Good News for Fruit Fly Truckers

Science has arrived at credible hypotheses to explain a number of complex waking behaviors. Yet an overtly simpler behavior—one that doesn’t vary much from situation to situation or person to person, and involves a minimal amount of physical and mental activity—baffles us, leaving us with a surfeit of hypotheses that seem to explain some aspect of it, but none that is sufficient to explain it as a whole.

That perplexing behavior is sleep. It comprises 1/3 of our lives, yet we don’t really know why. It seems to play a number of roles. It acts as a restorative influence on the body, bolstering the immune system and our overall feeling of restedness. It also seems to be very important during development, occupying most of an infant’s time as its brain is rapidly maturing. And indications are that it's an important part of memory consolidation.

But none of these purported reasons for sleep can explain on its own why it may have evolved. For example, it seems that the restorative functions of sleep could be achieved without putting ourselves in a state where we are oblivious to our external environment—something that is very dangerous evolutionarily. The necessity of sleep during development doesn’t explain why adults need to continue doing it, and, while it may be less efficient, memory storage is still possible after sleep deprivation.

The unsatisfying nature of each of these hypotheses on their own has caused some to support an explanation of sleep that stresses its adaptive importance in helping our ancestors remain safe from predators. Sleep incapacitates us at a time (in the dark) when we are most vulnerable, keeping ancient humans out of the paths of nocturnal carnivores. While this might be evolutionarily adaptive, however, it doesn’t explain why we experience minimal conscious ability to monitor our environment during sleep (why not just a restful but conscious state?), or why animals that are predators and not generally hunted sometimes sleep a great deal (e.g. lions).

In addition to lacking a clear purpose for sleep, we have yet to understand the physiological mechanisms behind it. This has caused some scientists to turn to the model organism Drosophila for answers. The sleeping state of Drosophila has much in common with that of mammals. It involves homeostatic and circadian regulation, consists of long periods of immobility, becomes more fragmented with age, etc.

While scientists still haven’t come to a consensus on the reason for sleep, Drosophila research has led to several findings that have aided in the elucidation of its physiology. For example, it has helped to explain the role of neurotransmitters, like serotonin, that play a key role in sleep regulation. Recently, it has led to an amazing discovery: a way to reverse the effects of mental fatigue due to sleep deprivation by manipulating gene expression.

The cognitive deficits that Drosophila develop as a result of sleep deprivation are similar to those exhibited by humans. The extent of the impairment is correlated with the amount of time spent awake. Learning in Drosophila has been found to be dependent on a structure known as the mushroom bodies (MBs)—thought to be somewhat homologous to our hippocampus—and a dopamine receptor called the dopamine D1-like receptor (dDA1).

Scientists at the Washington University School of Medicine recently investigated whether sleep-loss induced learning impairments could be reversed in Drosophila. They used a learning task that takes advantage of the flies’ predisposition to fly towards a light. The flies were placed in a T-maze with a lighted tunnel and a dark tunnel. The lighted tunnel also contained a piece of filter paper soaked in quinine, which has a bitter taste and repels flies. On repeated trials, the flies had to learn to resist their urge to fly down the lighted tunnel by associating it with the bitter smell of quinine.

Sleep deprivation led to a decreased ability to perform on the learning assay. Additionally, the researchers found that learning the task at all was heavily dependent on the functionality of the dDA1. When they studied mutant flies with a deficiency in this receptor, learning was significantly reduced. Thus, they manipulated dDA1 in the MBs to be over-expressed and surprisingly found that this caused learning deficits after sleep deprivation to return to baseline levels.

The authors of the study use these findings to make a couple of postulations about sleep and sleep deprivation. First, they suggest that, although sleep deprivation probably affects several pathways, it may target specific brain areas that are essential for functioning (in this case, the MBs). Also, they hypothesize that one of the functions of sleep may be to restore levels of neurotransmitters essential to proper functioning, like dopamine.

While this finding has already led to speculation on popular science sites about a pharmacological method of negating sleep-deprived cognitive impairments, it’s important to remember that this was a study done in fruit flies, and much work would have to be done to find if it is potentially applicable to humans. Regardless, while the overall purpose of sleep continues to be a mystery, this study does add one more piece to the puzzle in understanding its physiological mechanisms.

