Monday's Molecule #93

 
What's going on here? Your task is to identify the experiment that led to this result. It's a short step from there to this week's Nobel Laureate(s). You just need to switch species.

Here's a hint: This week's Nobel Laureate(s) and last week's Nobel Laureates have something in common.

You need to describe what you see in the figure as accurately as possible. Then identify the Nobel Laureate(s).

The first one to correctly identify the figure and name the Nobel Laureate(s), wins a free lunch at the Faculty Club. Previous winners are ineligible for one month from the time they first collected the prize. There are four ineligible candidates for this week's reward: Brad Hersh of Clemsen University, Alex Ling of the University of Toronto, Haruhiko Ishii, and Bill Chaney of the University of Nebraska.

THEME:

Nobel Laureates
Send your guess to Sandwalk (sandwalk (at) bioinfo.med.utoronto.ca) and I'll pick the first email message that correctly identifies the molecule and names the Nobel Laureate(s). Note that I'm not going to repeat Nobel Laureate(s) so you might want to check the list of previous Sandwalk postings by clicking on the link in the theme box.

Correct responses will be posted tomorrow. I reserve the right to select multiple winners if several people get it right.

Comments will be blocked for 24 hours. Comments are now open.

UPDATE:The figure shows the result of an experiment where human cells in culture were irradiated with X-rays (Scherthan et al. 2008). There are two obvious chromosomal rearrangements. Breaks and deletions are common in X-ray treated cells. The Nobel Laureate is Hermann Muller who won the prize for creating mutants using X-rays. He worked with Drosophila melanogaster. Only one person got this one right and that person is ineligible.

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[Figure Credit: The figure is from Scherthan et al. (2008)]

Scherthana, H., Hieberb, L., Braselmannb, H., Meinekea, V., and Zitzelsberger, H. (2008) Accumulation of DSBs in γ-H2AX domains fuel chromosomal aberrations. Biochemical and Biophysical Research Communications 371:694-697. [doi:10.1016/j.bbrc.2008.04.127]

Periannan Senapathy Publishes in PLoS ONE

 
I first encountered Periannan Senapathy on the sci.bio.evolution newsgroup. He was promoting the idea that the various animal phyla arose independantly rather than from descent from a common ancestor.

Here's one of his early postings on sci.bio.evolution from Feb. 13, 1995"

As a molecular biologist and genome researcher, I have enjoyed following the many ongoing debates in this and other forums over evolution theory -- both as a whole, and various aspects thereof. My own work in genome mechanics and genetic molecular structures has yielded much evidence pertaining to these debates, and over the years I have published several of my findings in PNAS, J Molec Biol, J Biol Chem, Nucleic Acids Research, Science and other journals.

Until recently I have published these findings separately, although clearly they are all related. Now, however, I am publishing a single unified theory that incorporates all of these pieces -- and an enormous body of other evidence as well. This new unified theory proposes a radically alternative explanation for the origin and diversity of life on Earth, asserting that most of Earth's organisms must have originated independently in one primordial pond, and that the natural-selection mechanism described by evolution theories could have produced only minor variations among essentially similar species. These conclusions surely will provoke a lively debate in the scientific community, but a fair reading of the theory will show that it easily explains all of the available evidence -- molecular, biochemical, organismal and fossil -- and notably accommodates all of the contra-evolution evidence that has dogged evolutionists since Darwin.

His ideas were eventually published in 1994 in a book with the following provocative title: "Independent Birth of Organisms. A New Theory that Distinct Organisms Arose Independently from the Primordial Pond, Showing that Evolutionary Theories are Fundamentally Incorrect." The book was published by "Genome Publications."

Periannan Senapathy owns a company called "Genome International Corporation" which funds his work. The PLoS paper, for example, is funded by "Genome International Corporation" and the disclaimer reads, in part, "This project is purely an academic project, fulfilling the academic interest of the corresponding author, who owns the company." I don't know if the book is self-published.

The research article in PLoS ONE is ..

Regulapati, R., Bhasi, A., Singh. C.K., and Senapathy, P. (2008) Origination of the Split Structure of Spliceosomal Genes from Random Genetic Sequences. PLoS ONE 3(10): e3456 [doi:10.1371/journal.pone.0003456]

It address an old problem about the origin of introns. The consensus among scientists these days is that introns arose late and they are derived from insertions of self-splicing RNA's into coding regions. These were subsequently copied into DNA and integrated into the genome. The similarity between Group II introns and spliceosomal introns lends strong support to this model.

