Dirty sex: the role of bacteria in sexual selection

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  • The potential ecological or environmental effects on sexual selection have attracted very little scientific interest in evolutionary biology research. All that is set to change as Dr Oliver Otti, from the University of Bayreuth in Germany, probes ever deeper into the influence of host organisms’ microbial communities on the co-evolution of the sexes. Using an unusual experimental model, Dr Otti is using diverse populations of bed bugs from around the world to uncover the effect of microbial communities on their reproductive success and host fitness.

    Male and female reproductive traits display some of the most rapid evolution observed in nature and it has been suggested that sexual conflict is a major driver of this diversity. Other evolutionary conflicts are also known to propel evolutionary change, such as host–parasite and predator–prey conflicts. However, these conflicts are thought to have little role in shaping male and female reproductive traits. Although some researchers have focused on these other selective pressures to explain reproductive diversity, no attempt has been made to quantify the relative significance of the different mechanisms in shaping reproductive traits.

    The evolutionary quickie
    Dr Otti has previously attempted to explain the phenomenon of post-ejaculatory variation in reproductive success in terms of the relative contributions of natural and sexual selection. This found large environmental (i.e., not sexually selected) sources of variation, leading him to conclude that, “by focusing purely on sexual selection we may be missing a crucial driver of the evolution of reproductive systems”.

    Microbial infiltrators
    Clearly an important component of an organism’s environment is the microbial community surrounding and inhabiting it. The classical way in which these microbes are perceived to affect reproductive traits is by sexual selection of those traits which enable an organism to overcome the microbial pressures inflicted on it (fitness). However, Dr Otti believes that microbial communities have a more direct impact.

    There has been no attempt to quantify the relative significance of the different mechanisms in shaping reproductive traitsQuote_brain

    Haemocoelic fertilisation process – red indicates where most of the sperm are in the female abdomen at a given interval of time

    In males, this impact is likely to be in the form of sexually transmitted and opportunistic microbes found on the genitalia or within the female reproductive tract. As contact with these microbes has the potential to damage sperm in either a lethal or sub-lethal manner, it is expected that males would evolve methods to protect their sperm. This is corroborated by the presence of antimicrobial substances in the ejaculate of several animal species. This is particularly pertinent in so-called competitive fertilisation (females are inseminated by multiple males, any of which might result in fertilisation), where reduction in the number of available sperm will reduce a male’s chance of successful fertilisation.

    The potential of Dr Otti’s work should not be underestimated,
    paving the way for a fresh look at the development of reproductive traits not just in bed bugs, but far beyondQuote_brain

    Related issues in female reproductive efficiency are mainly linked to the invasive nature of penetrative copulation in many vertebrates. Dr Otti highlights the increased risk of infection by the introduction of both sexually transmitted and opportunistic pathogens during pairing. If damage occurs, and particularly in the case of multiple matings where risk of such damage and microbial load are both increased, females are exposed to potentially high-risk infection with the resultant drop in reproductive efficiency. This has resulted in the observation of a link between reproductive and immune activities, as well as morphological adaptations by females, such as sclerosis (hardening) and elongation of the reproductive tract.

    Bugs in the bed sheets
    Dr Otti’s preliminary research, backed up by the small body of literature currently available on the subject, shows that there is a definite reproductive cost to both males and females from the presence of opportunistic sexually transmitted microbes. This has resulted in the coevolution of protective mechanisms between the sexes by differentially altering selection pressures. Dr Otti believes this is just the tip of the iceberg, and that other microbes are likely to shape reproductive traits and contribute to their variation. His research aims to address this gap in scientific knowledge because, as he states: “It is critical to understand how sperm–microbe interactions influence male and female fitness, and to define the functional role of antimicrobial components in seminal fluid.”

    Bedbugs mating – the insects provide an excellent model for studying the interaction between reproduction and immunity.
    Image reproduced with kind permission of Richard Naylor

    Dr Otti’s current project uses bed bugs (Cimex lectularius) because the open reproductive system where immunogenic factors are involved is similar to higher vertebrates. Sperm are initially inseminated into an immune organ and then migrate to the ovaries; this clear and established time course allows Dr Otti’s team to empirically test the interaction between reproduction and immunity in both the males and females.

    As well as this, bed bugs’ mating practices have been well documented, with males mating far more frequently than is necessary. Repeated wounding and subsequent infections can reduce female life span by up to 30%. As males are in control of any processes before and during copulation, any compensation by females must naturally occur after mating through the physiological and immunological control of fertilisation success. This has already been observed, with two components of the female immune system being shown to attack sperm within the immune organ. What is not clear is the cause of high sperm mortality during the next stage of their journey, when they encounter microbes in the haemolymph (the invertebrate equivalent of blood).

