Its Not You, Its Me - Sex, Fruit Flies, and Evolutionary Biology in Canada

I’m back!  Sorry I’ve been silent for the past week.  I just came back from five days in Banff, Alberta (with amazingly beautiful 20-degree weather the entire time, which is unheard-of in Alberta in May).  Unfortunately, I didn’t have much time to enjoy the scenery, as I was responsible for registration for the 2011 conference for the Canadian Society of Ecology and Evolution.  My supervisor, Sean Rogers, was in charge of the event, and pulled off what I think is unanimously considered the best CSEE conference to date.  Not that the ones in the past were bad, but this year’s venue at the Banff Center was simply perfect, and I am happy to have been a small part of it.

The conference was essentially a who’s-who of evolutionary biologists and ecologists in Canada (and beyond).  The featured speakers were Jerry Coyne (author of Why Evolution is True, and writer of his own popular anti-religious blog), the amazing Athabasca oil sands researcher David Schindler, palaeontologist Phil Currie, and the University of Calgary’s own bat expert, Robert Barclay.  But these were just the tip of the iceburg, with researchers like Dolph Schluter from UBC, Louis Bernatchez from Laval, Hopi Hoekstra from Harvard, Jeff Hutchings from Dalhousie and numerous other renowned and up-and-coming scientists presenting their research.   I myself presented a poster highlighting the current state of my Master’s thesis, which, unfortunately, was rather dataless.  Hopefully this will be rectified in June.

I definitely geeked out during the conference, and was able to get away from my job to listen to a number of incredible talks.  I wanted to take a minute today to share with you what for me was one of the highlights.  I neglected to take notes, so please forgive me if I forget some of the details.

The talk was one of the thirty-minute symposia (most talks were twelve minutes), given by Dr. Amanda Moehring from the University of Western Ontario, and was entitled ‘The genetic basis of behavioural isolation between species’.  It was a very elegant experiment with some amazing results.

Drosophila melanogaster is the Latin name for the common fruit fly, a small and relatively unimposing animal whose rapid life cycle has made it an ideal experimental organism throughout the 1900s. 

Have you ever seen a fruit fly suddenly stop walking and raise one of its wings in the air?  I always thought there was something wrong with those flies’ wings, but apparently not; apparently male Drosophila will exhibit this behaviour as part of an elaborate courtship display.  With the wing in the air, they vibrate it rapidly to produce a sort of love song to the female.  The male then walks behind the female and gives her backside a little lick.  If the female is receptive, she spreads her wings and remains still; the male mounts her to copulate.  If the female is not receptive to the male, all she needs to do is walk; the male is then unable to mount her.

Within Drosophila melanogaster there are choosy females who carefully pick their mates, and there are others who are a bit ‘looser’ and will sleep with most anyone. 

It turns out that such female ‘choosiness’ behaviour is also important in separating species.  There are different species of Drosophila other than D. melanogaster.  I wish I could remember which species she used, but I think it was D. simulans (confound my unpreparedness for note-taking!).  At any rate, there is an unusual fact about these species: while female melanogaster will readily mate with male simulans, simulans females are choosier and will reject melanogaster males.

We thus have an example of behaviourally-determined reproductive isolation between two species, with the isolation being asymmetrical.  (For non-scientists, reproductive isolation is considered one of the prerequisites for defining a species under the biological species concept.  If two populations readily mate with one another and produce offspring that are healthy and fertile, those populations will share genes with one another and essentially be quite similar.  But if two populations refuse to mate, or are physically incapable of mating, or produce infertile hybrids, or breed at different times of the year or the day, etc etc etc, they are said to be reproductively isolated.  Since they are unable to swap genes, changes in one population will not penetrate the other, and so over time they can become quite distinct.  In this instance, we have a female behaviour called choosiness which prevents two species of Drosophila from mating, but this is asymmetrical because only one of the species has this choosiness; Drosophila melanogaster females will often mate with Drosophila simulans males, but never the other way around).

What causes choosiness in female simulans but not in female melanogaster?  Is there a gene that is responsible?

This is where the experiment becomes quite elegant, but it requires a bit of knowledge on your part to make sense.  You need to know two things: what chromosomes are, and what dominant and recessive genes are.

I hope you all know that your cells contain 46 chromosomes, 23 of which came from your mom and 23 of which came from your dad.  In humans there are in fact only 23 kinds of chromosomes, meaning that you received a full set of 23 from one parent and a full set of 23 from another parent.  Each chromosome is given a number for its name, so you have one copy of chromosome I from your mom and one copy from your dad, etc.  Under a microscope, these 23 chromosomes look quite different from one another, being of different sizes and, sometimes, shapes.  These chromosomes are made up of DNA, and much (but by no means all) of this DNA is subdivided into genes.  Thus each chromosome variety contains a different number of genes, and those genes code for different things.  Some genes code for the proteins that make up your hair, some for the enzymes that break down lactose, some for the behaviours that you exhibit, etc.  Lots more could be said about genes, but that should be sufficient.