The fact that bacteria have very few introns is consistent with the idea that introns arose late in evolution. As is the fact that introns positions are not highly conserved, as one might expect if they arose early.

The Senapathy paper tries to make the case that introns are primitive and the human genome, with lots of introns, is more primitive than bacterial genomes, which have lost their introns.

Speaking of losing, this issue was resolved over ten years ago when the last holdouts for "introns early" conceded defeat. Senapathy has not come up with anything that even remotely rejuvenates that losing position.

On a more troubling note, it's beginning to look as if PLoS ONE is attracting the kooks who find it easy to get their work published in that forum. It won't be long before I stop reading papers, and abstracts, from PLoS ONE.

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It's the Sequence, Stupid!

The latest issue of Science has an article summarizing the research done by Wilson et al. (2008). The "perspectives" article, by Hilary A. Coller and Leonid Kruglyak has a provocative title It's the Sequence, Stupid!. The Wilson et al. paper casts doubt on one of the current fads in molecular biology, namely the claim that there's something called "epigenetics" that trumps DNA sequence when it comes to determining gene expression.

Here's the abstract from the Wilson et al. (2008) paper. You be the judge.

Homologous sets of transcription factors direct conserved tissue-specific gene expression, yet transcription factor–binding events diverge rapidly between closely related species. We used hepatocytes from an aneuploid mouse strain carrying human chromosome 21 to determine, on a chromosomal scale, whether interspecies differences in transcriptional regulation are primarily directed by human genetic sequence or mouse nuclear environment. Virtually all transcription factor–binding locations, landmarks of transcription initiation, and the resulting gene expression observed in human hepatocytes were recapitulated across the entire human chromosome 21 in the mouse hepatocyte nucleus. Thus, in homologous tissues, genetic sequence is largely responsible for directing transcriptional programs; interspecies differences in epigenetic machinery, cellular environment, and transcription factors themselves play secondary roles.

Grab some popcorn and a beer and sit back in your easy chair to watch how the evo-devo people talk themselves out of this one.

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Wilson, M.D., Barbosa-Morais, N.L., Schmidt, D., Conboy, C.M., Vanes, L., Tybulewicz, V.L.J. Fisher, E.M.C., Tavaré, S., and Odom. D.T. (2008) Species-Specific Transcription in Mice Carrying Human Chromosome 21. Science 322: 434-438. [DOI: 10.1126/science.1160930]

The Powerful 7%

 
Here's the result from a US Gallup Poll taken in May, 2007 [Religion].



Only 7% are potential atheists. How powerful are those people? PZ Myers has the answer from an article by Melanie Phillips on the financial crisis in America [Pharyngula: You mean it's all my fault?].

I see this financial breakdown, moreover, as being not merely a moral crisis but the monetary expression of the broader degradation of our values - the erosion of duty and responsibility to others in favour of instant gratification, unlimited demands repackaged as 'rights' and the loss of self-discipline. And the root cause of that erosion is 'militant atheism' which, in junking religion, has destroyed our sense of anything beyond our material selves and the here and now and, through such hyper-individualism, paved the way for the onslaught on bedrock moral values expressed through such things as family breakdown and mass fatherlessness, educational collapse, widespread incivility, unprecedented levels of near psychopathic violent crime, epidemic drunkenness and drug abuse, the repudiation of all authority, the moral inversion of victim culture, the destruction of truth and objectivity and a corresponding rise in credulousness in the face of lies and propaganda -- and intimidation and bullying to drive this agenda into public policy.

Makes you feel very sorry for the 93% who are being manipulated, doesn't it?

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Death to Apostates?

 
There was a conference in the UK where a number of former adherents of Islam talked about the fact that rejecting Islam is a very serious offense. Apostates can be killed.

A.C. Graying wrote an article about the conference. It was published in The Guardian and you can read it on RichardDawkins.net.

Now Nesrine Malik has responded in Friday's issue of The Guardian [Death for apostasy?].

Here's part of what she says ...

As a Muslim who has lived most of my life in Muslim countries, this picture is hard to recognise. I have several friends and family members who are non-believers and apart from some efforts to return them to the straight and narrow or at least go through the motions of religious observance, they have not come into any physical danger. A close friend – hitherto religious – only recently sent me a long, tortured email detailing his journey away from Islam and from all religion; he expressed no fears for his life or safety, merely trepidation at the prospect of acclimatising to this new God-free world view.

