Battle of the Sperms

Inside every Drosophila melanogaster (fruit fly) female is a battleground where sperm can jockey with each other for the honor of fertilizing her eggs. By sequentially mating females with males genetically-engineered to produce sperm that glow either green or red (see video below), scientists have been able to directly observe sperm from two different males competing in what resembles a fruit fly version of Tron. Color coding the sperm in this way allows researchers to distinguish one male’s sperm from another’s and determine who the winners are.

“Sperm from two different males genetically-engineered to express either green or red fluorescent proteins compete within the female reproductive tract of the fruit fly Drosophila melanogaster.”

Surprisingly, it’s not always the fastest sperm that wins.

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It’s a Male. It’s a Female. No…it’s a Gynandromorph!

Don’t eat me cuz I’m beautiful…

You could make a case for “looking weird” being an effective survival strategy. Just ask Calvin the lobster whose calico-patterned shell spared it from dining table destiny. Rather than being dunked into a pot of boiling water, Calvin is now on display at the New England Aquarium. Calico-patterned lobsters are extremely rare, occuring about 1 in 30 million. For comparison, the rarest are albino lobsters, which occur at a rate of about 1 in 100 million whereas blue lobsters are more commonly found (~1 in 2 million).

But even stranger looking lobsters are lurking out there in our oceans. Take for instance this two-toned lobster that looks like only one half of it was cooked :

As odd as this lobster looked initially, the more I looked at this picture the more familiar it seemed to me. Where have I seen something like this before? I paced around the lab a bit yesterday and it dawned on me. I’ve seen something like this happen in Drosophila. Every once in a while I’ll find a fruit fly where one half of its body is yellow and other half is brown–split right down the middle. Or more strikingly one eye is white and the other is red. And what’s even more peculiar, one half of the fly will be male with male specific structures like the sex comb and the other half is female. The genitalia of these individuals can vary from having “two complete sets of genitalia, one male, one female. Most of the time, you get weird mish-mashes of tissue that don’t look like male or female genitalia (h/t @DaveMellert).” This bilateral sexual asymmetry is a form of gynandromophy, where an organism abnormally displays both male and female characteristics and is not to be confused with hermaphrodites which are organisms that have both male and female sexual reproductive organs.

Drosophila gynandromorph

In Drosophila, sex is determined using the XX/XY sex-determination system (also used in humans) in which females have 2 X chromosomes and males have 1 X chromosome and 1 Y chromosome. Bilateral gynandromorphy in Drosophila occurs when there is a spontaneous, anomalous loss of an X chromosome during the first mitotic division in a female zygote. This results in one daughter nucleus* containing 2 X chromosomes (denoted as XX) and the other daughter nucleus containing only 1 X chromosome (denoted as XO). In this situation, cells derived from the XX nucleus will give rise to the female body plan in half of the fly while cells derived from the XO nucleus will give rise to the male half despite the lack of a Y chromosome. This is because in Drosophila the most important factor in sex determination is the number of X chromosomes. This phenomena also indicates that by the first mitotic division the left and right side of the Drosophila has been determined since one half will become male and the other female. All of the descendants of one cell will makeup the entire left side of the animal while all the descendants of the other cell will makeup the right side!

XX/XY sex-determination system in Drosophila

Although this explains how bilateral sexual asymmetry occurs, you might be wondering why in the Drosophila gynandromorph above one half of its body is yellow (left) and the other is brown (right) or why one eye is white and the other is red. This is because the genes that determine eye and body color are found on the X chromosome. So in the case of eye color, the original zygote was heterozygous for the gene controlling eye color–it has the recessive allele or gene variant for white eyes (w) on one X chromosome and the wildtype allele for red eyes (w+) on the other X chromosome. During mitosis, one of the w+ -bearing X chromosomes is lost and so cells derived from this XO nucleus will carry only the white eye gene, therefore giving rise to 1 white eye. The other eye is heterozygous for the eye color gene, but since w is recessive the eye will be red by virtue of also carrying the wildtype w+ allele. Of course, loss of the X chromosome can also occur after the first mitotic division, in which case the animal will be a mosaic gynandromorph having patches or regions that are male or female instead of the stark left-right division of male and female body plans.