    Experimenting with sex
    Through a complex, but well controlled, series of experiments, Dr Otti plans to achieve five distinct objectives:

    • Characterise the variation in microbe communities found in distinct populations of bed bugs, and their correlation to these populations’ specific reproductive traits (in males and females)
    • Determine the frequency of sexual transmission of potential pathogens and quantify the effect of these microbes on reproductive outcomes
    • Quantify the effect of microbes on sperm health in both males and females
    • Quantify the relationship between mating status and immune system regulation
    • Quantify the effect of microbes on sperm fitness

    In this way, Dr Otti will establish many novel insights into how individuals within the experimental populations deal with opportunistic infections and optimise reproductive investment. His background in evolutionary and behavioural ecology, and in particular his work on the conflicts between individuals and species, makes him well-placed to push forward this novel approach to the study of reproductive ecology. The potential of this work to bring together the studies of reproduction and host–microbe interactions should not be underestimated, paving the way for a fresh look at the development of reproductive traits not just in bed bugs, but far beyond.

  • Why are microbes such potent drivers of reproductive evolution?
    Microbes are immensely ubiquitous, they have colonised every possible habitat and organism. For bacteria, for example, all organic matter means food. Therefore, every organism will always be at risk of a microbial attack. Also, many organisms harbour microbes as commensals or mutualists and therefore have a rather friendly relationship with them. Nevertheless, microbe numbers need to be kept in check and the organisms have to deal with microbes that can become pathogenic when conditions are favourable. Many researchers concentrate on gut microbiota and their interactions with their hosts. But as already said, microbes are found everywhere, even in reproductive organs, and their impact on reproduction is basically unknown. So far, I only assume that they might be potent drivers of reproductive evolution.

    What makes the bed bug a good experimental species?
    As males can force copulation on females, they are in charge of all processes before and during copulation. This means processes by which females can manipulate fertilisation happen after copulation and are limited to female physiology. Such a reduction of processes is very helpful, when disentangling rather complicated interactions between males and females and host and parasites. Finally, a practical aspect is that bed bugs are small insects that do not live on a complicated diet or need a lot of space. We can have hundreds to thousands of them on a single shelf in a CT room.

    How will your work expand our understanding of reproductive ecology?
    My work will highlight the importance of the environmental conditions in which reproduction takes place, by elucidating how males and females deal with, or regulate, opportunistic microbes in their own reproductive organs or in the reproductive organs of the opposite sex. In addition, if males and females vary in their ability to regulate sexually transmitted opportunistic microbes, I hope to show that this has an impact on the variation of reproductive success between individuals. This would form evidence that opportunistic microbes have the potential to affect sexual conflict.

    What techniques are you using to identify microbes and their effects?
    In a first step, we combine an experimental approach with one of the latest gene sequencing techniques to make a catalogue of microbes for different reproductive organs and for virgin and mated individuals. Then we will use artificial infections and insemination, immune assays and mating experiments to identify effects of different microbes from the catalogue on males and females.

    What led you to the investigation of these effects on reproductive efficiency?
    I think it is a combination of aspects that have led me to this investigation. First, I was always astonished by how little variation in reproductive success is explained by the male and female genotype in studies on sexual selection, especially in sperm competition experiments. Second, an important review on sexually transmitted diseases in insects from 2004 (Knell and Webberley, 2004) seems to completely ignore bacteria. This might have been due to the lack of data, as at that time bacteria and other small critters were not easy to identify. In many discussions with my collaborator Klaus Reinhardt, we have then developed the idea that sperm–microbe interactions could affect reproductive success and that males and females should protect themselves if these microbes prove to be harmful. Therefore, we set up several initial studies investigating the importance of environmental, opportunistic microbes with regard to sperm viability.

  • Research Objectives
    Dr Otti is a biologist whose research focuses on the evolutionary and ecological aspects of sexual selection, host–parasite interactions and immunity.

    Funding
    Deutsche Forschungsgemeinschaft (DFG)

    Collaborators

    • Prof Dr Klaus Reinhardt (Technical University Dresden, Germany)
    • Dr Paul R Johnston (Free University of Berlin, Germany)

    Bio
    Dr Otti is an evolutionary biologist currently working at the University of Bayreuth. After completing his Master’s degree at the University of Bern in 2002, he studied a PhD at the ETH Zurich. In 2008, he moved to the University of Sheffield, where he began his current research programme with bedbugs.

    Contact
    Dr Oliver Otti
    Animal Population Ecology
    Animal Ecology I
    University of Bayreuth
    Universitätsstrasse 30
    95440 Bayreuth
    Germany

    T: +49 921 552 646
    E: oliver.otti@uni-bayreuth.de
    W: http://www.bayceer.uni-bayreuth.de/toek1/de/mitarbeiter/mit/mitarbeiter_detail.php?id_obj=106154

  • Dirty sex: the role of bacteria in sexual selection
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