Biology – Page 78 – David Bradley

Sperm and eggs

Adam Bjork and Scott Pitnick of Syracuse University have found a sexy paradox. But, don’t get too excited, it’s fruit fly sex we’re talking here and specifically sperm and egg production in Drosophila.

Previous work in the Pitnick lab showed that after sex, sperm competition takes place within the females (who mate with several males) can lead to decreased sperm quantities by favouring the production of larger sperm. In other words, never mind the quantity feel the length mode comes into play, because female fruit flies have evolved so that longer sperm have a greater chance of successfully fertilizing eggs. Fertilization failure is obviously important as individuals who fail are essentially an evolutionary dead end.

This leads to the ‘big sperm paradox’ because the idea that postcopulatory sexual selection could favour the evolution of giant sperm clashes with traditional sexual selection theory, which predicts that the most successful sperm competitors will be the males that produce many, tiny sperm. As males evolve to produce larger – and therefore fewer – sperm, eggs become less rare, and sexual selection should weaken, according to theory. The term ‘isogamy’ refers to the state at which males and females have equal investment per gamete (sex cell) when producing sperm and eggs. In a truly isogamous population, each sperm and each egg would have a chance to participate in a successful fertilization. In such a population, sexual selection would be extremely weak, as there would be little or no competition among males to fertilize eggs.

To investigate this apparent paradox between empirical data and traditional theory, Bjork and Pitnick set out to measure the strength of sexual selection in four Drosophila species of varying sperm length, ranging from the anisogamous D. melanogaster (in which a male produces 30 sperm in the time it takes a female to make one egg) to the nearly isogamous D. bifurca (where just six sperm are produced per egg). They found that, contrary to theoretical predictions, the level of competition among males did not decrease; the strength of sexual selection remained high as sperm size increased. Their results show that, once females evolve a preference for longer sperm, intense sperm competition can actually reverse the trajectory of sperm evolution so that the most successful males are those with the most female-like strategy of producing very few, large gametes.’The sperm of Drosophila bifurca is 20 times longer than the male that produces it,’ says Bjork.

‘To put that into perspective, if humans made sperm that long and you took a six-foot man and stood him on the goal line of a football field, his sperm would stretch out to the 40-yard line.’

While it is fascinating, the evolution of giant sperm is puzzling. Says Bjork: ‘Until recently, it was widely believed that selection generated by sperm competition favors males that manufacture the smallest gametes possible in order to maximize sperm number. In essence, sperm competition is attributed with the evolutionary maintenance of anisogamy. I became interested in understanding whether the very act of sexual selection, by definition, can limit its own ability to act.’

The next step is to investigate the details of the effects of sperm length evolution on the intensity of sexual selection.

Details were published in Nature under the banner “Intensity of sexual selection along the anisogamy—isogamy continuum”, which doesn’t give much away unless you’re in the field. The paper’s DOI reference number is 10.1038/nature04683. Use our DOI lookup tool to go straight to it (simply cut and paste the DOI and click DOI lookup in the right-hand toolbox) and don’t forget you can grab the simple script to add this and our other toolboxes to your website.

Chirality – panda thumb

The chirality of life, an issue I’ve discussed on numerous occasions in these and other pages, emerges as yet another source of pseudo-science for the intelligent design lobby. Apparently, the bias in handedness among the molecules of life – amino acids, DNA, etc, could not have arisen spontaneously without a guiding hand…

An interesting discussion on this very subject is underway at The Panda’s Thumb blog, it will be interesting to see where it leads. However, my own interviews with chemists on this subject over the years point to a wide range of natural phenomena that could have led to the emergence of the chiral bias with no need to invoke a supernatural hand.

Human to human bird flu

The World Health Organization has expressed concern that a recent cluster of deaths associated with the H5N1 virus in Indonesia may not have originated with an animal host, suggesting the possibility of human to human transmission of the virus. However, it also cautions that the analytical evidence suggests that the virus has not mutated into a human transmissable form, which means we are not just yet on the verge of a global bird flu pandemic after all.

The news media inevitably picked up on this warning and ran with it, but thankfully the BBC saw the double-edged nature of the WHO announcement points out with some degree of rational response that many people in Indonesia, as in other southeast Asian countries, live in such close proximity to their animals and not necessarily in the most hygienic of circumstances that the likelihood of catching bird flu is much higher in such an environment.

It is the lack of a mutated form of H5N1 among these victims that means we are not yet doomed to see the feathers fly globally.