Although the Council of Ex-Muslims and AC Grayling depict the threat to life and limb as an indisputable fact, in reality there are differences of opinion among Muslim scholars (ostensibly the hard core of the religion) regarding the death penalty for apostates.

This is not to say that Muslim governments – and Arab ones in particular – have a tolerant view of apostasy but the death threat is invoked only rarely and more for political reasons rather than religion ones: to set an example or to save face as a proxy punishment for challenging the social or political status quo. While this is in no way acceptable, it is an extension of the general lack of enshrined civic human rights and evolved political institutions and processes – a historical, social and geo-political reality in many Muslim countries that makes a mockery of any comparison to the experience of those renouncing Christianity or Judaism.

It's of little comfort to an ex-Muslim to learn that there is disagreement within the Muslim community about whether they should be put to death or not. They take only a little more comfort from the fact that the death penalty is only used "rarely."

I admire Nesrine Malik for stating that killing apostates is "in no way acceptable" but she weakens her case a great deal by making excuses. Just because killing apostates is part of a larger intolerant viewpoint is an explanation, but not an excuse. Personally, I don't much care how the members of a cultural group run their lives but I draw the line when they interfere with others. People who leave the group are making a choice and they should be left alone.

Malik claims that many ex-Muslims try to make themselves into martyrs by claiming they have been threatened with death. They are just using "the emotive power of 'death for apostasy' to serve their own ends, be they personal or political." That may or may not be true—it probably is—but that's not the point.

The point is that outspoken Muslims like Nesrine Malik should make their opposition to 'death for apostasy' much clearer than just throwing away a phrase like "no way acceptable" to be immediately followed with a reason why it's understandable.

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The Toronto Star Editorial Cartoon

 
Today's editorial cartoon in The Toronto Star is worth sharing. If Canadians were allowed to vote in the US Presidential elections, the McCain people would have waved the white flag months ago.

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The Toronto Star Opposes PR, Again

 
I don't get it. Why are the editors of The Toronto Star against proportional representation? Their reasoning doesn't make sense.

Last year the editorial board opposed the Ontario referendum on electoral reform. Their arguments were so stupid and factually incorrect that "Public Editor" Kathy English was obliged to defend the newspaper's editorial opinions [The Toronto Star Defends Its Editorial Policy on MMP].

She didn't do a very good job.

Today The Star has an editorial opposing any proportional electoral system. The paper notes that a nationwide proportional system of voting would have given a different distribution of seats than the result of last Tuesday's election. They note that the Green party would have probably gotten a number of seats.

The editorial states, correctly, that we can't just recalculate the results based on Tuesday's voting because if we had voted under a proportional system people wold have voted differently. They say ...

Furthermore, the analysis is backward looking – transposing last week's results onto a new system. In all likelihood, if Canada had a system of proportional representation, the outcome would be very different, given the demographical and geographical diversity of the country. The pro-life Christian Heritage Party, for example, might win enough votes to get seats. And new parties might emerge to win seats – say, an Alberta First party or even ethnic parties.

So Harper might be kept in power by entering a coalition with pro-life and Alberta First parties. Now that, indeed, is a scary prospect.

This is scary. It looks like the editors of The Star are afraid of proportional voting because (horrors!) some people might elect MP's who truly represent their points of view.

Now we certainly can't have that in a democracy, can we?

The scare tactics are not based on rational analysis of what happens in other countries. Most Western democracies have moved into the 21st century and they find it beneficial to let the people have their say. Sticking with an unfair first-past-the -post system only breeds disillusionment and bitterness with a system that disenfranchises a majority of voters. (Voter turnout on Tuesday dropped below 60%.)

It's time for The Toronto Star to find new editors—ones who are as progressive as the newspaper they work for. The current editors are clearly not up to the job.

But maybe we shouldn't expect too much from editors who put Madonna on the front page under a banner about a rape probe.

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Barack Obama Is my Cousin!

 
George Bush and I share a common ancestor. This isn't as surprising as it might seem. Just about anyone in North America who has relatives from the British Isles will be able to trace some of their ancestors back five or six hundred years if they dig hard enough.

There's an excellent chance that you can connect at some point to several well-known lineages, usually those involving kings and queens. In my case I eventually hook up with Mary Stewart (1380 - 1458) who is the daughter of Robert III, King of Scotland. She married George Douglas (1376 - 1402).

