Specific Research themes:
Sexual selection, conflict, and the evolution of visual mating signals
Many animals use colour signals for sexual signalling. Our interest is in understanding the evolution of the most exaggerated versions of these signals, such as the extremely bright and iridescent structural colours of tropical butterflies and other insects. These colours are often limited to males, or considerably more exaggerated in this sex, and are known to be used in various realms of sexual competition. In the case where such a colour badge is used to indicate mate quality, theory expects them to become highly exaggerated (and, hence, costly), sexually dimorphic, and strongly condition-dependent. Empirical studies across a range of animals have also demonstrated that these traits exhibit higher genetic variances than most other non-sexual traits, probably because their expression is strongly integrated into developmental and physiological processes.
We address questions at the forefront of sexual signalling theory by observing and manipulating morphology, behaviour and development, and by using breeding experiments to explore phenotypic and genetic sources of variation. We are also interested in the mechanisms of colour signal production, which we study using reflectance spectrometry and electron microscopy. The male-limited nature of these traits also offers opportunities for studying intralocus sexual conflict and the evolution of sexual dimorphism. Projects along these lines are currently underway using butterflies, harlequin bugs, guppies and lizards.
Predicting the direction of evolution in the wild
Part of our interest in animal colouration is to explore whether different types of signals are likely to be inherently favoured as components of sexual signalling systems. The field of colour-based signalling has been pre-occupied with how a select few pigments (mostly carotenoids) give rise to ‘honest’ signals because they must be acquired from the environment (rather than synthesized). However, there is growing recognition that colour signals generated by a broad range of non-mutually exclusive pigmentary and structural mechanisms will also be costly to express, and hence, potentially highly honest signals. The key question is whether certain ways of producing colour are likely to be consistently and universally favoured by evolution, or whether certain colours are favoured due to species-specific sensory drive, sensory bias (etc.), or whether colour traits are arbitrarily subject to elaboration in an idiosyncratic way.
We are presently studying these questions in guppies (Poecilia reticulata) and via ongoing collaborations with Prof. David Reznick (University of California, Riverside). Prof. Reznick heads a major research program whereby guppies from various Trinidadian populations have been transplanted to upstream reaches in which visually-mediated predation is markedly reduced. Sexually selected colouration becomes elaborated in these reduced predation populations, and we can chart the trajectory of such evolution over subsequent years.
Excellent opportunities for addressing colour signal evolution also exist in the realm of phylogenetically-controlled comparative studies.
Evolutionary/behavioural ecology of host-endosymbiont interactions
Our interest in endosymbiosis stemmed from the serendipitous discovery that one of our model butterfly species (Eurema hecabe) is host to the bacterium Wolbachia. This widespread endosymbiont of invertebrates is transmitted vertically, through the maternal line, and sometimes manipulates its hosts’ reproduction to apparently facilitate the rate of such transmission. One of the more intriguing manipulations is feminisation, whereby Wolbachia hijacks its hosts’ sex-determining machinery to cause genetic males to develop into viable female phenotypes (se-called “she-males”). While enhancing bacterial transmission, this disrupts host sex ratios from the Fisherian ideal (50:50), which sets the scene for a co-evolutionary arms race between host and endosymbiont.
We are studying co-evolution in this system by exploring the potential for counter-adaptations in the host, such as resistance (to feminisation) in infected lines and/or mate discrimination. This work is centred upon Eurema hecabe, but we are also keen to see whether this strain of Wolbachia has spread horizontally to closely related butterflies or other arthropods that interact with E. hecabe. The presence of feminising Wolbachia offers outstanding possibilities for gaining insights into functional and evolutionary aspects of sex determination and sex differentiation, including the role played by epigenetic factors (such as DNA methylation).
Predicting adaptive potential
We also have interest in understanding the extent to which evolutionary genetic factors can explain contemporary species distributions, and the potential for such populations and species to respond adaptively to environmental change. This work is done in collaboration with Dr. Carla Sgro (Monash University) and is based on findings in Drosophila suggesting that restricted tropical species may in fact lack adequate amounts of genetic variation for coping with environmental stress. Our interest is to study these phenomena in Eurema butterflies, which have quite different life histories than flies, yet which show similar distributional patterns and are also amenable to lab-based quantitative genetics.
It surely is nice!