Genes versus heat – a reptile sex trigger

High temperatures can make an Australian lizard that is genetically male develop into a female. The finding throws new light on how sex is determined in reptiles.

For most reptiles, a gene on a sex chromosome triggers an embryo to develop as either a male or a female. In some species, males have an X and a Y chromosome, while females are XX, as in mammals. In other species of lizards, males are ZZ while females are ZW, as in birds.

But for a third group of reptiles, which includes all crocodiles, alligators and marine turtles, temperature, rather than a gene on a sex chromosome, triggers either male or female differentiation. Extreme low or high temperatures generally lead to more females.

Now a team led by Alex Quinn at Canberra University in Australia has found that the central bearded dragon (Pogona vitticeps) is susceptible to both types of sex trigger, and that temperature can override its genetic gender.

Transitional form

When the team incubated eggs at relatively high temperatures – between 34°C and 37°C – the majority of embryos that had ZZ sex chromosomes (genetically male), hatched as females. The team thinks the bearded dragon represents a transitional form, in evolutionary terms, between the two main methods of sexual determination.

The research shows that, for the bearded dragon at least, the W chromosome is not necessary in producing a female. The team suspects that a double dose of a particular gene on the Z chromosome is instead crucial for maleness, and that this gene is inactivated by high temperatures.

“The possibility that there is a male-determining, dosage-dependent gene on the Z chromosome of bearded dragons is an important insight,” says Quinn, “because to date, scientists have discovered the master sex-determining gene only in mammals and a single species of fish.”

The team plans to hunt for that master gene in the bearded dragon. They also want to investigate how widespread the phenomenon of temperature sex reversal really is in reptiles.

If many other reptiles with sex chromosomes are also susceptible to temperature, this would broaden the number of species that could be vulnerable to climate change.

“The concern is that the current rate of climate warming could be too rapid for these species to adapt to, and this could potentially result in heavily skewed sex ratios, and even population crashes in some cases,” Quinn says.

Journal reference: Science (vol 316, p 411)

Low-salt diet prevents heart attacks and strokes

Eating less salt can reduce the risk of cardiovascular disease by 25% and cut the risk of death from all causes by a fifth, according to a new study.

The 15-year study of 2400 people demonstrates for the first time that cutting back on salt can reduce the risk of diseases such as stroke and heart attack, in addition to lowering blood pressure.

Volunteers in the study who were assigned to a low-salt regime had a 20% lower risk of death from all causes over the course of the study than their control counterparts. The findings should compel governments to take more action to reduce the salt content of processed foods, says Nancy Cook at the Brigham and Women's Hospital in Boston, Massachusetts, US, who led the study.

Numerous studies have documented how consuming foods high in salt can lead to high blood pressure. This happens because the salt draws more water into the blood, and the increase in fluid volume exerts more pressure on vessel walls. High blood pressure is known to contribute to heart disease, but few studies have shown a direct link between salty foods and the condition.

Salt snapshot

In the late-1980s and early-1990s Cook and colleagues collected urine samples from more than 3000 people with above-normal blood pressure. Analysing the urine samples collected over the course of a 24-hour-period gave the researchers a snapshot of the subjects' salt intake. On average, they were consuming 10 grams of salt per day.

Cook's team then randomly assigned half of these participants to attend weekly workshops that taught how to cook low-salt meals and read nutrition labels on packaged foods.

After approximately three months of this nutrition counselling, urine sampling revealed that the subjects reduced their daily salt intake by about 3 grams per day on average – the equivalent of about half a teaspoon.

Fifteen years later Cook's team was able to obtain follow-up health information about 2415 of the participants from medical records and telephone interviews.

Healthy choices

Phone interviews indicated that those who had received training on how to reduce their salt intake many years ago continued to consume less of it than their control counterparts. For example, 47% of those who received this intervention said they looked for reduced-salt foods in the supermarket, compared with 29% of the control group.

Of the 200 people who had developed cardiovascular disease – including heart attacks and stroke – in the past 15 years, 112 had received no dietary recommendations and 88 were in the group taught to reduce their salt intake.

After controlling for factors such as weight and age, the researchers calculated that reducing one's salt intake by 30% could decrease the risk of cardiovascular disease by 25%.