George Bush descends from Mary's sister Elizabeth Stewart who married James Douglas, brother of George Douglas [The Ancestry of George Bush].¹

Now, most of you might not be too excited about being related to George Bush but here's the good news ... I'm also related to Barack Obama according to Obama and Bush related.

That's pretty cool. It explains a lot.

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1. It's possible that Bush descends from Beatrice Sinclair and not Elizabeth Stewart—the website is confusing. Even if this is an error there are several other connections.

If you build it, will they follow?

I'm going to subject you to one of my pet peeves.

The University of Toronto is in the process of replacing all its pathways. I really like the new style of path even though it's probably very expensive. It will improve the look of the university.

That's not the peeve. Look at the photograph below. It shows the newly completed paths leading up from the subway stop on the corner of University and College (behind me). The ramp is the easiest access to my building. Several hundred people a day walk up the path from the subway stop and up the ramp into the building.

When the stone masons were building the path I mentioned that the old pathway didn't align with the much newer ramp and, consequently, people were cutting across the grass in order to save time. That's why the grass is worn away at the base of the ramp. The guys who were laying the stones agreed that the fancy new path should at least clip off the corner by the lamp post to encourage people to follow it.

They were overruled by their supervisor who claims he doesn't have the authority to move or change the paths. So they simply replaced the old paved pathway with the nice new stone blocks and moved on.

How ridiculous. My pet peeve is this. You should build pathways where people actually walk and not where you want them to walk.

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What Questions about Evolution Can Students Safely Ask?

 
Denyse O'Leary is upset about the fact that students who challenge evolution may be perceived as being unworthy [Intelligent design and popular culture: What questions about evolution can students safely ask?].

She was impressed with the suggestions made by some photographer so she reproduced them on her blog. Here's what Densye O'Leary thinks will stump the average Professor. This is just for amusement on a Friday afternoon.

Don't argue against him. Agree with him. Then ask a question like one of those below:

1. I’d like to shut up those stupid IDers once and for all. Please tell me where I can find a book that shows clearly all the transitional fossil forms between fish and amphibians or reptiles and birds or some such major transition. I’d like to see it spelled out in detail with pictures and measurements and explanations of each fossil so I can crush those idiots.

2. I know that evolution is the most solidly proven theory in all of science, so please show me the mathematical proof of how random changes create information. I’m sure there must be one because this is a fundamental truth of evolution.

3. I know that in any system like life on earth that is open and receives outside energy the system will steadily grow more and more complex but I don’t really understand the physics of this. Could you explain it to me?

Just in case you've forgotten, this is what passes as the best evidence for Intelligent Design Creationism. We should think up a name to describe these people.

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Jonathan Wells reviews the Christiane Nüsslein-Volhard and Wieschaus Experiment

In his book, Icons of evolution, Jonathan Wells has ten chapters devoted to refuting evolution. This is typical behavior for an Intelligent Design Creationist. There's no mention anywhere in the book of positive evidence for intelligent design.

One of the chapters is "Four-Winged Fruit Flies." The main point of the chapter is that most of the Drosophila developmental mutations are lethal or extremely deleterious so they can't be transitional states in evolution. Yet, according to Wells, the textbooks are full of misleading statements claiming that morphological mutations supply the raw material for evolution. Wells says that there's no evidence for any beneficial mutations in spite of the fact that they have been looked for.

Yet the evidence cited in these textbooks falls far short of supporting those sweeping claims. To be sure, biochemical mutations lead to antibiotic and insecticide resistance, and human beings carrying the sickle-cell trait are more likely to survive malaria as infants. But only beneficial morphological mutations can provide the raw materials for morphological evolution, and evidence for such mutations is surprisingly thin. As we have seen, four-winged fruit flies do not provide the missing evidence, despite their current popularity.

If textbook-writers have no good examples of beneficial morphological mutations, it's not because biologists haven't been looking for them About the time that Lewis was studying Ultrabithorax, German geneticists Christiane Nüsslein-Volhard and Eric Wieschaus were using a technique called "saturation mutagenesis" to search for every possible mutation involved in fruit fly development. They discovered dozens of mutations that affect development at various stages and produce a variety of malformations. Their Herculean efforts earned them a Nobel prize (which they shared with Lewis), but they did not turn up a single morphological mutations that would benefit a fly in the wild. [my emphasis]

The experiment that was performed by Christiane Nüsslein-Volhard and Eric Wieschaus was designed to detect recessive lethal mutations that affected development. These kind of mutations are likely to identify genes that are essential for development. I described the experiment in a separate posting [Balancer Chromosomes].