Gynandromorphy occurs in other animals also, although the specific details depend on the species in question. In butterflies, gynandromorphy is a result of sex chromosome aneuploidy as well but their situation is reversed: XX cells are male and XY or XO cells are female.

In birds, however, gynandromorphy occurs by a different process. Sex determination in birds is different from insects and humans: females have a Z and W chromosome whereas males have 2 Z chromosomes. This chicken is a mosaic of both normal male (ZZ) and female (ZW) cells, with male cells concentrated on 1 side and female cells concentrated on the other.

The exact details of this phenomena in birds is unknown. One hypothesis is that it is due to an error that “occurs in the formation of an egg, which normally carries one chromosome to unite with the single chromosome carried by the sperm. But if an egg accidentally ends up with two chromosomes — a Z and a W — and if this aberrant egg is fertilized by two Z-carrying sperm, the bird that results will have some ZZ cells and some ZW cells, he explained.” Another hypothesis is that the egg is abnormally fertilized by two sperm. 

Coming back to lobsters, bilateral gynandromorphs can occur in crustaceans as well. As for humans, however, despite Conrad Lycosthenes’s claims that a bilateral human gynandromorph existed, Natalie Reed explains why this phenomena in all likelihood would not happen, “Back to this not happening in humans: yes, intersexual chimerism can happen in humans. You can even end up with human beings who have one ovary and one testicle. But given that almost all sexual differentiation is a result of hormones, which are more or less evenly distributed throughout the body, you would never see any kind of stark split down the middle of a human with, say, a breast on one side and a flat chest on the other.”

*I say nucleus rather than cells because Drosophila do this strange thing during early embryonic development where the chromosomes duplicate and segregate into separate nuclei without the normal cell division (cytokinesis) that occurs after mitosis. This results in a cell called the syncytial blastoderm that has multiple nuclei without cell membranes separating the nuclei from each other. After 13 rounds of nuclear divisions, cell membranes are finally erected (cellularization) to partition the nuclei into individual cells.

Here is a video of mitosis in the early Drosophila embryo. Note the lack of cell membranes.

Here is a video of cellularization.

A man, a mouse, and a fruit fly walk into a bar: Genetic approaches in understanding Alcohol Use Disorder (AUD)     The recent buzz surrounding the drinking habits of Drosophila, from dealing with rejection to killing parasites by self-medicating, got me thinking: How might fruit flies be used for research in understanding the cause of alcohol use disorder (AUD)?  AUD, which encompasses both alcohol abuse and dependency, affects millions of Americans and is a complex disorder influenced by a myriad of environmental and genetic factors. Not surprisingly, efforts to identify the genes responsible for AUD in human and animal models are fraught with limitations. Human studies comparing the genetic variability between affected and unaffected individuals have identified many potential genes responsible for AUD, but only with weak statistical support, thus leaving scientists to ponder which genes are bona fide candidates. On the other hand, more feasible experiments performed in animals and cell culture have identified genes involved in ethanol response, but whether these genes are relevant to a complex, human disorder such as AUD is unknown.

     In a study published last December in G3: Genes, Genomes, Genetics, biologists from UCSD and UCSF used a multispecies approach to work around these obstacles to identify and test potential genes involved in alcohol-induced behavioral responses (Josyln et al., 2011). Behavioral changes in response to ethanol can be indicative of later AUD development. To prune the list of potential genes from the entire genome, the researchers analyzed available mouse genetic data to identify regions of the mouse genome associated with ethanol-induced ataxia (loss of coordination). These genomic regions represent clusters of genes that could influence AUD. Despite differences in overall structure and number of chromosomes, many of these gene clusters have been maintained in the genomes of many organisms–this is known as synteny. The researchers exploited this fact to cross-reference the mouse-identified regions to the corresponding sections in the human chromosomes, allowing them to focus on smaller regions rather than the daunting task of searching across the entire human genome.

Corresponding genomic regions between mouse and human genomes are color-coded.

