Thursday, April 30, 2015

Evolution of U/V sex chromosomes

Posted by Jessica Abbott

Homalothecium lutescens, by

For next week's lab meeting I thought it might be interesting to read a paper about the evolution of sex chromosomes in haploid species. I was recently on the examination committee for Frida Rosengren, who has worked on mosses, a typical group with haploid sex chromosomes. These haploid chromosomes are usually called U and V, to distinguish them from diploid ZW and XY system. The fact that they usually exist in a haploid state leads to some interesting predictions, for instance that degeneration of the male V chromosome should not occur. I haven't read this paper yet but I think it sounds interesting!

Title: The evolution of sex chromosomes in organisms with separate haploid sexes

Abstract: The evolution of dimorphic sex chromosomes is driven largely by the evolution of reduced recombination and the subsequent accumulation of deleterious mutations. Although these processes are increasingly well understood in diploid organisms, the evolution of dimorphic sex chromosomes in haploid organisms (U/V) has been virtually unstudied theoretically. We analyze a model to investigate the evolution of linkage between fitness loci and the sex-determining region in U/V species. In a second step, we test how prone nonrecombining regions are to degeneration due to accumulation of deleterious mutations. Our modeling predicts that the decay of recombination on the sex chromosomes and the addition of strata via fusions will be just as much a part of the evolution of haploid sex chromosomes as in diploid sex chromosome systems. Reduced recombination is broadly favored, as long as there is some fitness difference between haploid males and females. The degeneration of the sex-determining region due to the accumulation of deleterious mutations is expected to be slower in haploid organisms because of the absence of masking. Nevertheless, balancing selection often drives greater differentiation between the U/V sex chromosomes than in X/Y and Z/W systems. We summarize empirical evidence for haploid sex chromosome evolution and discuss our predictions in light of these findings.

Immler & Otto (2015) Evolution 69(3):694-708.

Chance and direction in research

This is re-posted from Andrew Hendry's Eco-Evo Evo-Eco blog. By Jessica Abbott.

Since Andrew Hendry was kind enough to write a guest post about his career path to date, I was invited to return the favour. As with most researchers I know, my career path has been considerably influenced by chance events. In fact, now that I think about it, you can see this effect pretty much as far back as you want to go. Andrew started his story with his MSc work, but I’ve decided to put a bit more focus on the things that got me started on the road to research. I regularly give lectures for high school students, and one of the things they’re often interested in is how I decided to become an evolutionary biologist. Besides, all you have to do is look at my CV to get an idea of the things I’ve done during and after my PhD.

Some people you meet in science ended up there despite the fact that it was never their childhood dream. Others always wanted to be researchers. I fall into the second category. Ever since I was a kid I was interested in science, especially biology and astronomy. I first became interested in evolution when I read a book about it in 6th grade. At that time I didn’t really realize that you could be a professional evolutionary biologist, though, so I never really considered it as a possible career.

By the end of high school I had settled on marine biology as an interesting field. But I didn’t want to work with dolphins! At some point I’d seen a lecture by a local researcher from Trent University, who talked about the development of new cancer treatments from naturally-occurring chemicals (for example taxol, which is derived from yew trees and can be used to treat ovarian cancer). She also mentioned marine sponges, and how they might be a promising subject for similar research since they have effective but relatively non-specific immune function. This sparked my interest as a way to combine research in marine biology with some practical applications. I therefore decided to study marine biology at the University of Guelph during my undergraduate degree.

Suberites domuncula, by Guido Picchetti. Charismatic, no?
It was my first-year introductory zoology class that really made me start thinking about evolutionary biology. Ron Brooks taught the class and basically seemed to completely ignore the material that was supposed to be covered in the course, at least judging by the information we covered in the labs. Instead he talked a lot about evolution and told everyone to read The Selfish Gene. I was a good student, so of course I read it. And it made me realize that this was the sort of thing that I really wanted to work with.

I also wanted to broaden my horizons on a personal level, so I applied to go on an international exchange for my third year. My destination, Lund University in Sweden, was pretty random. I had originally applied to go to Aberdeen or Sydney, because they were the only two places that had marine biology programs (at least among the universities that Guelph had a reciprocal exchange agreement with). But because both these locations were highly popular (meaning only one semester abroad was allowed) and I wanted to go for a whole year, the exchange office suggested some other options. Lund seemed to have the most interesting selection of courses, so that’s where I decided to go, despite knowing basically nothing about the country or the university.