Now, since you have two copies of the same chromosome variant, one from your mom and one from your dad, this means that you have two copies of the same gene.  Sometimes these copies are identical, but sometimes they are different.  These different versions of the same gene are known as alleles.  What happens if your body contains two different alleles of the same gene?  A number of different things can happen, but the simplest scenario is when one allele’s product is shown, and the other’s is not.  The visible allele is said to be dominant while the invisible one is said to be recessive.  If you have two copies of the recessive allele, you will show the product of that recessive allele.  For example, the allele that gives peas a smooth skin is dominant over an allele that gives peas wrinkled skin.  If a pea gets one smooth allele from its mom and one from its dad, it will be smooth; if it gets one wrinkled allele from its mom and one from its dad, it will be wrinkled; but if it gets one smooth allele from one parent and one wrinkled allele from the other parent, it will have smooth skin.  Smooth is dominant to wrinkled; wrinkled is recessive to smooth.

Back to the fruit flies: it was observed that the lack of choosiness in melanogaster females was dominant to the choosiness in simulans females.  How did they discover this?  They bred melanogaster with simulans to produce a hybrid.  All of the hybrid offspring got half of their chromosomes from the one parent (melanogaster) and half from the other parent (simulans).  All of the hybrid females were willing to breed with either parent, indicating that the ‘choosiness’ allele from simulans must be recessive to the ‘loose’ allele in melanogaster.

Still with me?  Because this is where it gets good.  The lab wanted to identify the genes responsible for choosiness.  But there are four chromosome types in Drosophila, containing over 27 000 described genes.  So how would you even begin to narrow in on one gene?

Behaviour is remarkably complicated and is likely based on a large number of genes.  This means that there was a good chance they would find a responsible gene on any chromosome that they looked.  They chose a large region in chromosome 3 to begin their search. 

They performed their search by taking advantage of the decades of genetics research done on Drosophila melanogaster (considerably less has been done on simulans).  There is literally a catalogue of melanogaster mutants that can be ordered from the warehouse – their lab ordered every mutant type that had a known deletion in part of chromosome 3.

Basically, workers at this warehouse had used some interesting genetics techniques to remove entire segments of chromosome 3, and had recorded the segments that had been removed.  For example, if a chromosome contained genes ABCDEFGHIJKLMNOP, deletion mutants could look like ABCMNOP or AGHIJKLMNOP, etc.  What is the value of removing a segment of melanogaster’s chromosome?  Well, by crossing one of these deletion mutants with simulans, the hybrid would only have one copy of certain genes – those genes missing from their deletion-mutant melanogaster parent.  The single copy they would have to have acquired from simulans. 

For example, if the melanogaster mutant ABCMNOP was crossed with a normal simulans, the offspring would inherit two copies of genes A,B,C,M,N,O,P, but only one copy of genes D through L.  D through L would all be from the simulans parent.

Since choosiness was recessive, if any of the hybrid females refused to mate with melanogaster males, this would mean that the previously hidden simulans choosiness allele was being expressed.  It could only be expressed if the dominant allele was not present; the dominant allele could only be missing if it had been deleted from melanogaster.  Therefore the gene had to be found in the section of the missing chromosome.

In the above example, if the hybrids were refusing to mate with melanogaster males, we would know that the simulans choosiness allele was being expressed, and that it had to be located somewhere within genes D-L.  Genes A,B,C and genes M,N,O,P would therefore have been eliminated from our search.

So, Dr. Moehring’s lab carried out the experiment.  They used different mutants that covered the entire span of the section of chromosome three that they were interested in, and sure enough they found a few segments of chromosome three that, when deleted, unmasked alleles in the hybrids that caused them to be choosy.

They then chose one of these regions to narrow in on by using overlapping deletions.  For example, if the choosiness gene had to be one of genes D-L, they could use mutants missing D-L and A-F; if neither of the hybrids were choosy then the area of overlap, containing genes D,E, and F, could be eliminated as a possibility and more overlapping deletion mutants used.  By doing this they narrowed in on the gene responsible for female choosiness.

To their surprise, the gene had no readily apparent relationship with behaviour.  Instead, it was a gene that was responsible for producing a microtubule binding protein.

At first they were perplexed by this.  Microtubules are structural elements within cells.  They are highly important, but how do they cause choosiness?  It turns out that mutations in these microtubule binding proteins can lead to dramatic changes in brain structure, and changes in brain structure almost certainly affect behaviour.  Thus it would seem that simulans have a mutation in their microtubule binding proteins that change their brain structure and thus their reproductive behaviour, while melanogaster microtubule binding proteins cause a slightly different brain structure that produces a dramatically different reproductive behaviour.

They named this gene Its not you, its me.

My friend absolutely loathes the Australian accent, while I love it.  Our joke for the week was that we had differences in our microtubule binding proteins.

Yes, we are geeks.

But I defy you to tell me that this experiment is not classy.

Images from http://abugblog.blogspot.com/2010/09/celebrity-flies.html

http://ghr.nlm.nih.gov/handbook/basics/howmanychromosomes

http://www.topophonien.de/Neue%20Website/mainpages/underpages/brennesselgarten_artikel_en.htm