 

SEUGNET, L., SUZUKI, Y., VINE, L., GOTTSCHALK, L., SHAW, P. (2008). D1 Receptor Activation in the Mushroom Bodies Rescues Sleep-Loss-Induced Learning Impairments in Drosophila. Current Biology DOI:10.1016/j.cub.2008.07.028

Bisexuality in Drosophila

The fruit fly, like many organisms, has a stereotypical courtship ritual that precedes mating. After noticing a female, a male fly will follow her with a persistence that is strangely reminiscent to me of behavior that can be observed in any local pub on a busy night. The male will then tap the female with his foreleg, which allows him to sense her pheromones through chemoreceptors on his leg, and verify whether she is sexually receptive. If so, he will extend one wing and vibrate it, producing a species-specific courtship song. He also licks her genitalia to further test her pheromones. Of course these last few steps aren’t as noticeable at the local bar, and if they are you may be in the wrong place (perhaps a strange fetish pub). If she doesn’t reject him, he mounts her and attempts to copulate.

See the ritual here:

A fruit fly’s ability to discriminate between males and females is based on visual, auditory, and chemical cues, such as the pheromones 7-tricosene and cis-vaccenyl acetate (cVA). Flies that don’t produce these pheromones are deemed female and courted by other males. Mutant flies that cannot sense the pheromones attempt to copulate indiscriminately with males and females. Normally, however, homosexual behavior in Drosophila is relatively rare.

Earlier this year, a joint research team from France and America set out to determine what the biological difference between bisexual and heterosexual flies is. Is it that bisexual flies have difficulty sensing pheromones like 7-tricosone and cVA, or that they are sense the pheromones and are attracted to the opposite sex? What is the mechanism that causes that difference in attraction?

The group identified a mutation in drosophila that drastically increased homosexual encounters. They named it genderblind (gb) due to the resulting phenotype, which exhibited bisexual behavior. They determined, using an immunoblot, that the gb mutation causes a reduction in gb protein quantity. An immunoblot is also known as a western blot, and involves separating proteins with gel electrophoresis and then probing for specific proteins with antibodies that have been raised against them (presence of the protein will invoke an antibody response).

In order to determine if homosexual behavior in flies was simply a result of the misinterpretation of sensory cues, the group manipulated visual and chemosensory cues and measured fly response. They found that, although reducing the availability of visual cues affects the ability of the fly to discriminate between sexes, it was not enough of an effect to explain gb behavior. When they exposed the gb flies to mutant males that did not produce 7-tricosene and cVA, homosexual behavior was reduced to wild-type levels. When they applied these pheromones topically to the mutants, however, homosexual behavior from the gb flies was restored. This suggested that gb flies sense the pheromones, but interpret them differently than wild-type flies.

The group was able to identify the genderblind protein as a glial amino-acid transporter subunit and a regulator of glutamate in the central nervous system (CNS) of the fly. One function of glutamate is to reduce the strength of glutamatergic synapses through desensitization. The gb mutants had reduced genderblind protein levels and lower levels of extracellular glutamate. This resulted in increased glutamatergic synapse strength in the CNS. A glutamate antagonist administered to gb flies caused them to revert back to wild-type sexual behavior, indicating that the stimulation of glutamatergic circuits is responsible for the homosexual behavior. Additionally, inducing the overexpression of glutamate in the CNS of the fly caused an increase in homosexual behavior in both gb and wild-type flies.

Amazingly, the homosexual behavior could basically be turned on or off by manipulating glutamate transmission. The researchers suggest that this implies there is a physiological model for drosophila sexuality in which flies are pre-wired for both heterosexual and homosexual behavior. The homosexual behavior, however, is normally suppressed by genderblind proteins. A similar model has been proposed for mice.

So, the natural question is: what, if anything, does this say about homosexuality or bisexuality in humans? Well, the authors of the study state that genderblind has a high homology to a mammalian protein, the xCT protein. This is a cystine/glutamate transporter and may be an important regulator of glutamate in the CNS, similar to genderblind in the fly.

Despite this similarity, however, in my opinion it is improbable that a relationship between xCT protein levels and bisexuality/homosexuality that is similar to the one in drosophila and genderblind protein exists in humans. This isn’t to say there couldn’t be a correlation, just that the direct connection seen in fruit flies would appear too simple to be a basis for human sexual orientation, which is probably governed by a number of gene-protein relationships. So, while glutamate levels could play a part in suppressing homosexual behavior, they probably couldn’t act like a “bisexuality-switch” they way they do in the fruit fly.