Lund is lovely in the spring.
Once I got to Lund, I really liked it. The classes were small and the material was interesting. Swedes were hard to get to know, but nice once you knew them. It was fun learning a new language. And of course I met my future husband. So rather than go back to Guelph I registered as a student in Sweden for the next year. And near the end of my second academic year in Lund I started a master’s project with Erik Svensson. My choice of project was also somewhat random. Because I was interested in evolutionary questions in general, I wasn’t so picky about the type of study organism. I asked around to find out who had a project that needed a student, and just went with the one that sounded most interesting. That’s how I ended up working on Ischnura elegans. When the opportunity arose to continue working with Erik in the same system, I took it.

As I neared the end of my PhD I started thinking about what to do next. I was never especially enamoured with field work, so I thought it would be fun to try working with a lab-based system. I was interested in the evolution of sexual dimorphism (I’d done a bit of work on sexual dimorphism during my PhD), but also in genetic conflicts. I’d run across Bill Rice’s work on intralocus sexual conflict (then often called ontogenetic sexual conflict) which combined both of these things, but at that point there weren’t so many people working in that area, so it wasn’t really on my radar. Then I went to ESEB in 2005 and saw a talk by Russell Bonduriansky about intralocus sexual conflict. It made me realize that this could be a viable option after all. I therefore got in touch with Adam Chippindale to see about doing a postdoc with him.

Adam’s response was a pretty typical one – he’d love to have me as a postdoc but didn’t have the money to hire me himself. But he was happy to help me out in designing a project so that I could apply for my own funding to go to Queen’s University. I applied to both NSERC and the Swedish Research Council (VR), and was successful with VR. That’s how I got started working on experimental evolution, and Drosophila, a method and a system which I still use today.

When we moved to Kingston we had hoped to stay longer than the two years of my VR fellowship, but when I applied for an NSERC postdoc again (my last chance) I wasn’t successful. The choice was between returning to Sweden with a new repatriation fellowship from VR, or being unemployed and living in my parents’ basement. I think you can guess which was the more attractive choice. That’s how I ended up in Uppsala, working with Ted Morrow. I took my fly populations with me and continued the stuff that I’d done at Queen’s in Uppsala.

I liked the fact that there were a bunch of sexual conflict people in Uppsala, and I liked working with Ted. When my one-year repatriation grant was up, I was lucky enough to be offered a one-year postdoctoral stipend by Klaus Reinhardt, funded by the Volkswagen Foundation. During that period I continued to work in Uppsala, but on a collaborative project with Ted and Klaus. The stipend kept me going until I was successful in obtaining a Junior Researcher Project grant from VR.

Macrostomum lignano mating, by Lukas Schärer.
The Junior Researcher grant let me start up my own small group, and start work on a new study organism, Macrostomum lignano. (The story of how I decided to do a project on Macrostomum is also interesting and much influenced by chance events, but I won’t go into details here. This post is long enough already.) Although I considered staying in Uppsala, in the end I decided to move back to Lund, both for personal and professional reasons. I liked having a lot of people that shared my interest in sexual conflict in Uppsala, but the downside was that it meant that I was just one of many, and that I wouldn’t necessarily bring anything new to the department. Lund was also closer to old friends and my husband’s family. I’ve been working here since 2012.
Looking back, it’s clear that both chance and direction have played a role in my career path. In many ways, I’m exactly where I had hoped I would be at this stage, when I imagined my future as a teenager. I imagined myself working at a good research university (preferably abroad), in a good relationship (maybe kids – not essential), combining research, teaching, and popular science in an enjoyable mix. These things are all true (that’s where the direction part comes in). However exactly what I’m working on and where I am are different than what I expected (that’s where the chance part comes in).
It’s also been a lot harder than I had expected it to be. It’s not like I thought being a researcher would be easy. But being a postdoc with no option to plan long-term, no job security, and a family, was much harder than I had expected. A common theme when senior scientists talk about their career paths is “I just worked on whatever I thought was most interesting, I never tried to think strategically”. I know that PhD students and postdocs can find this a bit frustrating – even if this approach is perhaps a necessary condition for success, it’s probably not sufficient. There’s probably just as many people out there (or more!) who followed their hearts but didn’t get that tenure-track job or key big grant, as the ones who did. I can understand this frustration, because “just do what you think is fun” is not very helpful advice. However, one can also look at it another way. It’s good to have long-term goals in mind (direction), so that you can take the right opportunities as they come up (chance). But if you’re not really enjoying your work while you’re working on it, what’s the point? Don’t spend a lot of time doing things you don’t like just because you think they’re strategic. You might get hit by a bus tomorrow.

Friday, April 24, 2015

Labmeeting: Talk by Rosa Sanchez on Robertsonian fusion systems in mice on April 28

 Posted by Erik Svensson on behalf of Rosa Sanchez

Next week's lab-meeting will consist of a talk of Spanish postdoc Rosa Sanchez, who is more known for her work on damselflies, but who has also worked on mice during her last postdoc. Time and place as usual:

Time: Tuesday, April 28, 10.30

Place: "Argumentet", 2nd floor, Ecology Building

Any fika-volunteer?