Note that the experiment was specifically designed to detect deleterious mutations—lethal being about as deleterious as you can get. It could not possibly have detected beneficial mutations, as Wells claims, since these would have been discarded early on when the mutant lines were established.

Was the true purpose of the experiment a well-kept secret known only to insiders? Hardly. Everyone who read the papers knew that the screen was for recessive lethals. In her Nobel lecture Christiane Nüsslein-Volhard says,

In 1979, Eric Wieschaus and I, at that time in the EMBL, Heidelberg, had developed the methods for the large scale screening for embryonic lethal mutations in Drosophila. The screening procedure focused on the segmented pattern of the larval epidermis (8). In this and subsequent screens, a number of new genes acting in the embryo and required for the formation of a morphologically normal larva were discovered (9-11).

Should Wells have known this? You be the judge.

Wells has a Ph.D. in biology, molecular and cell biology, from the University of California, Berkeley (USA). He worked on embryology and evolution as a graduate student and subsequently as a post-doctoral fellow in the laboratory of Carolyn Larabell at Berkeley. He published two papers on development in 1996 and 1997.

It's safe to assume that Wells understands the basic principles of genetics and developmental biology.

Could Wells have misunderstood the purpose of the Nüsslein-Volhard & Wieschaus experiment? No, Wells may be an IDiot but he's not that stupid. When Wells makes an issue of the fact that Nüsslein-Volhard & Wieschaus did not find any beneficial mutations there's only one rational conclusion: Wells was deliberately misrepresenting the truth.

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[Photo Credit: Jonathan Wells from Conservapedia]

Way to Heaven

 
This woman was greeting people at the subway stop outside of the Pharmacy Building on the university campus. You don't see this very much aroound here so I thought I take a picture.

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Parliament

 
I was cleaning up the photos on my cellphone and I found this. The buildings in the background are Canada's Parliament Buildings on "the hill." It's where all the MP's elected on Tuesday will be meeting in a few weeks. Does anyone recognize where I was when I took the picture?

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Tangled Bank #116

 
The latest issue of Tangled Bank has been published on Pro-Science [116th Tangled Bank].

Welcome to the 116th edition of the Tangled Bank. As usually, we have a lot of good stuff for you all. Unlike some former hosts, I am not a very creative writer, and I'll spare you all from any attempts of making some kind of theme for this Tangled Bank. So, without any further ado, let's get to the posts.

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Send an email message to host@tangledbank.net if you want to submit an article to Tangled Bank. Be sure to include the words "Tangled Bank" in the subject line. Remember that this carnival only accepts one submission per week from each blogger.

Balancer Chromosomes

Monday's Molecule #92 was a depiction of an inversion in a Drosophila chromosome (right).

The chromosomes shown here are the large polytene chromosomes of the salivary glands. They are made up of 1000-2000 aligned stands of DNA that form when successive rounds of DNA replication are not followed by separation of cell division. Flies that are heterozygous for a wild type chromosome and one with a large deletion inversion, will form the loop structure shown in the diagram.

In normal cells, you won't see this structure as the chromosomes align during mitosis and meiosis, but it still exists. What the structure tells us is that the presence of an inversion, or any other type of chromosomal rearrangement for that matter, doesn't have much effect on chromosomal alignment and segregation during cell division.

Today we want to focus on another point. Imagine that a recombination event (crossover) occurs when the chromosomes are aligned like this. If the crossover takes place in the inverted region then each of the recombined chromosomes will be missing some genes and the cells that are produced from such an event will die.

Imagine that the crossover occurs between point C and D. If we trace the new chromosome staring from A on the black chromosome (the AR chromosome) then you get A B G F E D on the black chromosome followed by C B A on the normal white chromosome. The other product of the crossover will begin with A B C from the normal white chromosome and end with D E F G B A from the black homologue.

There won't be any viable crossovers in the region covered by the mutation. We will see what this has to do with balancer chromosomes in a minute.

Imagine that you are working with an important mutation (x) that affects embryonic development in Drosophila. Flies that are homozygous for the mutation (x/x) are blocked at a particular stage of development and the visible phenotype of the mutations tells you a great deal about the genes that control development. These mutations are recessive lethals. The heterozygous flies with one mutant chrmosomes and one normal chromosome (x/+) are viable.

