males. It could be that females just give in to some amorous partners because the energetic cost of resisting those advances outweighs the cost of just conceding to the act–a behavior biologists call the convenience polyandry hypothesis. In this case, however, females may still get the final laugh since the males they first mated with and most likely preferred will have the greater chance of actually triumphing as the father of their children. “[Embryonic cannibalism] may allow female sand tigers to engage in convenience polyandry after mating with preferred males without actually investing in embryos from these superfluous copulations,” the researchers speculate.
While the females did invest in initially developing those doomed embryos, those investments are much smaller than what would be required to bring multiple embryos to full term. Those smaller embryos also represent resources allocated to the stronger, dominate embryonic winners, which thus have a better chance of surviving and passing on their mother’s genes than if she had spent the energy to instead birth multiple, weakling babies. In a way, the mother shark is providing nourishment for her strongest babies by producing multiple embryos that the most robust can eat.
“This system highlights that competition and sexual selection can still occur after fertilization,” the authors write. For example, the first embryo to implant may not end up being the the one that survives the gladiator arena of the sharks uterus. While this new research still needs to delve into the details of the competition that takes place within the uterus, a picture is emerging based upon these initial findings: Females may chose which males to mate with or may be coerced into reluctantly mating, but male sperm fitness and the quality of the embryos they produce could also carry significant weight in which animals ultimately wind up as winners in this system.
“This competition can play an important and probably under-appreciated role in determining male fitness,” the authors conclude.
April 24, 2013
Who you’re physically attracted to might seem like a frivolous, random preference. In recent years, though, science has told us that our seemingly arbitrary tastes often reflect unconscious choices that are based upon very relevant biological traits.
In general, we find symmetric faces more attractive, likely because they reflect a healthy underlying genome. Women typically prefer men with more distinctively masculine facial features because they indicate high testosterone levels and physical strength, while men prefer women with exaggerated youthful features, possibly because of the evolutionary advantages a male gets when coupling with a younger mate.
Despite all this research into our visual appearances, though, scientists have done relatively little digging into our auditory preferences when it comes to sexual attraction. Why do we find certain peoples’ voices attractive–and why do we sometimes find other types of voices such a turn-off? Specifically, why do women generally prefer men with deep voices, and men prefer women with higher ones?
At least according to a paper published today in PLOS ONE, the explanation is relatively simple: It’s all about body size. Researchers from University College London found that, at least among a sample of 32 participants, high-pitched female voices females were found to be attractive because they indicated the speaker had a small body. Deep male voices, on the other hand, were judged as more attractive because they conveyed that the speaker had a large frame—but were found to be most attractive when tempered by a touch of “breathiness,” suggesting the speaker had a low level of aggression despite his large size.
The group, led by Yi Xu, figured this out by playing recordings of digitally manipulated voices to the participants. The males in the study heard a computer-generated female voice saying phrases such as “I owe you a yo-yo” in which the voice was manipulated with a number of digital alterations in terms of pitch, formant (the particular peaks and valleys in a sound’s frequency spectrum) and other qualities.
The specific manipulations either conveyed a smaller body size or a larger one, based upon previous research that matched various voice qualities with different body sizes in humans. When asked to rate the voice’s attractiveness on a 1 to 5 scale, the men preferred the voices that suggested a smaller female. Past a certain point, though, higher voices were judged as no more attractive that slightly deeper ones. Listen to the most and least attractive (both, admittedly creepy) voices below:
The female participants’ voice preferences were similar, but slightly more nuanced. On the whole, they preferred deeper voices, which signaled a large body size, but another trait was also crucial: “breathiness.” The researchers hypothesized that this breathiness effectively takes the edge off a voice, making a man with a presumed large frame seem less aggressive and angry. They also polled the participants on whether they thought the simulated voices sounded angry or happy, and the breathy deep males voices were generally perceived as much happier and less angry than the less breathy (i.e. “pressed”) deep ones. Listen to the most and least attractive male voices below:
Beyond explaining the popularity of Barry White, the researchers say these findings correspond to much of what we know about voice preferences in the rest of the animal kingdom. Birds and other mammals, it turns out, have long been known to advertise their physical characteristics via the sound qualities in their mating calls.
All this points to an obvious question, though: Why would males prefer smaller females, and females prefer larger males in the first place? The researchers don’t attempt to address this question, but this duality reflects the sexual dimorphism present in most animal species. These differences generally result from sexual selection giving incentive to different mating strategies—so in this case, our voice preferences suggest that women benefit, in evolutionary terms, by mating with larger, but less aggressive men, while males benefit from mating with smaller females.
As the same time, what we commonly consider attractive varies dramatically over time and location—for example, dozens of prehistoric “Venus figurines,” discovered all over the world, portray extremely voluptuous female figures. So, if we tested the preferences of all humans throughout history, we might find a less obvious trend. This preference for small-voiced females and big-voiced males, then, might simply be an artifact of our contemporary cultural concepts of “attractiveness,” rather than a deep-seated evolutionary choice after all.