Below is information from Rosa about the content of her talk:

The Spanish Robertsonian fusion system on the House mouse Mus musculus domesticus
This lab meeting I am going to talk about my work on the house mouse Mus musculus domesticus about the mechanism(s) responsible for the formation and maintenance of Robertsonian (Rb) s in natural populations.
The house mouse Mus musculus domesticus is arguably the best-studied and understood model of variation of Rb fusions in nature. I have studied Spanish Rb system, which occurs in a 5,000 km2 area of Barcelona province and is characterized by a high level of chromosomal polymorphisms [i.e., seven different Rb chromosomes including Rb (3.8), (4.14), (5.15), (6.10), (7.17), (9.11) and (12.13) showing non-geographically coincident clines, and low recombination rates. 

Friday, April 17, 2015

Lab Meeting on Quantitative Genetics

This week in lab meeting we will have a discussion on quantitative genetics. Anna Nordén will present some preliminary results from her PhD work in the hermaphroditic worm, Macrostomum lignano. She is conducting a quantitative genetics study in which she investigated the heritability of fitness through female and male sex roles. Additionally, I suggest that we discuss the following paper that attempts to synthesize quantitative genetic studies with studies of DNA sequence variability in Drosophila.

Hope to see you there! I'll bring fika.

Figure from Bank et al. 2014: Two hypothetical Distributions of fitness effects (DFE) of all possible new mutations.

Causes of natural variation in fitness: Evidence from studies of Drosophila populations

Brian Charlesworth

    DNA sequencing has revealed high levels of variability within most species. Statistical methods based on population genetics theory have been applied to the resulting data and suggest that most mutations affecting functionally important sequences are deleterious but subject to very weak selection. Quantitative genetic studies have provided information on the extent of genetic variation within populations in traits related to fitness and the rate at which variability in these traits arises by mutation. This paper attempts to combine the available information from applications of the two approaches to populations of the fruitfly Drosophila in order to estimate some important parameters of genetic variation, using a simple population genetics model of mutational effects on fitness components. Analyses based on this model suggest the existence of a class of mutations with much larger fitness effects than those inferred from sequence variability and that contribute most of the standing variation in fitness within a population caused by the input of mildly deleterious mutations. However, deleterious mutations explain only part of this standing variation, and other processes such as balancing selection appear to make a large contribution to genetic variation in fitness components in Drosophila.

      Friday, April 10, 2015

      Natural selection on thermal performance in a novel thermal environment

      Next lab meeting paper will be:

      Natural selection on thermal performance in a novel thermal environment

      Michael L. Logana,1, Robert M. Coxb, and Ryan Calsbeeka
      Tropical ectotherms are thought to be especially vulnerable to climate change because they are adapted to relatively stable temperature regimes, such that even small increases in environmental temperature may lead to large decreases in physiological performance. One way in which tropical organisms may mitigate the detrimental effects of warming is through evolutionary change in thermal physiology. The speed and magnitude of this response depend, in part, on the strength of climate-driven selection. However, many ectotherms use behavioral adjustments to maintain preferred body temperatures in the face of environmental variation. These behaviors may shelter individuals from natural selection, preventing evolutionary adaptation to changing conditions. Here, we mimic the effects of climate change by experimentally transplanting a population of Anolis sagrei lizards to a novel thermal environment. Transplanted lizards experienced warmer and more thermally variable conditions, which resulted in strong directional selection on thermal performance traits. These same traits were not under selection in a reference population studied in a less thermally stressful environment. Our results indicate that climate change can exert strong natural selection on tropical ectotherms, despite their ability to thermoregulate behaviorally. To the extent that thermal performance traits are heritable, populations may be capable of rapid adaptation to anthropogenic warming.

      Where: Seminar room "Argumentet", 2nd floor (Ecology Building)
      When: Tuesday, April 14, 10.30
      Fika: An assortment of cookies and fruit. 

      Sunday, April 5, 2015

      Lab-meeting: is there a physical limit to fitness?

      Posted by Erik Svensson

      I hope all ESEB-members have had a Happy Easter. I post somewhat late for the lab-meeting tomorrow, but we will start lightly with a short and fairly recent paper in Science from Rich Lenski's laboratory, which adresses a fundamental question in evolutionary biology: is there a limit to how much fitness can increase? You will find a link to the paper here, and the Abstract is posted below.

      Time and place as usual:

      Where: Seminar room "Argumentet", 2nd floor (Ecology Building)
      When: Tuesday, April 7, 10.30

      Long-Term Dynamics of Adaptation in Asexual Populations