 

Grosjean, Y., Grillet, M., Augustin, H., Ferveur, J., Featherstone, D.E. (2008). A glial amino-acid transporter controls synapse strength and courtship in Drosophila. Nature Neuroscience, 11 (1), 54-61. DOI:10.1038/nn2019

Sweet Dreams, C. Elegans

Sleep is sometimes a vexing subject for scientists. We spend about 1/3 of our lives doing it. Yet, despite all the progress that has been made in discovering the reasons behind myriad other human behaviors, there is still no consensus on why we sleep. Some believe it has a recuperative effect on the body, allowing energy stores to be replenished. While a good night’s sleep may certainly allow us to feel more rested, this theory doesn’t explain the necessity of sleep, as the same result presumably could be obtained by lying still for eight hours. Others suggest sleep is an evolved, adaptive behavior that protected our ancestors from too much activity during the night—a dangerous time due to their inability to spot nocturnal predators. This also is an unsatisfying concept for several reasons, one being that some animals have evolved methods to enable sleep even though the act itself puts them in danger (e.g. bottlenose dolphin). Another hypothesis is that sleep is a necessary part of memory consolidation and mental functioning. While there is a great deal of debate over the particulars of this theory, it has more evidential support than the other two hypotheses.

Researchers at University of Pennsylvania School of Medicine are hoping to learn more about the purpose of sleep by studying Caenorhabditis elegans, a roundworm used as a model organism for many of the same reasons I outlined in my post about drosophila research. The group is the first to show that nematodes do experience sleep. They discovered a period in the worm’s development, which they termed lethargus, that is similar to sleep in other animals. The fact that sleep can be found in organisms so evolutionarily distant from us is further support for the idea that sleep is necessary, and not just an evolved, adaptive behavior.

But the changes that occur in the worm during lethargus may also give some clues as to the purpose of sleep. While C. elegans is in this phase of quiescence, numerous synaptic modifications take place within its nervous system. Thus, the researchers at Penn postulate that lethargus is a period in roundworm development that is necessary for nervous system growth. As synaptic changes occur during sleep in mammals as well, this lends support to the idea that sleep is necessary for proper brain functioning and development.

The group also identified a gene in C. elegans that regulates sleep. It has a human homologue that, although previously known, has not been studied in relation to sleep. The researchers hope these findings in the roundworm will eventually provide insight into the human sleep process and bring us closer to solving the mystery of why we spend so much of our lives in a seemingly nonproductive state.

Warning to Homophobes: Don't Drink With Fruit Flies

It’s common knowledge that drinking alcohol can lower our inhibitions, causing some of us to occasionally do things we regret the next sobering (in more ways than one) day. One common cause of alcohol-induced remorse is the weakening of sexual restraint. It can lead to a sexual liaison with someone you normally wouldn’t consider sharing your bed with, whether it be a co-worker, friend, or someone you’re just plain not attracted to (hence the scientific term “beer goggles”). While all of this is common knowledge, scientists don’t really understand why it happens. So a group of researchers at Penn State is attempting to make sense of it by studying Drosophila melanogaster, more commonly known as the fruit fly.

It may seem strange to study a fruit fly to gain a better understanding of human beings. Fruit flies, however, have been an integral part of science since the beginning of the twentieth century. At that time Thomas Hunt Morgan was trying to comprehend Gregor Mendel’s pea plant experiments. He wanted to figure out what molecular mechanisms could be responsible for inheritance. One reason he chose to study fruit flies is they produce a new generation about every two weeks. While Mendel had to wait a year for traits to be passed down from his pea plants, Morgan could go through over twenty generations in that time. Morgan had set out to prove Mendel wrong, but ended up winning the Nobel Prize in 1933 for demonstrating that inherited information was passed down on chromosomes (confirming Mendel’s hypothesis of inheritance).

Drosophilas have been widely used in science ever since. Their quick rate of reproduction allows researchers to make genetic manipulations and study the effects shortly after. In addition, the genome of the fruit fly has been sequenced, and surprisingly they share up to 77% of disease-causing genes with humans. These include genes for Parkinson’s, Huntington’s, and Alzheimer’s disease. Their similarity to humans makes them popular for genetics research, but our good understanding of the Drosophila genome has also made them common subjects for studying behavior. Fruit flies have been used to study (among other things) memory performance, longevity, sexual orientation, and alcohol and drug abuse.

The team at Penn State gave fruit flies a daily dosage of ethanol (the intoxicating agent in alcohol) and observed the results. They saw that ethanol increased male fruit fly courtship of females, as would be expected. But to their surprise, they found it also resulted in increased instances of inter-male courtship. Inter-male sexual relations rarely happen in Drosophila without the influence of ethanol, but with ethanol the tendency for them to occur rose steadily after the first few encounters. The researchers studied the molecular mechanisms behind this behavior and found a couple of factors, one being dopamine transmission, that were necessary for the decreased sexual inhibition. The study wasn’t undertaken to examine homosexuality in fruit flies (although researchers in the past have identified a gene, called fruitless, that can be manipulated to cause homosexual behavior), but to find a physiological basis for sexual disinhibition. The results are something future researchers can build upon to understand not only why we sometimes wake up next to a face we are surprised (or mortified) to see, but--more importantly--why more unprotected sex and sexual assault occurs when alcohol is involved.