     Next, an alcohol challenge study was performed in human subjects. Study participants were given ethanol to consume and the researchers then measured how much ethanol affected the participants inclination to sway to the left and right (another measure of coordination similar to ataxia). Using DNA collected from participants, a genetic association analysis of the human genome corresponding to the regions of the mouse genome associated with ethanol-induced ataxia was performed to identify genes linked to ethanol-induced body sway. The logic behind genetic association is that differences in DNA sequence (genetic variability) amongst individuals underly the variability in human traits (phenotypes, i.e. hair color, eye color, and in this case disease). Thus the researchers looked for a change in DNA sequence (genetic variant) that was common to individuals with the most pronounced ethanol-induced body sway (phenotype). This analysis revealed glypican 5 (GPC5) as a candidate gene involved in ethanol response in humans. GPC5 belongs to a class of genes that encode cell surface proteins, known as glypicans, that act like cellular antennas to receive protein-encoded messages from other cells (see figure below). These protein messages can induce specific responses in the cell, such as turning on the expression of specific genes or alter cellular metabolism. This form of cellular communication is known as signal transduction and is vital for the proper development of an organism.

Adapted from U. Häcker et al., 2005

     To test whether glypicans can affect ethanol response the researchers turned to Drosophila. The researchers found that mutations in the equivalent (homologous) Drosophila glypican genes, dally and dally-like (dlp), affected fruit fly behaviors sensitive to ethanol exposure. Normally, when exposed to ethanol vapor, fruit flies are initially startled and display elevated locomotor activity that becomes increasingly uncoordinated until the flies eventually becomes sedated. Mutations in both dally and dlp affected ethanol-induced locomoter activity and decreased the time it took for the fruit flies to become sedated. Interestingly, wildtype Drosophila become tolerant to ethanol since a second exposure to ethanol vapor takes a longer time to induce sedation. Only the mutation in dally displayed an inability to develop this second exposure tolerance suggesting that dally and dlp have different roles in developing tolerance to alcohol. These results confirmed that glypicans influenced alcohol-induced behaviors in Drosophila.

Bar Fly from Science News on Vimeo.

     The authors of the study reasoned that the convergence of data obtained from their mouse, human and fly studies “provides strong support to the hypothesis that GPC5 is involved in cellular and organismal ethanol response and the etiology of alcohol use disorders in humans.” In further support of their hypothesis, they point to other research indicating that the signal transduction pathways regulated by glypicans are also involved in ethanol response. While the role of GPC5 in the etiology of AUD requires further study, this research provides an example of a powerful, combinatorial genetic strategy that may prove useful in identifying causative genes in the context of other complex, multifactorial diseases such as cancer, metabolic syndrome, or heart disease.

* April is Alcohol Awareness Month

Joslyn, G., Wolf, F., Brush, G., Wu, L., Schuckit, M., White, R., & Hall, I. (2011). Glypican Gene GPC5 Participates in the Behavioral Response to Ethanol: Evidence from Humans, Mice, and Fruit Flies G3: Genes|Genomes|Genetics, 1 (7), 627-635 DOI: 10.1534/g3.111.000976

When Species Invade

Adapted from photo by Michael Durum 

     This week’s World Wide Wednesday* features 3 stories about invasive species, which are non-native plants, animals, or other organisms that have been introduced to a particular ecosystem usually through human activity.  Invasive species generally cause ecological and economic damage to their new habitat by competing for resources or preying on native species. This week we’ll explore examples of harmful invasive species as well as an invasive species that’s playing nice with its new neighbors.

Beware soft fruits, the Drosophila suzukii are coming!

D. suzukii male

     Most Drosophila species, including the popular lab strain D. melanogaster, are attracted to rotting fruit in which to lay their eggs, but not Drosophila suzukii. Otherwise known as the spotted-wing drosophila, D. suzukii prefer to inject their eggs directly into the flesh of fresh fruit. Once hatched, D. suzukii larvae will eat the fruit from the inside and is therefore an economic threat to fruit crops such as cherries, blueberries, raspberries, blackberries, peaches, nectarines, apricots and grapes:

D. suzukii larvae in a strawberry. (adapted from photo by Hannah Burrack)

     Unlike other Drosophila species, female D. suzukii have a rather diabolical (to fruit at least), saw-like adaptation of their ovipositor, the external organ that deposits eggs, which allows it to pierce the skin of soft fruits:

Left: D. suzukii ovipositor. Right. D. simulans ovipositor. Adapted from image by Martin Hauser.