Sex and Vomeronasal Attraction

The vexing question of whether sex pheromones play a role in human sexual attraction raises its ugly head once again, this time in a posting on The Register. Most scientists would say that there is little evidence that humans rely very much on pheromones for sexual attraction directly. However, others scientists suggest that a tiny organ in the nasal cavity, the so-called vomeronasal organ (VNO), or Jacobson’s organ, can detect chemical attractants that pass between people and are not apparent to us at the thinking level. Moreover, anecdotal evidence would point to smell having a very strong effect in sex whether or not we believe it’s pheromonal or not.

The VNO definitely plays a role in the lives of other animals from cats to snakes and from elephants to mice. In humans the organ seemingly all but disappears even before birth, leaving just a few people with a tiny pit in the septum that might have some vestige of pheromonal responsiveness.

Until, we find specific chemicals that trigger sexual attraction when sniffed, in double-blind controlled tests, it is likely that for the foreseeable future there will be no genuine, working product.

Touch Wood – Short History of Viagra

Just a reminder that the Sciencebase archive of science articles is stuffed full of interesting tid-bits (to use the PC term). Among the most popular pages (attracting the most readers in other words) is the repro of a feature article I wrote some time ago for Tomorrow’s World magazine on the subject of Viagra.

Said item was illustrated with a large banana and two strategically placed apples. I was going to dig out the original paper cutting and scan it for this item, but didn’t fancy getting a search engine rude-filter slapped on the page.

The Viagra article is here.

Zoo Poo

This is one of those lovely science stories that is sure to catch even the tabloid media’s attention as well as give people the opportunity once again to complain about “what scientists do”.

According to CSIRO Livestock Industries scientist Andre-Denis Wright, droppings from rhinos and elephants at Perth Zoo are being put under the microscope to help scientists determine whether or not various strains of protozoa, which help break food down in the guts of “exotic” animals can be introduced into the guts of sheep and cattle to help them thrive in the harsh conditions of the Australian outback.

As part of the Zoo-Poo project scientists have so far sequenced the DNA of protozoa found in a range of faecal samples from rhinos, elephants, orangutans, and red pandas. The study will reveal the origins of various protozoa and identify common protozoal species found in animals from different parts of the world.

Wright is collaborating with rumen microbiologist Burk Dehority of Ohio State University. Dehority has previously found some of the same protozoa in different grazing animals from South America, Africa and Alaska. However, the protozoa from the guts of marsupials in Australia appear to be quite distinct. He has now received poo samples from the zoo animals including a “grab sample of faeces” from a rhino, collected by a zoo handler before it hit the ground. [Nice]

“Very large mammals like the rhino and the elephant eat voluminous amounts of roughage and they are able to take the soluble nutrients out and ferment them in the cecum and then they just pass the roughage out,” explains Dehority, “hey exist on the fact that they just eat volumes.”

He said little was known about how the guts of large animals worked and the research would assist in developing better nutrition for zoo animals as well as livestock.

Phylum Arthropoda

A common search carried out by sciencebase visitors is “Phylum Arthropoda”. Innocent enough, of course, there are bound to be lots of followers of the site interested in insects, arachnids, and crustaceans.

Anyway, the best starting point to get plenty more info on the arthropods is, as usual, Wikipedia. But, if you’re after science lessons on arthropods then check out the K5 science lesson plans page and follow the link to the Columbia Education Center in Portland, Oregon, to get to the specific listings. If you’ve got a particular creature in which you’re interested, please use the sciencebase search box top right, we can’t guarantee that you’ll find what you’re looking for among the sciencebase pages themselves but there are usually other links on the site you can follow to find out more.

Bremelanotide MSDS

Readers interested in sexual chemistry will have spotted the recent item on bremelanotide (Sex Gets Up Women’s Noses, April 24, 2006), which is soon to enter Phase III clinical trials for female sexual dysfunction (see also PLoS Medicine on the subject of disease mongering).

Anyway, recent sciencebase visitors have been trying to locate the material data safety sheet (MSDS) for this compound (judging from the recent spate of searches on the site for that term). Anyway, ChemSpy.com has excellent access to several MSDS sources here. If it’s listed anywhere you should be able to find the bremelanotide MSDS there.

A blogger on another site discussing my short bremelanotide article, suggested that the fact this drug is odourless and colourless represented a serious risk in terms of men spiking a woman’s drink, but I wonder…this drug doesn’t knock you out or give you amnesia it just makes you horny, so if Mr B. Nomates can’t score under normal circumstances when any number of potential mates may be horny or not, it won’t seriously boost his chances will it?