You want to maintain a stock of these flies so you can have mutant flies whenever you need to do an experiment. If you put a heterozygous male and female together in a fly bottle and leave them for a few weeks, you won't be surprised to find that there are no flies that are homozygous for the lethal mutation. However, repeated crossings of heterozygotes will result in 25% wild-type flies (+/+) and these flies will continue to mate with each other and with the heterozygous flies. You won't be able to tell which flies carry your valuable mutation.

One way around this is to mark your mutant chromosomes with a visible marker. Let's say that your mutation is on chromosome 2. (There are three autosomes and one pair of sex chromosomes in Drosophila.) You will need a dominant marker for reasons that will soon become apparent so let's choose detached (Dt), a mutation that effects the vein pattern on the wings. The chromosome carrying your valuable developmental mutation (x) will also carry the Dt allele.

Now all you have to do is look in your stock bottle for flies with the detached phenotype and you know that those flies should be heterozygous for x (Dt x/+ +). Flies that have normal wings can be recognized and killed. (Drosophila genetics is the ultimate blood sport.)

Problem solved, right?

No. It won't be long before a recombination event separates Dt and x, especially if they are far apart on chromosome 2. After a while you still won't know which flies carry your valuable mutation.

This is where balancers come in handy.

Let's look at the experiment done by Christiane Nüsslein-Volhard and Eric Wieschaus in the late 1970's. This is the experiment that won them the Nobel Prize.

In one of their experiments they were looking for mutations on chromosome 2 that affected embryo development.

They started with a line of flies that had eye color markers on chromosome 2; cinnabar (cn) and (bw). This is the red chromosome in the diagram. They treated males from this line with the potent mutagen ethyl methanesulfonate (EMS) and crossed them to a strain carrying a balancer chromosome (black) and a non-balancer homologous chromosome (blue).

The balancer chromosome is called CyO and it has several interesting features. First, it has a large inversion called In(2LR)O that covers most of the chromosome leaving only the ends intact. Second, it carries a dominant mutation called Curly (Cu) that produces flies with curly wings. This is a homozygous lethal mutation. Third, it carries another homozygous lethal mutation called dumpy-lethal (dp^1vI). Finally it carries cn. (Also purple (pr).)

The blue chromosome carries an allele called DTS-91. DTS stands for dominant temperature sensitive. Flies that carry even a single copy of this alleles will die at high temperature. DTS also stand for David T. Suzuki, the man who created these alleles but that's just a coincidence.

The first cross is done at high temperature. The flies produced from the first cross are the F1 generation. None of them will carry the blue chromosome because those flies will be killed at high temperature. All of them will carry one mutagenized chromosome 2 and the CyO balancer chromosome. These flies are crossed again with DTS-91/CyO flies at high temperature to get the second generation (F2) of flies that all contain one mutagenized chromosome 2 and CyO. The second cross helps eliminate other mutagenized chromsomes so that the workers will only be looking at mutations that affect chromosome 2.

Now the cn x bw/CyO flies are allowed to mate with each other until it's time to look at the effect of the mutations. The stock will never produce homozygous cn bw flies as long as the mutated chromosome carries a recessive lethal. Stocks that have flies with cinnibar eyes and not curly wings are discarded.

The stock will never produce flies that are homozygous for the balancer chromosome since it carries two recessive lethal mutations. All flies will have curly wings because they carry the balancer chromosome with Cu. There will never be a recombination event that transfers the developmental mutation to the balancer because the balancer contains a large deletion inversion.

This is why balancer chromosomes are so important n Drosophila genetics. They are essential for maintaining fly stocks carrying homozygous lethal mutations. Such mutations have been extremely important in sorting out fly development.

Christiane Nüsslein-Volhard and Eric Wieschaus created thousands of lines carrying recessive lethal mutations on chromosome 2, and thousands on the X chromosome and chromosome 3, each of which have their own balancers. Then they examined each line to look for embryos that were blocked during early development. (25% of the eggs will be homozygous for the mutant chromosome.)

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[Lower Figure credit: St Johnston (2002)]

St. Johnston, D. (2002) THE ART AND DESIGN OF
GENETIC SCREENS: DROSOPHILA MELANOGASTERNature Reviews: Genetics 3:178-188. [PDF]