     Originally identified in Japan and Asia in the early 20th century, D. suzukii first appeared in California in 2008 and quickly spread across the Pacific Northwest. It has since found its way to  Southeastern US states, Michigan, Wisconsin, and even Maine. More recently, the spotted-wing drosophila has been spotted in European countries as well. The spread of D. suzukii is most likely due to the export and import of affected fruits.

A smug nutria asks, “When is an invasive species no longer invasive?”

Screenshot from Hi. I’m a Nutria. 

     Last week, the New York Times posted this Op-Doc animated short by filmmaker Drew Christie that explores the existential question facing all invasive species,  “How long does it take to become a native?” The short video provides a brief history of how nutria were originally brought to America to start fur farms and rightfully lays the blame on human activity for the introduction of the over-sized rodents to the Pacific Northwest. As nutria fur demand declined, many farmers simply released nutria into the wild. Drawing comparisons to other invasive species to the Pacific Northwest such as the American bullfrog, grey squirrels, house sparrows, and oh yeah humans, the nutria asks, “why do I get all the grief?”

Photo credit: Petar Milošević

     The answer of course lies outside of the video. Nutria are capable of destroying vast areas of wetland through their destructive feeding and burrowing habits. In sensitive ecosystems, such as the Louisiana wetlands, the havoc that nutria wreak can exacerbate existing degradation thereby increasing the threat and potential damage of flooding due to hurricanes and sea level rise. Various population control methods have been implemented with varying success. The Louisiana Dept. of Wildlife and Fisheries runs a nutria hunting and trapping incentive program.  Nutria has also been marketed for human consumption (although that has not gained much traction) as well as “guilt-free” fur.

Not all invasive species from Asia are bad…

Asian Shore Crab (Hemigrapsus sanguineus)

     You might remember from the end of last year the story of Samantha Garvey, the recently (at the time of the story) homeless 18-year-old Brentwood High School senior whose research project made her an Intel Science Talent Search semifinalist. The subject of her research? How the presence of the Asian shore crab, an invasive predator, affected the thickness of the shells of its prey, the ribbed mussels native to Long Island Sound. Although at first glance this might make the Asian shore crab seem like another bad invasive species story, research from Brown University scientists suggest otherwise. Appearing on US shores over 20 years ago, most likely by piggybacking on commercial ships originating from Asia, the Asian shore crab has populated almost the entire eastern seaboard without disrupting native ecosystems. In fact, the Brown University researchers found that the success of the Asian shore crab did not come at the expense of indigenous inhabitants since the study found a positive correlation between the number of invasive crab and a greater number of native species.

     Asian shore crabs also make great bait for tautog, one of my favorite fish to catch in RI:

     Til next week!

Related Reading:

Rats the Size of House Cats Invade the Florida Keys

CABI invasives blog

Toxicomania: Poisonous Invasive Plant Protects Australian Lizards from Poisonous Invasive Cane Toads

Globe-trotting hitchhikers: invasive species assault U.S. waters

*I was strongly advised to change the name to World Wide Wednesdays.

Drug Design and Disease Models: We can do it better in Drosophila.

Dr. Ross Cagan

     Last night’s “Future Advances in Drosophila Research” session of the Drosophila Research Conference concluded with Ross Cagan‘s eloquent defense of using Drosophila for better drug design and better disease models. He argues that Drosophila has significant advantages, including genetics, generation time, and tools, over other organisms (sorry mice) to model complexity (and complex diseases) He started with the premise that single genes that drive diseases such as cancer are not always the best therapeutic targets. Why? Because often times targeting their activity is highly toxic to the cell. He then went on to describe a Drosophila model for medullary thyroid carcinoma (MTC). About 95% of all MTC cases are due to genetic mutations that activate the RET kinase. Despite the fact that Drosophila do not have thyroids, Dr. Cagan was able to model MTC in the fly by directing expression of RET kinase mutants responsible for MTC in the eye which led to tumor growth:

Model for MTC in the Drosophila eye

     Furthermore, when he directed the expression more globally, the flies died before reaching adulthood. He then went on to describe a drug that was highly effective at rescuing these phenotypes known as AD1. Surprisingly, AD1 was not a very effective inhibitor of RET kinase activity or in his words the “world’s worst kinase inhibitor.” It wasn’t very specific and it wasn’t very effective. But this broad spectrum behaviour of AD1 was precisely why it was effective at rescuing the defects in the fly. RET kinase is a protein that regulates the activity of other kinases such as RAS, Src, PI3K (all of which are involved in cancer). Guess what? AD1 was targeting all of them. When the activity of other structurally similar compounds, AD2 and AD3, were compared to AD1 they were less efficacious because they were unable to target all of those other kinases. More importantly, since AD1 was “the world’s worst kinase inhibitor” it displayed little toxicity to the animal as a whole indicating that tumors might have lower tolerance thresholds for drugs than the entire animal. All of which to say is that a focused approach on a single target over a multitarget approach is not always the best course given that cancer is a complex disease.

     This multitarget approach segued nicely into the second part of his talk where he focused on building better models for disease in Drosophila. He highlighted the work done by a post doc in his lab, Erdem Bangi (a former graduate student the Wharton lab where I work in now), who is designing a better model for colorectal cancer. Treatment for colorectal cancer is an unmet need in the healthcare. Building on the idea of multiple targets and the fact that tumors often exhibit mutations in multiple closely associated genes rather than just one gene, Dr. Bangi is creating mutant flies with different combinations of mutations in different genes. We’re talking double, triple and quadruple mutants. In these multiple mutant flies, Dr. Bangi found that drugs that were previously effective in models where only one particular gene was mutated were effectively useless. This harkens back to the original premise that targeting the single gene driving disease is not always the best approach. Dr. Bangi is now using these multiple mutant flies to design and discover more effective drugs.

     The work highlighted by Dr. Cagan last night is important in that it represents a shift in how we discover drugs and how we should think about treating diseases. Rather than just taking a chemical and testing to see if it can rescue a particular phenotype, this research underscores the importance of knowing the role of multiple gene targets in disease progression. His talk highlighted not only the value of basic research in drug discovery and design but also the intrinsic advantages of Drosophila as a model organism. Can we do it better in Drosophila? Come on everybody altogether: Yes we can! (and my pandering to Chicago continues…)

#DROS2012 comes to Chicago!


     Every year, hundreds maybe even thousands of biologists converge on a lucky US city like fruit flies swarming on yeast. The reason? The Drosophila Research Conference, or as people in the know call it…The Fly Meeting. This year, the annual meeting rolls into Chicago. The Windy City. Chi-Town*. The City of Big Shoulders. Home of such notables as Subrahmanyan Chandrasekhar, winner of the Nobel Prize in Physics, Vladimir Drinfeld, mathematician, winner of the Fields Medal, President Barack Obama, and of course Kanye West:

'Ye wearing my Drosophila-inspired shades. Got room for me at DONDA?

     In terms of conferences I’ve been to, the Fly Meeting is like the Pitchfork Festival of scientific conferences. (I was going to go with Bonnaroo or Coachella at first, but I went with pandering instead.) Over the next 5 days, Drosophila researchers are going to get down and dirty, immersing themselves in the latest research and scientific techniques, catching up with old colleagues, establishing new collabo’s, attending concurrent research talks and poster sessions. Oh the poster sessions! Imagine an auditorium filled with rows upon rows of posters and hundreds of scientists buzzing from one poster to the next and the atmosphere is thick and dank. The uninitiated might mistake the odor for lack of hygiene, but us pros know…it’s the sweet smell of science.

Surprise! Drosophila was the most often used word in the Drosophila Research Conference Abstracts.

     This year’s conference is particularly exciting (and nerve wracking) for me since I’ll have the opportunity to share my research during one of the platform sessions. Imagine trying to cram about half of your thesis work into a 12-minute talk. Sounds fun doesn’t it? The title of my talk: The classic fibrodysplasia ossificans progressiva mutation reveals the latent kinase activity of the Drosophila BMP type I receptor Saxophone. Sort of a mouthful. Also, I’ll be attempting to live tweet the conference. You can join/follow the conversation: #DROS2012. So if things go sort of quiet on the blog over the next few days, you’ll know why.

*Did anyone else grow up thinking Chi-Town was just short for Chinatown?