Turning Sperm Heads

Size really does matter! In fact a micro device that can analyse even the smallest of the small could help solve one of “man’s” greatest mysteries – what turns a sperm’s head and sends it in the direction of the egg for that fertilizatory encounter?

Sperm are well-known for turning their microscopic heads and changing direction (at least to those with a microscope who like to view such tiny events). Previous research (about which I wrote in 1991 under the heading “Not every sperm is sacred”) revealed that sperm turn in response to chemical signals, a process termed chemotaxis, and even have their own olfactory receptors. Such chemical messages may play key roles in the fertilization process. Defects in sperm chemotaxis may be a cause of infertility, and sperm chemotaxis could potentially be used as a diagnostic tool to determine sperm quality to treat male infertility.

However, Milos Novotny and Stephen Jacobson of Indiana University have developed a new tool to probe exactly how sperm chemotaxis occurs. In the current issue of Anal Chem, they describe the initial tests on their microfluidic device for studying sperm chemotaxis: “An advantage of the microfluidic platform over conventional chemotaxis assays is the ability to create chemical gradients with temporal and spatial stability, leading to greater repeatability in the experimental conditions.”

They add that microfluidic devices provide a convenient, disposable platform for conducting chemotaxis assays.

http://dx.doi.org/10.1021/ac052087i

Sex Gets Up Womens Noses

Spray-on sex could usher in an age of McNookie, according to an article in The Observer on Sunday. PT-141 is billed as libido in an atomiser, says the paper, and could finally offer women the chance to turn on their sexual desire as and when they need it.

“A dose of PT-141 results, in most cases, in a stirring in the loins in as little as 15 minutes,” reports Julian Dibbell, “Women, according to one set of results, feel ‘genital warmth, tingling and throbbing’, not to mention ‘a strong desire to have sex’.”

So, what is PT-141?

It’s an odourless and colourless synthetic chemical that you inhale deeply through a small, white plastic inhaler. The compound, produced by Palatin Technologies and currently undergoing regulatory assessment, is a melanocortin-based therapy that seems to work directly on the brain rather than simply stimulating the loins as is the case with Viagra.

The drug’s market name is Bremelanotide and it’s a word you’re sure to see a lot more of in the near future as spammers start offering it and variations on the theme (watch out for Bremelan0tide in your email subject lines). Its mode of action is poorly understood, but unlike thousands of years of rhino horn, tiger dick, and oysters this one definitely seems to work.

Molecular structure of Palatin's PT-141 (Bremelanotide)

“It’s not merely allowing a sexual response to take place more easily,” Michael Perelman, co-director of the Human Sexuality Program at New York Presbyterian Hospital and a sexual-medicine adviser on the PT-141 trials explains, “It may be having an effect, literally, on how we think and feel.” What is known is that it acts on at least one of five known melanocortin receptor subtypes in the brain. These are chemical receptors that regulate a diverse array of functions including sexual arousal, appetite, energy maintenance and inflammation. The company is working on drugs to affect these receptors and so help with sexual dysfunction, obesity and cachexia (disease-related muscle loss).

One has to wonder though whether the advent of drugs like PT-141 is simply another example of the kind of drug mongering I discussed a few years ago my Alchemist column on ChemWeb. There now seem to be dozens more “diseases” than there ever were. Is it just coincidence that problems that were not considered problems are now categorised as syndromes and disorders have emerged at just the time when the patents are expiring on the billion dollar staples of the pharmaceutical industry? Maybe, maybe not. Some of those disorders, such as restless legs syndrome, may sound like spurious ailments invented by a desperate industry, but they certainly aren’t for those who suffer from them. Even shyness and anxiety might one day succumb to drugs, and why not? If these disorders are debilitating then who are we to suggest that sufferers have no right to a treatment that “cures” them?

Sometimes there is no relief for a whole range of problems without pharmaceutical intervention. Whether or not we should allow people the option to control their sexual desire chemically is a moot point, the fact is humans have used all kinds of methods to loosen up a libido for millennia, and will continue to do so whether that’s with a regulated drug or something else. Palatin seems simply to be hoping to take a slice out of the action.

Anyway, for those interested in such things Palatin gives the basic structure of its product as a short cyclic peptide with the sequence – Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH. It’s currently in Phase III clinical trials, so it shouldn’t be too long before those spams start arriving.