Cook says that the results of the study should encourage governments to "work with the food industry to come up with lower sodium foods", and notes that salt content is highest in processed and fast-foods. "People generally consume much more salt than what is biologically needed."

In 2006, the American Medical Association urged the US Food and Drug Administration to revoke the "generally recognised as safe" (GRAS) status of salt and to adopt stricter salt guidelines.

Current US dietary guidelines recommend that people consume less than one teaspoon of salt per day.

Journal reference: BMJ (DOI: 10.1136/bmj.39147.604896.55)

Nuclear Weapons - Fission Wreapons

Nuclear weapons exploit two principle physical, or more specifically nuclear, properties of certain substances: fission and fusion.

Fission is possible in a number of heavy elements, but in weapons it is principally confined to what is termed slow neutron fission in just two particular isotopes: ²³⁵U and ²³⁹Pu. These are termed fissile, and are the source of energy in atomic weapons. An explosive chain reaction can be started with relatively slight energy input (so-called slow neutrons) in such material.

Isotopes are 'varieties' of an element which differ only in their number of neutrons. For example, hydrogen exists as ¹H ²H and ³H -- different isotopes of the same chemical element, with no, one, and two neutrons respectively. All the chemical properties, and most of the physical properties, are the same between isotopes. Nuclear properties may differ significantly, however.

The fission, or 'splitting' of an atom, releases a very large amount of energy per unit volume -- but a single atom is very small indeed. The key to an uncontrolled or explosive release of this energy in a mass of fissile material large enough to constitute a weapon is the establishment of a chain reaction with a short time period and high growth rate. This is surprisingly easy to do.

Fission of ²³⁵U (uranium) or ²³⁹Pu (plutonium) starts in most weapons with an incident source of neutrons. These strike atoms of the fissile material, which (in most cases) fissions, and each atom in so doing releases, on average, somewhat more than 2 neutrons. These then strike other atoms in the mass of material, and so on.

If the mass is too small, or has too large a surface area, too many neutrons escape and a chain reaction is not possible; such a mass is termed subcritical. If the neutrons generated exactly equal the number consumed in subsequent fissions, the mass is said to be critical. If the mass is in excess of this, it is termed supercritical.

Fission (atomic) weapons are simply based on assembling a supercritical mass of fissile material quickly enough to counter disassembly forces.

The majority of the energy release is nearly instantaneous, the mean time from neutron release to fission can be of the order of 10 nanoseconds, and the chain reaction builds exponentially. The result is that greater than 99% of the very considerable energy released in an atomic explosion is generated in the last few (typically 4-5) generations of fission -- less than a tenth of a millisecond.

This tremendous energy release in a small space over fantastically short periods of time creates some unusual phenomena -- physical conditions that have no equal on earth, no matter how much TNT is stacked up.

Plutonium (²³⁹Pu) is the principal fissile material used in today's nuclear weapons. The actual amount of this fissile material required for a nuclear weapon is shockingly small.

In the Fat Man (Nagasaki) weapon design an excess of Pu was provided. Most of the remaining bulk of the weapon was comprised of two concentric shells of high explosives. Each of these was carefully fashioned from two types of explosives with differing burn rates. These, when detonated symmetrically on the outermost layer, caused an implosion or inward-moving explosion.

The two explosive types were shaped to create a roughly spherical convergent shockwave which, when it reached the Pu 'pit' in the center of the device, caused it to collapse.

The Pu pit became denser, underwent a phase change, and became supercritical.

A small neutron source, the initiator, placed in the very center of this Pu pit, provided an initial burst of neutrons -- final generations of which, less than a microsecond later, saw the destruction of an entire city and more than 30,000 people..

Nearly all the design information for weapons such as these is now in the public domain; in fact, considering the fact that fission weapons exploit such a simple and fundamental physical (nuclear) property, it is no surprise that this is so. It is more surprising that so much stayed secret for so long, at least from the general public.

A neutron reflector, often made of beryllium, is placed outside the central pit to reflect neutrons back into the pit. A tamper, often made of depleted uranium or ²³⁸U helps control premature disassembly. Modern fission devices use a technique called 'boosting' , to control and enhance the yield of the device.

Today's nuclear threat lies mostly in preventing this fissile special nuclear material (often referred to as SNM) from falling into the wrong hands: once there, it is a very short step to construct a working weapon.

source : simplethinking